Contents:
Sam's Repair Briefs - Complete: 1 to 99
Copyright (c) 1994, 1995, 1996, 1997, 1998
All Rights Reserved
Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:
This series presents case studies of selected repair problems from my archives. (Archives sounds more impressive than scribbled notes and schematics, doesn't it?). I hope that these summaries are useful for those of you are interested in tackling your own repairs. These will NOT be of the form: "replace C418 when vertical size is reduced on HyperTech TV chassis HT312". Rather, they will document specific but common problems with TVs, VCRs, CDs, computer peripherals, etc. The symptoms, testing, diagnostic procedure, repair procedure, and comments will be included so that you can learn from my approach (and my screwups). If you want specific solutions to well known or repeat problems with your RCA or Zenith, then the repair professionals who frequent this group will be in a better position to help since they work on many of the same line of equipment on a regular basis. However, if you want to develop a general diagnostic approach, then this series may provide some tips and insight based on theory and experience. Not all of these are exciting cast-of-thousands repairs. Many tend to be mundain but address common problems with consumer electronic equipment. You will no doubt recognize some as directly pertaining to some appliance or electronic device that you had repaired (or junked) in the past. There will probably even be some of those 'dogs' that we all hate - the problems that never seem to go away. In any case, these will all be based on my true experiences with minimal embellishment.
Note: These articles are archived at: * http://www.repairfaq.org/ under the heading: Assorted Repair Briefs. These are maintained by Filip Gieszczykiewicz. Repair Guides are also available at this URL for devices and problems similar to those covered in the Repair Briefs(c) series. ************************************************************************ 1 * Daisy Wheel Printer - Carriage gets stuck * 2 * Panasonic PV1461 VCR with Dead Power Supply * 3 * USR Data Modem Won't Dial * 4 * RCA 25" TV Blows Fuse * 5 * Pioneer CD Changer will not Recognize Disc * 6 * Dead Power One 400 W Multiple Output Switcher * 7 * Goldstar CR820U TV with no Color * 8 * Panasonic PV3720 VCR with Erratic Horizontal Video Noise * 9 * Original ATT Touch Tone Phone will not Dial * 10 * Pioneer PD5100 CD Player Trashed * 11 * Yamaha R8 Receiver with Tuning Problem * 12 * Sylvania TV with no Horizontal Sync * 13 * Dead Microwave * 14 * Tandy Color TV/Monitor with hum bars * 15 * Realistic CD Player Randomly Shuts Off * 16 * Sony CDU31A CD ROM Drive Failure * 17 * Panasoic Color TV Shakes and Pulsates * 18 * Dead Zenith color TV * 19 * Goldstar VCR caught in infinite eject loop * 20 * Nintendo Game Console will not Reset * 21 * Sharp VC7864U VCR Erratic * 22 * Magnavox Console TV Deflection Problems * 23 * Hewlett Packard AN/USM281A Oscilloscope with Multiple Problems * 24 * Sony Servolock Turntable Erratic * 25 * Kenwood KX-55C Cassette Deck with dead Transport * 26 * Kenwood CD Changer with erratic behavior * 27 * Panasonic PV 4820 VCR with multiple problems * 28 * JVC HR-D860U VCR mangled tape * 29 * Panasonic VCR with video noise, audio hum, and erratic operation * 30 * JVC 6 Disc CD Changer gets stuck * 31 * Mitsubishi HS-318UR VCR Dropped * 32 * Toshiba T1000XE PA8706 AC Adapter * 33 * Mac Plus with no Video * 34 * RCA EFR485 (CTC111) Color TV Part 1: Messed up Colors * 35 * Panasonic PV1414 Closed Circuit TV Camera with no Video * 36 * Heathkit IM2202 Digital Multimeter - Dead * 37 * RCA EFR485 (CTC111) Color TV Part 2: No Picture * 38 * SYLVANIA RXX170-WA01 Color TV Clicks - No Power * 39 * Sony Compact Stereo Model HP-179-P - FM Dead * 40 * Mitsubishi HS-U53 - Bad Tracking * 41 * Mitsubishi FA2100-CW FAX Machine will not Send * 42 * Mitsubishi HS3381UR VCR Tracking Bands * 43 * Panasonic PV2812 VCR - no Play or Record * 44 * JVC HRD-550U VCR - Tracking Problems * 45 * SEARS (Goldstar) VCR Part 1: Broken Cassette Loader * 46 * Beckman Model 310B Digital Multimeter with Random Display * 47 * Quasar TT4259WW Color TV - Dead * 48 * SEARS (Goldstar) VCR Part 2: Shutdown in Play and Record * 49 * HP DeskJet Professional Printer - Part 1: Dead * 50 * Panasonic PV1545 HiFi VCR - Eats Tapes * 51 * HP DeskJet Professional Printer - Part 2: Missing Lines * 52 * Aztech CDA-268-01A CDROM Drive - Drawer Continuously Closing * 53 * Craftsman Electric Drill - Worn Motor Bearing * 54 * NEC CDR-260 Double Speed CDROM Drive - Intermittent * 55 * RCA FPR560ER Color TV - Erratic Behavior * 56 * Toshiba M4200 VCR - Weak Intermittent Audio * 57 * HP DeskJet Professional Printer - Part 3: Print Fades Out * 58 * Magnavox FD2000-SL01 CD Player - Dead * 59 * GE 13AC1504W Color TV - Dead (with other problems) * 60 * Canon FaxPhone 80 - More or Less Dead * 61 * Sony D14 Portable Compact Disc Player - Smoked * 62 * Zenith System 3 Color TV - Intermittent Blue Fog * 63 * Sony Boombox CD Player - Erratic Shutoff * 64 * Book Tape Player - Missing Channels 3 and 4 * 65 * Aiwa CSD-707 Boombox CD Player - Doesn't Recognize CDs * 66 * Sony KV-13TR-20 Color TV - Erratic Reception * 67 * Magnavox Phonograph - Record Changer Problems * 68 * Clocks, Clock Radio, and Cordless Phone - Smoked * 69 * GE Portable Color TV - Dropped * 70 * Nutone RF Wireless Chime - No Response to Button * 71 * Tandy Color TV/Monitor - Intermittent Shutdown * 72 * Braun Quartz Clock - No Movement * 73 * Sony D88 Portable CD Player Part 1 - Does Not Recognize Discs * 74 * Sony D88 Portable CD Player Part 2 - Erratic Audio Noise * 75 * Panasonic CT-714 Color TV - Dead * 76 * Emerson VGA Monitor - Dead Doggy * 77 * Sony KV-2675R Color TV - Will Not Power Up * 78 * Magnavox 31 Inch TV - Dropped and Fried * 79 * ConairPhone Desk Phone - Almost Dead * 80 * Sharp 13KM15 Color TV - Dead * 81 * Sony KV-19TR20 Color TV - No Reception * 82 * Emerson CGA Color Monitor - Dead * 83 * Tatung CM1495 Multisync Color Monitor - Dead * 84 * Sony Portable CD Player - Dead * 85 * Lambda LUS-8-12 Switching Supply - Fried * 86 * Fisher Power Amp with Blown Channel * 87 * Mac Plus and Original Apple 20 MB Hard Drive - Multiple Problems * 88 * HP DeskJet 500C Color Ink-Jet printer - No Printing * 89 * Craftsman Eager 1 Lawn Mower - Rod Disaster * 90 * Harman-Kardon Model 520 Stereo Receiver - Multiple Problems * 91 * Multifunction Desk Phone - Erratic Operation * 92 * Aiwa Sports 250 Boombox - Dropped * 93 * Kenmore Microwave - No Heat and Dead Buttons on Touchpad * 94 * Hoover Vacuum Cleaner Doesn't Pick Up * 95 * Subaru Auto AM/FM/cassette - Intermittent Reception * 96 * GE Frost-Free Refrigerator - Reduced Cooling * 97 * Panasonic VCR Power Supply - Comedy of Errors * 98 * Kenmore Window Fan - No Breeze * 99 * JVC 20 Inch Color TV - Flat Line * ************************************************************************
Patient: Panasonic KX-P3131 Daisy Wheel PC printer.
Symptoms: After some random amount of printing, the carriage would get
stuck and just twitch instead of moving across the platen.
Testing: Using its internal test function, I let it print out until the
problem ocurred.
Once the problem ocurred, the carriage would just kind of twitch back
and forth. Grabbing it, it was obvious that the stepper motor was providing
no effective power. Since this is usually indicative of a missing phase
to the stepper motor, I immediately suspected either:
1. Bad connection - cable or soldering.
2. Bad driver.
3. Bad motor.
Since it was intermittent with no relationship to time, heat, position, etc.,
this is most likely (1).
I then removed the cover to gain access to the circuit board and motor
connections. It was necessary to defeat the cover interlock to get
it to come one. Now, to get it to screw up again.
I left it printing out the ASCII character set and got a byte to eat.
When I came back it was busy gouging a hole in the paper. So now for
the critical test: Will pressing on the stepper motor connector cause
a change? The answer is --- Yes! The carriage started moving again meaning
that it is likely a bad connection.
Of course, to gain access to the underside of the circuit board required
removing a zillion screws and the entire mechanical assembly. But once
this was accomplished - immediate gratification. There were obvious
bad solder joints around several pins of the stepper motor connector.
I resoldered these and few others that were suspicious and inspected the
rest of the board. If only I could remember which screws went where!
Apparently, the continuous vibration of the assembly eventually caused
the connections to fail. This is not likely a heat related problem though
it could be just plain bad quality control.
Once reassembled, I left it happily printing out page after page of ACSII
characters. Then, just to be sure, I connected an old laptop and printed
a few pages of Repair Notes.
Comments: This is one of those dream problems since their solution is so
obvious and so definitive. There is no doubt that the cure will last.
Unfortunately, the tough 'dogs' are the ones you lose hair over.
Patient: Garage Sale Panasonic PV1461 VCR (Asked $10, offered and accepted $5).
Symptoms: The power supply had been identified as being bad by the seller.
Testing: I plugged it in - nothing as expected. This was to verify that
all functions and display were dead.
I removed cover and found that the main fuse had been removed. Hmmm, this
usually means catastrophic power supply problem. This is the typical Panasonic
switching power supply. Well, I have completely rebuilt these, so no biggie.
(Of course, there was one I blew up, but that is another story).
The hardest part was removing the power supply. It is buried underneath
the bottom circuit board necessitating the removal of this board and the
front panel. Do you think they design these things this way to discourage
tampering? There was plastic in specific places to prevent removal from
the top even though it would have been trivial to design for easy removal.
I disconnected power supply from VCR. Fortunately, this was just a connector.
I tested across switchmode power transistor with ohmmeter - dead short C-B-E.
I looked in ECG for 2SC3890 - ECG379 with that infamous # indicating an
electrical but not mechanical match.. Since I had some BU406s, I looked
this number up in ECG guessing that the BU406 would be have similar ratings
and be usable in a small switcher - guess what, ECG379. The difference is
that the 2SC3890 is totally plastic while the BU406 is a metal tab TO220. OK,
so I cut out a bit of mica to serve as an insulator and used a nylon screw.
This is temporary as I intend to get the proper replacement (2SC3890 - $2.15
from MCM Electronics).
I also checked continuity from the main filter cap to the C and E leads
of the transistor to rule out a blown fusable emitter resistor. I checked
other semiconductors as well - all fine as far as my VOM was concerned.
Fortunately, the only casualty seems to have been the transistor and the
fuse was fast enough to prevent any damage due to its shorting.
To power up the supply, I initially used a Variac with a 25 W light bulb
in series with the line. Note that since I do not have any 1.6 A
fuses, the fuse is shorted. The light bulb will provide the current limiting
for now. I use my dual outlet widget box plugging in the supply to one
outlet and a lamp with the 25W light bulb into the other (the outlets are
wired in series for exactly this sort of application). This whole rig
was plugged into an isolation trasformer for safety.
I then identified the primary output and connected my VOM to this. It would
be in the range 5-15 V, probably 12 V based on the filter capacitors (16 V).
All set? Crank up power. Output comes up to about 13 V at around 50 VAC
in. Light Bulb hardly flickered. If it had not stopped in the 12 V range,
this would either indicate that there was a problem in the regulation or
that a load was needed. Had this happened, I would have put a 22 ohm 5W
resistor on this output and retested. Cranking up the Variac to full
voltage causes no noticeable change to output.
OK, connect VCR. Lights, VCR, power! Nothing. Light bulb is now
glowing but there is no indication of life from front panel and no action
from motors.
Hmmmm.
Examining the nameplate, the expected power consumption is 29 W which is
way more then one can get through a 25 W light bulb. (Expect to be able
to draw maybe half of the bulb's ratings). So I go hunting for a 60 W
light bulb.
This time the motors twitch and the front panel comes alive.
Inserting a tape and holding my breath - tape starts to load but
then aborts with poweroff. Go for it! The Variac had been set at
about 90 VAC, so I crank it up to 120. Now everything works. I go
dig up a TV and verify that the basic functions are ok. Doesn't appear
to even need much cleaning. Even the idler tire appears to be in good
condition.
Add a 2SC3890 and box of 1.6 A fuses to my next MCM order.
Comments: this is the sort of repair that might not pay for a professional
shop to undertake. The time to disassemble the VCR, identify the problem,
replace the transistor and fuse, and verify correct operation could be
excessive, or at least has the potential to be excessive. If it turned
out to be more than a transistor and fuse or was beyond repair, they might
have to eat the cost in time and materials. In addition, there is no real
way to guarantee that other marginal components won't cause the problem to
repeat a week or month in the future. Upgrade/repair kits are available for
these supplies and would probably represent the lowest risk investment for
a permenant repair. No doubt, the previous owner had taken it in for repair
and been quoted a rediculous price to replace the entire power supply module.
Patient: USR 14.4 K baud external data modem.
Symptoms: Modem works in most respects but when it goes to dial, the tones
are superimposed on the dialtone which never goes away.
Testing: I plugged it into an old laptop kept for the specific purpose of
testing of random external peripherals. Indeed, the AT commands
worked just fine but tone dialing did not work. Interestingly,
pulse dialing would work 90% of the time but a connection was
never completely established.
At this point, I put a scope on each side of the 600 ohm coupling transformer.
A normal 2 or 3 volt p-p signal was present on the logic side of the
transformer during dialing. However, a much attenuated signal was present
on the phone line side - probably .2 V p-p or so. I probably could have
used a set of crystal headphones to just listen to the relative amplitudes
of the logic and phone line side signals instead of a scope. (Magnetic
headphones would have too low an input impedance.)
On a hunch, I did try replacing the transformer with one from my junkbox
but as expected, this produced no change.
The phone line is a nasty place for electronic components - 90 V ringing
signal, lightning strikes, pickup of EMPs (from nuclear bombs), etc.
The first place to look for fried components is therefore the phone line
circuitry. For a modem and no schematics, with the possible exception
of the power supply, it is also probably the only place where there is
any real chance of finding a problem.
Since there are a manageable number of discrete parts that connect the
phone line to the transformer, an ohmmeter check was in order. Unfortunately,
many of them were really itty bitty surface mount parts. After a couple
of go-arounds, this proved to bear fruit as an SO23 device marked on the
circuit board as a diode turned out to be shorted though I seem to have
missed it on my first pass. Carefully unsoldering the almost microscopic
part confirmed that it had turned into a dead short.
Note: markings for these devices were not always complete. Nonetheless,
basic ohmmeter checks could be made with enough confidence to tentatively
eliminate all except the single shorted diode.
Removing the diode and retesting proved that dialout was possible and that
normal communications at 14.4 K baud was normal. The markings on the diode
did not permit me to identify whether it was a simple diode or a zener.
I still do not know what function the diode actually served.
I replaced it with a 1N4148. The modem has been tested to confirm that
it is not damaged by the ringing voltage. Since the application does not
require dial-in access, I do not know if there is still a problem with
this mode.
Comments: Where a problem can be narrowed down to a small section of
circuitry, ohmmeter tests can prove successful in identifying parts
that have failed shorted or open. This may be the only option when
confronted with a device for which obtaining a schematic would be
difficult or impossible - or not worth the effort. While we might
consider a modem to be a throwaway item these days, the time need to
do basic testing of the phone line side components is minimal and,
as in this case, may be all that is needed.
Patient: RCA 26" TV, Chassis CTC131A/M. Garage sale acquisition ($5).
Previous owner explained that it failed to work after power
was restored following a power outage due to a thunderstorm.
They were more than happy to have me remove it from their garage.
Symptoms: Dead. Located fuse - blown due to short (not overload). Blackened
inside glass. Note: appearance of blown fuses is very significant.
Testing: Replaced fuse. This was in hindsight a mistake without using
a series light bulb to limit the current. The new fuse did not
blow but the lights dimmed momentarily. Apparently a fusable
resistor had sacrificed itself to protect the fuse.
Without schematics, I decided to trace the AC line circuits. These were
partially buried inside a metal box with some big fat resistors on a
separate board.
Measuring across the power resistors revealed that one 2.7 ohm resistor
was now open. That explained the new failure. Some further searching
located a TO3 power transistor inside the metal shield. Measuring C-E came
up 0 ohms. This was not the horizontal output transistor but was rather
in the power supply - a flyback switching supply separate from the deflection
circuits.
The transistor was marked with an RCA part number. My handy dandy ECG
Semiconductor Master Substitution guide listed it as a typical 350/800 volt
switching transistor. I did not have an exact replacement but figured
that a horizontal output transistor would probably work at least temporarily
in this application, so I used my favorite BU208A in its place.
As a temporary substitute for the fusable 2.7 ohm resistor, I put in
a regular power resistor with the understanding that a safe replacement
would be installed before the TV was buttoned up.
Now for the test. This would be classic use for a series light bulb and/or
variac for the initial power on. However, for whatever reasons, I did not
bother - and lucked out in this case. The TV came on just fine. The only
adjustments needed were to focus and horizontal position - totally unrelated
to the original problem.
The recommended ECG replacement for the chopper transistor was an ECG385,
a 350 V 10 A switching transistor. I elected to put in an ECG386 to give
myself a little extra margin. The ECG386 is rated at 500 V and 20 A.
Other specifications were similar or superior.
Comments: Since most modern consumer electronics are powered all the time
even if the power switch is off, it is always a good idea to unplug everything
during a lightning storm or if a blackout occurs. A nearby lightning
strike can easily impose huge transients on the AC line. When power is
restored following a failure, the initial power-on may not be clean including
mini-brownouts, spikes, multiple cycles, etc. These are all hard on
switching supply based equipment. Now, I fully realize that few of ua
actually follow this advise. (Of course, other screwups can result
in similar damage. I once was given a bag of dead stuff from a friend
of a friend who had been doing a little wiring in his house. Somehow
he managed to connect 240 V to a 120 V circuit - only for a second....)
This is one example where a failure with the most catastrophic impact
on performance (it was dead, after all) has among a very simple solution
(transistor, resistor, fuse). I much prefer these to the 'color noise
on channels in the UHF band' variety.
Patient: Pioneer PDM601 6 Disc CD Changer rejects all CDs.
Symptoms: Disc is loaded from cassette, disc spins at various speeds
for about 5 seconds, then it gives up, unloads this disc and
loads next until there are no more.
Testing: Testing consisted of removing cover and observing behavior.
Everything appears to happen normally except that disc directory
is never read.
This is one of those problems that has an obvious cause and solution once
experienced, diagnosed, and repaired the first time. I will outline my
approach the first time I came across this one describing this
particular case history from the perspective of the novice wannabe
repair expert.
Since the disc is loaded and spins, it is likely that the laser and focus
servo are functioning (though perhaps no guarantee, but I hope to get to that
in another episode).
At first, I suspected (incorrectly) that an adjustment was needed, so
I did what I always now warn against - turning any of the internal controls.
I thought that they had been returned to the original settings but was
not positive.
Pioneer CD players usually include a test function - a button on the main
circuit board marked 'TEST'. Normally, test functions are invoked either
by simply depressing the button or by holding it depressed when power is
turned on. In this case I discovered that if this button were depressed
when the unit was switched on, the display would change and certain front
panel buttons would now function controlling the servo circuits directly.
(Note that the approximate duration of the previous paragraph was about
1 year as I put the thing aside unable to make any headway during the
first go-around.)
After some experimentation with the front panel controls, I came to the
conclusion that:
* STOP turns all servos OFF.
* TRACK FWD enable FOCUS servo (and loads disc 1 in changer).
* PLAY enables SPINDLE servo.
* PAUSE enables TRACKING servo (and PLAY mode).
* MANUAL SEARCH FWD or REV to move Laser head.
Playing with these for awhile revealed that it was possible to move
the laser pickup, engage the focus servo, and make the disc rotate, Further
experimentation confirmed that the internal controls were more or less
in their correct positions - tracking offset could make the pickup slew from
one end to the other, for example.
All this was leading nowhere until I accidentally happened to engage
all servos in the middle of a CD - and the track display suddenly appeared.
I reached for my headphones and confirmed that the CD was playing, just fine.
A little further investigation allowed me to determine that the CD would
play fine from about the middle to the end but would get progressively
noisier when moving toward the starting track and would be totally
unplayable at the very start.
Now, what could depend so fundamentally on track position? Well, there
are two possibilities: spindle motor speed and PLL frequency. A little
careful tweaking of PLL center frequency had little effect.
One thing I noticed was that one of the servo driver ICs was running
quite hot. This should have raised a red flag but again, this was
the first time I had seen this problem. I also observed that putting
a heatsink on the IC and blowing on this would permit the disc to play
error free much closer to its start.
So now, you are saying, "what a moron, everyone knows that Pioneer CD
spindle motors are crap". The confusing thing here was that the spindle
motor was not dead, just marginal. So, all basic observations came
up negative.
Anyway, back to the saga. Suspecting the driver IC, I obtained a replacement
from MCM Electronics and swapped it. No change. Measuring motor voltage
showed a maximum of 1.7 V or so at any time including startup. Since the
driver is known now to be good and power was confirmed to be stable, I
started to suspect the motor.
Disconnecting the spindle motor and cycling the player revealed that the driver
was putting almost 10 V on the motor terminals but the motor was no doubt
partially shorted and dropping this to less than 2 V with the consequential
high power dissipation in the driver.
Now for the long shot. While the player was attempting to spin up
and read the disc directory, I gave the motor a squirt of degreaser
through its ventilation holes. The motor took off - went totally
overspeed. Power off. Wait for degreaser to evaporate. Try again.
Now, the directory came up the first time even though my internal controls
were still no doubt not perfectly tweaked. All functions worked perfectly.
For the first time is about 2 years, the player was producing music (without
the help of TEST mode!
I then performed a normal alignment of the internal controls (but this
is another story).
Measuring the motor voltage now showed greater than 5 V at spinup and a
range of 2-.5 V between start and end of disc.
Comments: At this point the proper course of action would be to replace
the spindle motor. However, since this is my CD player and replacing spindle
motors is sometimes a pain, I will just keep an eye on performance.
A pretty good indication of the motor's state is the time to spinup. If
this deteriorates again I will be forced to replace the motor. For now,
it continues to be satisfactory.
The initial confusion here was due to the fact that the motor was not
totally dead, just weak enough to cause a problem with the inner tracks
and more importantly, the directory. This is one of those cases where
the old style turntable with a bad weak motor would have been much easier to
troubleshoot,
Patient: Power One multiple output 400 W switching power supply.
Symptoms: Totally dead. No schematics available.
Testing: Applying power produces no output. Only observation is that
lights flicker indicating the input filter capacitors are
charging so this eliminates a blown line fuse as a possibility.
Unfortunately, this unit is not what I would call a 'simple switcher'.
In addition to the main switchmode transistor, there are 2 other power
transistors, a uA723 IC regulator, countless discrete transistors, resistors,
capacitors, some components I cannot identify. This is all on the primary
side of the transformer. Tracing the circuit is virtually out of the
question due to its complexity. The only good news is that I have
several identical units so I can compare readings between the bad one and a
working unit. Powering with a Variac produces similar lack of any output.
This is not a case of the outputs shutting down - there is simply no startup.
First check: power to main filter caps, continuity of thermal protector,
power to uA723, power to switchmode power transistor. These all are fine.
Next, checked components around input including power transistors, large
resistors (suspecting a startup problem), capacitors, etc. All ok.
Finally, about to give up, I decide to just test resistance across a more
or less random selection of components. Everything is identical until
I put my meter across a 6.8 M ohm resistor. On the good unit, this measures
above 1 M ohm. On the bad unit, it measures about 40 K ohms. I
unsolder components around this resistor until I located a 2N4124 transistor
that makes a difference when removed. Testing on the x1 range of my VOM
it tests fine but on the x1K scale, there is significant leakage in the
reverse biased junctions. Comparing with a nearby 2N4124, this is definitely
not the normal characteristics of a 2N4124. The 2N4124 is a general purpose
transistor so I replaced it with a handy 2N3904.
Powering with a Variac, the supply now comes up fine. I have no idea
of the function of the bad transistor.
Comments: This might be called a blind repair. Like bad connections, the failure
mechanism and function of the bad part will probably never be known. It
is not known whether the transistor was marginal to begin with and its
characteristics just drifted over time, or whether it went bad.
The basic assumptions which permit this technique to work at all
are that for a sudden change in behavior in a system with mostly discrete
parts, one of these parts has changed its resistance enough that an ohmmeter
check has a chance to find it and that the circuitry is interconnected
enough that checking a relatively small subset of node combinations has
a good chance of locating the bad part.
Patient: Goldstar model CR820U 19" color TV. Paid $1 at garage sale. If
I can repair it cheaply, it will be given to someone in need free
or for a nominal charge.
Symptoms: Basically operational except absolutely no color - not even any
color noise between channels. Color and Tint knobs have no
effect. In addition, brightness control has very limited range -
brightness slightly too high. Several other minor problems
including cracked tuner knob, dirty tuner, broken antenna wires.
Testing: This involved tuning local strong channels, adjusting fine tuning,
tweaking AGC, etc. Under no circumstances was there any hint
of color in the picture or between channels.
Fortunately, such symptoms narrow down the possible area of investigation
to the chroma decoder circuitry. Searching the circuit board for a likely
subsystem, I found that the set uses a TA7608P chroma chip. Fortunately,
this chip is listed in my ECG Semiconductor Master Replacement Guide with
a cross to ECG1532. Naturally, I suspect the IC at this point but know better
than to just go out and find a replacement - I have been burned in this way
with an RCA TV color problem - maybe I will say more on that in another
Repair Brief. While the ECG does have a pinout, this does not really provide
enough information to probe the circuit.
Off to the library to obtain the SAM's Photofact for this set. $0.75 poorer,
I copied the complete schematic and another interesting page - a chart showing
the resistances to ground for all pins on all of the intergrated circuits
used in the TV. One item I forgot to look for was the block diagram that
may have been included of the TA7608P chip. Oh well.
One thing I did try once armed with the SAM's was to attempt to tweak the
subbrightness control. Even this had very little effect. OK, on to the
fun stuff.
First test: confirm that the resistances of the circuit match those printed
on the resistance chart. The chart specifies a DMM that applies less than
.1 V on the ohms scales (to prevent forward biasing of any semiconductor
junctions). Hopefully, my DMM satisfies this requirement. First step:
make sure the main filter capacitor is discharged before making any resistance
measurements. Done. Unfortunately. these tests do not reveal anything amiss.
Tests of this type are not guaranteed to find any problems. However, there
is a fair chance that a shorted or open part would show up as a bad reading
in the near that part. In my case, if there were any bad parts,
the circuit topology prevented this simple resistance test from detecting
them.
Now for the live tests. I don't have a color bar generator so I just
have to hope that a broadcast channel will provide a signal that is close
enough.
In the interest of safety, all the following tests are made with the set
powered off of my isolation transformer.
Voltage measurements were inconclusive. Although some where off by 20%,
this could be due to my non standard input signal. There did not appear
to be any particular pattern.
Next, I used a scope to look at the testpoints for which SAM's supplied
waveforms. Again, these will be different than the ones using a standard
colorbar signal but the overall appearance should enable me to determine
if a particular output is dead or the amplitude is way off.
All the waveforms looked reasonable except one - the output of what I
deduce to be a gated chroma amplifier. Its output is almost dead. The
color reference oscillator (3.58... MHz) looks fine as do the chroma input
and color burst gating pulses. All the supply voltages and decoupling
pins look fine as well. This doesn't look good for the chip. The signal
seems to be getting in but the chroma amp would seem to be dead. Fortunately,
I could not locate an inexpensive replacement from my usual sources. The
TA7608P is probably obsolete. Even the ECG1532 is not available from
MCM Electronics or Dalbani. I did not get to the point of trying ECG
directly. You will see why I say 'fortunately' in a moment.
When confronted with a situation of this type, I usually try some experiments.
What would happen if I apply the chroma signal directly to the point in the
circuit that is the output of that dead chroma amp? I take a 10K resistor
and jump the input of the chip to the chroma amp output (and input to
the chroma demodulator.) Now, I have colored stripes on the screen indicating
that the chroma demodulator circuit is probably functioning. (Without the
color burst to phase lock, I could not hope for anything more). So, it
still looks like that bad amplifier. Well, one last desperate effort....
I use my sam's patented magic spit. This has served in on numerous occasions
mostly when locating clock noise, marginal timing, or glitches in high
speed digital systems but hey, the world is really analog anyhow. I am
not joking. Those who have done serious debugging know exactly what I
am talking about. Moisten your finger. Run it up and down the pins on a
suspect device. If something changes you have either (1) found a
particularly critical or high impedance but normally behaving circuit
(for example the frequency determining LC network of an oscillator) or
(2) something that is open or on the edge.
Magic spit to the rescue: running my finger (one hand in my pocket, isolation
transformer, etc.) over the two rows of pins on the chroma chip proved to
yield immediate results. Around the low number pins, the color suddenly
appeared grossly overloaded but with some indication of correlation with
the picture. This is the first time I have seen any indication of picture
related color. Hum.... OK, now to narrow it down. I take an insulated wire,
and strip both ends. Holding one bare end with my fingers, I touch each
of the pins in turn of the chip. Touching pin 6 has the most dramatic
effect producing the distorted colors. What is connected to pin 6? According
to the schematic, it is a 2.2 M ohm resistor to pin 3 and a decoupling
capacitor to the power supply. It is unlikely that the capacitor would
fail in such a way as to cause this behavior. So it must be the resistor.
Rummaging around in my resistor cabinet I come up with a 2.2 M ohm resistor.
and tack it across pins 3 and 6 (with power off!).
Now I get distorted color but this looks a lot more like a color TV than
what I had before. Time for a better antenna. That helped a little but
the picture is way too dark. Well, I did fiddle with the sub-brightness
control back when it had minimal effect. Locating the sub-brightness
control again, it now functions as expected having more than a sufficient
range. When adjusted to produce a picture of normal brightness (with the
user control mid-range), the color appears normal. Some additional fiddling
with the Color and Tint controls yields a fine looking color picture.
Removing and testing the original 2.2 M ohm resistor confirms an open
circuit. Soldering in my replacement completes the repair.
Now to clean the tuner, wipe down the case, repair the knob and antenna....
Total cost $1.77 including original purchase price (I threw in a hypothetical
$.02 for the resistor).
As I am writing this, I am watching the Goldstar TV - my trusty RCA
has just died after 14 years with no picture (raster and sound ok - that
will be another Repair Brief).
Comments: It is always tempting to suspect the expensive or unavailable
part first. Very often, as in this case, this proves to be erroneous.
Had the TA7608P been readily available at reasonable cost, I would have
probably replaced it only to find no change in behavior. This would,
however, have saved time.
Be warned that the 'sam's magic spit' approach must be used with caution.
You must understand the safety implications of touching *any* live circuit
especially with moistened fingers. I use an isolation transformer for
debugging. However, even with this precaution, I would think twice
before doing this on a live chassis (the Goldstar signal circuits are
isolated from the power line).
Patient: Panasonic PV3720 3 head VCR.
Symptoms: Band of what looked like tracking noise would come and go
depending on tape being played, speed of tape, whether at
start or end, etc. The noise was confined to the top 1/3
of the picture. Its height could vary from just a couple of
video lines to a band occupying 20 % of the screen.
Testing: Several tapes were played initially. Problem would be nearly
absent with some but severe on others. It was generally worse
with EP recorded tapes compared to SP tapes.
I generally do not like problems of this type because one of the more
likely possibilities is of a worn video head. This is one of the classic
symptoms yet it could have a number of other causes. The approach must
be to eliminate as best as possible the alternative causes until the risk
of purchasing a new video is minimized.
Alternate #1: dirty heads. Head cleaning with a wet cleaning tape
followed by a manual cleaning had little effect.
Alternate #2: tape path alignment. Visual inspection of the tape
movement showed nothing out of the ordinary. Tape motion was very
smooth and uniform with no wiggling, wavering, or wondering. All tape
guides were properly positioned, perfectly vertical (where appropriate)
and the tape appeared to be riding at the correct height on the video head
cylinder.
Alternate #3: backtension. Insufficient backtension could result in
similar problems. Inspection seemed to indicate that backtension was
normal. Manually increasing backtension by gently pressing the backtension
level to the left made a slight improvement. Increasing the spring tension
did the same. However, these were not dramatic effects and backtension
is not a critical setting to obtain a clean picture (though it is important
to be accurate to minimize head wear and clogging).
Alternate #4: roller guide height. Although visual inspection of the
of the tape path alignment proved negative I decided to confirm roller
guide height by careful adjustment of the supply side roller guide -
carefully noting its original position. (Problems at the top of the
picture would be related to the supply side roller guide.) Optimal
position for both EP and SP was at the original setting.
This left the video head as a likely candidate and at this point based
on the age of the machine, a new video head cylinder (MCM Electronics,
ERH433) was ordered and installed. Success! There was no doubt about
the improvement. The noise bards completely disappeared and the normal
backtension provided more then adequate head-tape contact.
Comments: Subtle problems that eventually point to the video heads are
among the more difficult to diagnose with enough confidence to risk ordering
an expensive video head and find out that the problem was elsewhere.
This was a case of video head wear (as opposed to a mechanical or
electronic failure of the heads). The chance of having an identical
video head available to swap - which is the best test - is quite small,
especially for a 3 head type. If this were a 4 head machine, some meaningful
comparisons could be made during playback since a different set of
heads is often used depending on tape speed and mode.
Thus, unless there is visible damage to the video heads or something
like an open winding that could be revealed by simple testing, it
comes down to eliminating as best as possible the alternatives until
only the head remains a likely possibility.
Patient: Classic ATT Touch Tone Wall Phone. 'Battlewagon' usually considered
indestructible.
Symptoms: Everything worked fine except dialing. For some buttons, dial
tone would not go away. For others, tones would be accepted but
would be erratic and incorrect digits. Certain tones sounded
weak or single frequency.
Testing: All buttons were tested. It was found that the problem was not
even consistent as some buttons would not work all the time.
While the internal wiring of one of these old phones is intimidating,
the basic tone dialing circuitry is an amazing example of simplicity.
About the only things that fail yet still permit some tone generation
are the pot core coils that determine tone frequency. Therefore, this is
the first thing to check.
Sure enough, the core that deals with rows has split where the two halfs
are joined. This seems to be a common problem due to both the age and brittle
cement used on some revs of this model phone, and probably, as a result
of rough treatment when hanging up the handset.
These cores must be aligned before being glued back together. In addition,
there is an adjustment plug which may need to be tweaked. I align by ear as
follows: Put a known good tone dialing phone and the bad phone on the same
phone line. Depending on which core is bad, depress either an entire (same)
row or column of buttons on both phones. (Adhesive tape is handy to
hold down the buttons unless you have four hands.) By depressing the entire
set of buttons, you are disabling the other tone generator so you hear a pure
tone. Without turning the fine adjustment plug (assuming it was not disturbed;
if it was, set it mid-range or the same as the one in the other core), rotate
the loose core top until a zero beat is obtained. As your rotate the core, you
will hear the tone change. As it approaches the correct setting, you will
hear the tones beat against each other. When you are set correctly, the
pitches will be equal and the beat frequency will go to zero. Mark the
position of the core with a pen or pencil and then glue with Epoxy or
other general purpose adhesive (around the outside - not on the mating
surfaces as this will affect the tone frequencies). After the glue sets,
confirm and adjust the plug core if needed. These cores use a strange
triangular core tool - I made mine by filing down an aluminum roofing nail
(do not use a ferrous material).
Comments: Those classic ATT touch tone phones are virtually indestructible.
However, broken cores (or actually, just broken joints on the cores) are
common but easily repaired once you know what to look for. Setting the tones
by referencing a known good phone seems to be a very reliable technique
as the zero beat permits an adjustment to better than .1%. Note that if
the reference phone is a more modern (and flimsy digital one), then pushing
multiple buttons may not work as it does with the old analog models. Setting
the frequency using the normal dual tones will work - it is just not as easy..
Patient: Pioneer PD5100 single disc CD player in really sad shape. Garage
sale acquisition paid $2. Probably paid too much. This episode
is more aptly called a restoration rather than a repair. Perhaps
a feature length saga. The challenge was irresistible.
Symptoms: Tray wasn't even on track, just sitting inside; Flapper ripped
off of mountings, electronic condition unknown. It is obvious
that the owner had attempted something - it would be generous
to call it a repair - and was unable or did not bother to get it
back together.
Testing: Not applicable at this point. With loose parts removed, power
was applied to determine if there was any hope at all. At least
the front panel came alive and pressing Eject resulted in the
tray loading motor spinning.
In order to attempt to play a disc, the controller needs to think that the
tray is closed. It will then go through its startup cycle. In the case
of this player, there is a limit switch - somewhere. Rather than trying
to locate it, I decide to put the tray back on its tracks. This is
easy but there is still something wrong as it jams when the Eject button
is pressed. So be it, leave that for later. At least the limit switch
will be activated. Rummaging around in the pile of lonely parts removed
from the carcass, I locate the clamper cover with the magnet. I pop in
a garbage CD, put the clamper cover on top (make a mental not to press
Eject under any circumstances as the tray, disc, cover, and anything else that
is not screwed down would probably fly across the room) and press the
power button. Some success - the disc spins and the directory is correctly
displayed. The display came up rather quickly indicating that most of the
optics and servos cannot be far out of alignment. This is quite remarkable!
With mounting anticipation, I connect the audio outputs to my amplifier
and press play. The disc spins and makes repeated attempts to start
playing at track 1 but it is obvious that something is terribly wrong.
Attempting to play other tracks results in similar behavior.
The pickup appears to actually move to the general vicinity of the
correct track but is unable to locate and lock onto the time/track that
is selected. Pioneer CD players perform a very audible search to home in
on the correct disc location; there was no evidence of this search.
I next attempt careful adjustment of the servo controls. Note that I do
not expect this to help the problem based on how quickly the directory
was displayed. However, the tracking could still be off and with care,
there should be little risk of making things worse. Who knows what controls
the owner touched in a misguided hope of performing a miracle. First, I marked
the *exact* position of each control with a felt tip pen. This will get me
back to the supposedly good positions no matter what. The only controls
that would likely have an effect are those related to tracking. Careful
tweaking of tracking balance, tracking offset, and tracking gain have
no detectable effect. I put them back in their original position and
verify that the player still recognizes the disc. So far so good?
At least the moron who butchered this thing does not seem to have
touched the electronic adjustments.
At this point what do we know? Well, we know that all of the major
components of the optical deck work including the laser, photodiode
array, fine focus and tracking voice coil actuators, and spindle motor.
These are all needed to read the disc index. The spindle motor, a
common problem in Pioneer CD players is fine as its toughest task is
at disc startup where the speed is greatest. Since the disc index is
located at the very inner extent of the disc, we do not know if the
sled servo (coarse tracking) is working correctly, only that it is
doing something - it resets to the inner track if manually moved away
and it does move to the approximate position of the selected track.
Well, Pioneer CDs have a TEST mode. Where is the button? I hunt all over
for the little button and am about to give up when there it is! Hidden by
the cables to the front panel.
OK, press TEST while switching on power. Now I have control of the
servos. A little experimentation confirms that focus and spindle rotation
seem to be functional: (of course, we knew that, right?) With no disc in
place, the focus search routine is initiated by pressing TRACK FWD. The
disc will only spin if focus lock is achieved and this is confirmed with
a disc in place. So far nothing new. I am able to move the pickup
back and forth on its tracks by pressing SEARCH FWD and REV.
However, when entering the correct sequence to play at an arbitrary
point on the disc, weird things happen. If I use the SEARCH FWD and REV
buttons to move the pickup to a particular spot on the disc, press
TRACK FWD to close the focus servo, PLAY to start spindle rotation, and
then PAUSE to actually start playing, the track and time info is only
displayed for an instant. Then, the pickup seems to move toward and bump
against the inner limit. Sometimes, a couple of times are displayed in
rapid succession which are not sequential as they should be. In fact, they
nearly always are far apart and the second is usually a lower time than
the first. Then the display is blank.
Hum, I don't have a schematic so this could be the end of the line. But,
I do see one chip on the circuit board that is getting unusually hot and
I know from past experience that it is a servo driver - TA8410K. I have
absolutely no idea if it is related in any way to the problem or really,
for that matter, what the problem is. I only know that (1) it has only
10 pins and is easy to replace, (2) I have a replacement in my parts
box, and (3) it is getting hot (which may or may not be a fault since
I know these type of chips to run at least warm).
Getting to the bottom of the circuit board proves to be a bit harder
than anticipated requiring removal of most of the snap type connectors.
I guess these are cheaper than real connectors for Pioneer but a pain
for servicing (cables are terminated in tinned wires and placed in the
connector housing, then a cover is pressed down to lock them in place).
I manage to only mangle one of these (cosmetic damage only).
Replacement goes smoothly. Getting all the connectors back in place
is loads of fun but the effort is worthwhile! Now, the disc plays
on the first attempt. There are still some tracking problems but
this is a distinct improvement. In all honesty, I am not sure that the
chip made the difference - it could have been a bad connections at one
of the connectors. The new chip runs warm, perhaps not quite as hot
as the old one, I am really not positive. I put a heatsink on it in any
case (as I always do with these chips - just for insurance.
Next I tackle the mechanical restoration. First step: get the tray
to move smoothly. Without going into terrible detail, the tray consists
of two parts whose relative motion raises and lowers the disc. There
appears to be something missing which controls when this raising and lowering
takes place as the disc is lowered even before the tray moves into the
machine. Sometimes there is a ball that controls this and a little
examination reveals a grease trail where such a ball could have been.
A corresponding hole in the tray bottom confirms this. I didn't notice
any such ball in the parts pile but it could have easily been lost (I later
found it near a corner of my workbench) but for now, I located a similar
sized steel ball in my steel ball collection. With the ball in place,
the tray now moves smoothly in and out and the disc is raised and lowered
at the proper time.
Now for the clamper.
This is a much sorrier affair as the clamper is mounted to the deck
sheetmetal with a couple of plastic standoffs that have been totally
snapped off at their bases. First I try simply glueing them but this
does not appear to be solid enough. In addition to the glue, I am able
to clamp one down with a metal scrap that I carefully shape and screw
down. For the other, I made a splint using a screw through a drilled
hole into a neighboring strut. Now the clamper moves up and down at the
proper time but the cover disk with the magnet seems to hit the tray.
The part that seems to help out has totally disappeared so I take a brass
rod and mount it in its place. Even without the rest of the mechanism,
this seems to work fine. This rod, wrapped with electrical tape to prevent
damage to the disc, prevents the disc from flopping around too much.
Disc loads; disc unloads; all is well.
I then went through the electrical servo adjustment procedure as outlined
in the CD Player Notes, final tweaking by maximizing the amplitude and
stability of the 'eye' pattern. I made the mistake of attempting to touch
the 'tangential adjustment' (at least that is what I think it is - without
a proper alignment disc, this appears to be very difficult) and spent
some frantic minutes until I was able to restore it to its original
position. Beethoven's Ninth Symphony comes in handy as it runs almost
to the edge of the disc (74 minutes) necessary to access the tangential
adjustment. I even risked careful adjustments of the LD - laser power just
to determine that it was not at the limit of its power. It was not.
I am fairly confident at this point that the adjustments are pretty much
where they should be - and they are very close to their original position.
Now the CD player works fairly well though it does not seem to have
as much disc defect tolerance as I would expect. I do not know if there is
still a fault either optical, mechanical, or electronic as all tests
that I can perform without service info seem satisfactory. Considering
what the player went through, this has still been a rewarding experience.
Comments: I consider this to be more of a learning experience than a repair.
At the outset, I did not expect to be able to get nearly as far as I did.
It was fun as such things go.
While I am in favor of home repairs, this is an example of a situation where
whoever attempted the repair of a problem due most likely to the bad servo
driver IC, totally destroyed any possibility of a professional even going
beyond looking at the unit and stating: "Yup, that was a CD player once upon
a time long long ago. To whom should I send the flowers?"
Patient: Yamaha R8 stereo receiver, about 4 years old at the time of the
failure.
Symptoms: Fm reception is totally dead. Station numbers change erratically,
not possible to save presets. Some AM stations work but most do not.
This happened without warning - turned it on one day and it was sick.
Testing: This involved methodically checking to see what functions are
operational. Incrementing and decrementing of FM station frequencies
is not operational in seek mode, only in manual. There is not
reception on any FM station frequencies. Incrementing or
decrementing AM station numbers across certain boundaries (I forget
the exact locations) causes a sudden jump of 800 KHz and may actually
jump to an illegal station frequency. Various other modes are non
functional including saving of memory presets. Even the hard reset
does not store the factory presets.
I purchased the service manual for this unit - a nice piece of documentation
and very reasonable - about $12. However, this is an example of modern
technology where even schematics, pin descriptions of the various LSI
chips, parts lists, etc. are not really adequate when so much depends
on firmware (in 3 microcontrollers) which is not provided. It turned out
to be difficult to even determine where each function is centered.
Some electrical tests that were performed:
Power supply voltages were verified.
Waveforms were checked on frequency synthesizer chip (LC7210).
Function of PLL charge pump was verified in both AM and FM. Output
(VCO control voltage) was consistent with frequency display when
reception was possible but not at other times. However, this could not
be a problem with the charge pump, only the digital control.
Intermediate 4 bit busses were checked for stuck-at faults - there were none.
The first real clue is that since even some manual tuning functions are
faulty, this is probably a digital fault. Presumably in manual, the
station display is driven by the microcontroller that drives the
synthesizer chip rather than being returned by that chip after a station
search. Even in this mode (for AM), there is the issue of the 800 KHz jump.
This is not approximate but exact and probably due to a stuck bit
representing the 800 place value. The question then became: where
was the bad bit? It is not on one of the intermediate busses as these
were tested.
Could it be in the tuning microcontroller? Maybe, but then I would expect
other functions controlled by this chip to be faulty (like mode setting,
etc.) This is not the case. Could it be in the frequency synthesizer
chip? Probably as only station tuning functions are defective. Could it
be elsewhere? There do not appear to be any other busses or digital
control lines that could cause the set of problems that are present.
However, not confident enough of the diagnosis of the faulty LC7210
synthesizer to spend the $25 or so that Yamaha would probably charge and
not finding this part in any of my normal mail order sources, I set the
receiver aside for a while. I dig out my garage sale NAD for use in
the meantime.
A couple of MCM catalog editions later - what's this? LC7210 - $6. I will
spring for that. Next MCM order arrives, solder in a socket as I always
do where possible. Replacement chip cures all problems!
With 20/20 hindsight, it is almost possible to identify the place
inside the LC7210 where the 800s bit bus fault occurs based on the
symptoms and the rudimentary block diagram provided in the service
manual.
Comments: Although not evident from the description above, this was a
frustrating experience even with the service manual because there really
was not enough information present to make the logical inferences needed
to come to a definitive conclusion as to the defective part. Modern
consumer electronics include more and more microcontrollers where the
intelligence is buried in firmware and not the hardware itself. Without
firmware listings, a microprocessor is just a black box even with pins
listings and internal block diagrams. It would be nice if the service
manual would at least provide better indications of which functions is located
where - identifying the functions of each of the components. (It would also
be nice if they were written or at least edited by Americans (in the case
of a manual destined for the U.S. market). Some of the translations are,
well, a bit strange.
Patient: Sylvania Color Television, about 20 years old.
Symptoms: Suddenly, the picture lost *all* horizontal hold. There was no
evidence of any kind of attempt at lock in. I do not know whether
this happened at power-on or while in use.
Testing: With strong signal, it was determined that horizontal hold had
no effect. It is as though the H sync is not making it to the
lock circuitry. Adjusting horizontal hold makes picture move
across screen. Angle of sides of picture changes but there is
no lock - even incorrect - at any setting.
Using my isolation transformer, I prepare to scope the relevant signals.
I obtain the SAM's for the set. I check for the sync signal at the input
to IC400 (I think). It is there. This should be a snap - bad IC! Well,
that is exactly what happens - a careless slip of the scope probe and not
only a snap, but a crackle and a pop - and now I have no video, no HV, no
deflection - nothing.
OK, so what started out as a simple signal problem is now a major (at least
cost and pride wise) power supply problem.
Checking the first TO3 transistor I can locate - short - one dead
transistor. This is the power supply series chopper.
Checking the horizontal output transistor (HOT) with an
ohmmeter - short - second dead transistor.
After removing transistor, I check for rectified line voltage at the
input to the chopper - nothing. Tracing this back I soon locate an
open fusable resistor.
So, whatever I touched probably caused the HOT to fail (forced on for
too great a time can blow the HOT as a single shot event). The shorted
HOT probably then took out the chopper transistor.
This is not fun. It is not likely to be inexpensive either. It does seem
that no other parts have been sacrificed. Fusable resistors and driver
transistors seem ok in so far as my meter is concerned. I still assume
that the original problem was caused by a faulty IC400 but this point it is
impossible to confirm this since the set id dead-dead.
Damage:
Chopper transistor - $10, horizontal output transistor - $6. IC400 - $15,
fusable resistor - $1.
After replacing the components, making sure to use mica insulators
and silicone heat sink compound for the transistors, the set comes
back alive. Sync is fine. A little touchup of the video background
and gain controls (unrelated to the sync problem) and we are done. Ouch.
Comments: The lessons learned here came at a cost - but mostly to my
pride. Cascade failures are all too easy to induce through carelessness.
Power supply circuits are not forgiving. One would think that probing
the sync signal would not be able to kill anything. However, the design
of power supply and deflection systems share some common characteristics.
One of these is that a single instance of an improper drive waveform
can blow the switching transistor as a single shot event - excessive
current or excessive flyback voltage. This is a matter of exceeding the safe
operating area of the transistor.
What you learn: if possible, make all connections to your test equipment
with power off. Insulate all but the last mm of your probe so that any
slip cannot cause a short. Work methodically, think things through, don't
be over-eager, don't take shortcuts.
Patient: Microwave oven (don't recall the brand). Symptoms: Totally dead - no front panel display or anything else. Testing: Plugged unit into live outlet confirms description of problem. First step: remove cover. Second step: confirm that HV capacitor is discharged. Although the unit has been unplugged for several days, it never hurts to be careful. Discharge with high value high wattage resistor (well insulated) and confirm with HV voltmeter. WARNING WARNING WARNING etc. Microwave ovens are probably the most dangerous piece of consumer electronic equipment in terms of potential for electrocution while being repaired. Much more so than TVs, for example. * There is up to 5,000 V negative with respect to the chassis while powered and stored in the capacitor when shut down. The energy stored in this capacitor is roughly ten times that stored in the CRT second anode of a TV or monitor. A malfunctioning oven that does not blow a fuse is potentially even more of a hazard since the capacitor cannot discharge into the magnetron load after shutdown. * The available current exceeds 1 A. Depending on the transformer, this may flow until the fuse blows or circuit breaker trips. * An isolation transformer (assuming you had one large enough) or GFCI will not protect against HV shock since the HV is already on the secondary side of the HV transformer and, as noted, the HV return is the chassis. * A circuit breaker or fuse is not sensitive enough and too slow to provide any protection for you. You might think at first that the possibility of microwave exposure is the principal danger. However, unless the door seal, oven chamber, or waveguide or its seals are damaged, there is no way for microwave radiation to escape. This still means that proper precautions should be taken - don't operate the oven for longer than needed with the cover off and don't stand too close. Third step: test fuse. Open. Since these have ceramic bodies, it is not easy to determine if the fuse died due to an overload or a short by visual examination. A microwave oven can blow a fuse for several possible reasons. Some of these are: 1. Shorted Magnetron. 2. Shorted capacitor. 3. Shorted rectifier. 4. Defective door interlock switches. 5. Bad Triac (possibly causes transformer core saturation due to operating as half wave rectifier or SCR). 6. Bad transformer (shorted turns cause overload). Fortunately, all but the last two are generally easy to identify using just an ohmmeter. Sometimes, an intermittent or short that occurs only at full voltage will prove elusive, however. Some quick checks reveal that the capacitor is a dead short. When replacing a microwave oven capacitor, it is important to get a fairly close match for the capacitance. The uF rating of the capacitor affects the microwave power output. Note that the 'working volts' rating on a microwave oven capacitor is not the same as on common capacitors found in other electronic equipment. It is not the maximum voltage permitted across the capacitor but closer to the VRMS rating of the HV transformer. And of course, before you start pulling wires off (1) mark down where they go and (2) discharge/check for voltage on the cap one more time. Replacing the capacitor with one from MCM Electronics brings the oven back to life. Comments: It is highly likely that the capacitor failed due to a defect in manufacture rather than some other underlying problem in the circuitry. When one thinks about how a capacitor is constructed - rolled up layers of foil and dielectric - it is amazing that capacitors do not fail that often. Any nick, thin spot, etc. represents a point of excess stress and can fail as in this case after considerable use - resulting in a short circuit, dead oven, and unhappy chef.
Patient: Tandy color TV/monitor found curbside. Unit appears to be in
good physical condition. Remote is missing. Well, you cannot
have everything.
Symptoms: When first turned on, TV appears to function normally. Why
was it tossed? Well, after 30 seconds or so, a pair of hum bars
begins to appear in the picture gradually getting worse until
horizontal width and sync are affected.
Testing: Using a Variac, there is a point below normal line voltage where
set operates perfectly. OK, so I will keep a Variac attached
to the unit!
After removing the cover, the first thing to suspect is the main filter
capacitor. If this should dry up and lose some of its value, these would
be the exact symptoms. Jumpering (with power off) of a known good capacitor
I keep for this purpose doesn't change anything. But what is this? A
discolored resistor catches my eye. Maybe it is changing value as it heats
and causing these symptoms. I wait a reasonable time for the set to cool
and measure the resistor - 360 ohms. OK, replace with new one. Expecting
this to cure the problem I am disappointed when there is absolutely no
change.
Off to the library for the SAMs Photofact. Darn - SAMs does not have a
service folder for this model. Nor for any similar models that I can
determine (the librarian was very cooperative).
Well, the problem seems to be heat related. I get out my trusty can
of cold spray. After going through nearly the entier can, it would seem
that there is only one part that has an effect on the hum bars when it is
chilled. It is the SCR that is part of the power supply regulator. Rather
than simply obtaining a replacement, I decide to trace the circuit to
determine, if possible, the possible cause of the problem figuring at this
point that the SCR is simply sensitive to heat.
During normal operation, an IC drives the gate of the SCR but what is this??
Until the secondary supplies kick in and provide power to the IC, the SCR
is driven by - you guessed it - the mysterious resistor. The other end of the
resistor goes to the raw DC on the main filter capacitor. Now that is
odd....Since I do not believe much in coincidences, I now start rethinking
the significance of this. Maybe that resistor is not quite what it appears
to be.
First, I remove it and see what happens: nothing. Power on, power off,
nothing.
Next, I momentarily touch the resistor to the circuit pad - the set comes
alive. Then I remove it. The set remains alive. And, after several
minutes, no hum bars. Hum....
I then try increasingly larger values of resistance until turn on is not
reliable - 15 K seems marginal, so I will go with 8.2 K ohms = that is
over 25 times what I measured! No wonder there were hum bars indicating
regulation problems - that low value resistor was totally overwhelming
the poor IC in driving the SCR.
The set is used daily and has been operating without further problems
for over 5 years since reviving it. It works great with a $10 universal
remote control.
How did the resistor get damaged in the first place? I have no idea - maybe
its wattage was slightly underrated and it just finally decided to poop out.
I have no way of knowing what the original value was supposed to be or even,
for that matter, the wattage.
Comments: resistors can and do change value, sometimes, as in this case,
quite dramatically. Without a schematic, there is no easy way to determine
when and if this has happened - and what the original value should have been.
However, any discoloration, burn, or scorch marks should arouse suspicion.
With 20/20 hindsight, these signs may indicate the presence of carbon -
a fairly low resistance substance and thus reduced resistance is likely.
And yet again, a $.02 part brings a complex renders a complex piece
of equipment inoperative.
Patient: Realistic portable CD player, about 4 years old.
Symptoms: Player will shut off at totally random times or sometimes will
not recognize the disc. There is a clicking associated with
the problem - probably focus search failing.
Testing: Attempting to play various CDs to completion provided no
indication that the particular CD or power adapter made the
slightest difference. There were some false leads with respect
to the latter but these turned out to be strictly coincidence.
The lens was inspected and cleaned anyhow with no change.
The first hint of the source of the fault came as a result of an
observation that pressing on the cover would sometimes either cause
the player to stop in the middle of a disc or allow it to recognize and
begin playing a disc when it would not otherwise cooperate.
Perhaps, the interlock switch was not being pressed in far enough. So,
rather than open the unit (I really don't like messing with the insides
of portable CDs if I can help it - you will see why in a few moments),
I glued a bit of plastic to the post that pokes the switch.
This seemed to help. For a few weeks, the problems had for the most
part gone away and the owner was a happy camper. Not surprisingly,
this fix was only temporary.
Since the quick fix had some effect, it is very likely that I am on
the right track. I will have to open it and deal with the switch
face-to-face.
This is not too bad except that it is necessary to remove the main
circuit board to access the switch which is mounted on a little board
of its own. Four screws (large enough to actually see without a
microscope) to get the bottom off, another couple to remove
the main board. One more and I can remove or at least extend the
switch circuit board far enough to inspect its solder connections and
get at the switch.
The solder doesn't look too bad but there might still be hairline
cracks that are not readily visible. A little reflow and they
should be fine. (Problems with solder joints here are not related to
heat as in a TV or monitor but rather due to the mechanical stress that
is applied to the switch every time the lid is closed.)
Now for the switch. It appears that the cover of the switch can be
snapped off relatively easily. The contacts appear somewhat gummy
so I clean these and pop the cover back on.
Tests with an ohmmeter now show the switch action to be solid. Wiggling
the switch lever and/or the entire switch has no effect.
Great, put it back together and I am done.
After replacing the switch board, main board, and bottom cover - the test.
Fanfare please!
Nothing. The player is dead as a door nail. It now will not even focus
and gives up almost immediately.
Off come the screws. Almost immediately, it is obvious what has happened.
In replacing the main board, I accidentally squashed one of the printed
cables linking the optical pickup and main board, partially severing
the cable. In fact, 2 of the 4 conductors are cut. This is the focus
and tracking drive cable so it is pretty important. What a pain!
Fortunately, luck is on my side with respect to the location of
the break - it is at a non-flexing part of the cable. Therefore,
repairing the cable should not be that difficult since once the conductors
are connected electrically, they can be coated with a sealer and flexing
will not be a problem.
To repair a cable of this type, I have two options: I can attempt to
jumper the break with some fine strands of wire or I can go point-to-point
from the circuit board to the destination on the optical pickup. However,
the latter connections are nearly hidden and would be difficult to solder.
I opt for the first. Using an Xacto knife, I carefully scrape the
orange mylar coating from both sides of the break. Then with #30
wire, I carefully solder across the break for each of the conductors.
A spring clothespin holds the wire in place during the soldering.
The entire affair is then coated with some clear sealer to reinforce
it mechanically and provide insulation. It isn't pretty, but it
will work fine. For added protection, I add a layer of plastic electrical
tape.
Now, finally, reassembling the unit keeping cable routing firmly in
mind, there should be no problem.
And, as expected, the player comes back to life and is rock solid with
respect to playing and recognizing discs. The oops should have no effect
on the expected longevity of the player.
Comments: we all can point to those minor disasters where we have overlooked
something where we should have been more careful. Whenever reassembling
anything, it is imperative that lead dress (ok, fancy term for how the
cables are routed) is kept firmly in the forefront of your mind. It seems
that with more and more miniaturization, this is an increasingly
important and at times, frustrating consideration. First of all,
it is very tempting to say when disassembling the unit 'this is obvious,
no need to write it down'. Bad move. Often, it appears much less
obvious when putting everything back in its place. I have never quite
figured out how they do it during manufacturing - correctly most of the time.
Ignoring cable routing can lead, as in this case, to severed wires. It can
also result in shorting between wires or between wires and sharp metal
brackets or shields. Broken wires can usually be repaired if they can
be located. Shorted signals can result in additional hard-to-locate
collateral damage which can really turn your hair gray.
What is even scarier is that with line connected electronics or appliances like
vacuum cleaners and even toasters - this can lead to electrically live
parts accessible to the user. Sometimes, the plastic insulation on typical
internal wiring will not fail immediately but will cold flow and cause
problems later. So, one should always make every effort to assure that
no wiring is being pinched and for metal cased appliances, check that the
case is not electrically live - has a high resistance (usually
infinite, but at least a few M ohms to both wires of the AC line
with any on/off switches in both positions) after the repair is completed.
For non-heating appliances or electronics, a little electrical tape goes
a long way. For heating appliances you really need to make sure that bare
wires are routed far from any exposed metal of the case taking into account
as well any motion that may occur during normal operation or due to being
knocked about or dropped.
Patient: Sony CDU33A CDROM Drive given to me supposedly brand new but broken.
Symptoms: Drive behaves the same as a similar working drive until it
is accessed. Then, there is no response by any DOS or
Windows software. No CDs are recognized, always get the message:
Abort, Fail, Retry?
Testing: I keep an old (well, what other type are there?) 286 PC clone
system around for the primary purpose of testing peripherals.
Installing the drive and software confirms the reported
behavior. I was given two similar drives. The other one was
reported as being intermittent but seems to work fine in my
test system. This one was indeed dead.
Since it is impossible to observe the behavior of the pickup and, in
particular, the lens with the cover on, the first step is to get at
the guts.
Fortunately, the CDU33A is quite simple to disassemble.
There are only two major components: the Printed Wiring Board (PWB) where all
the active electronics are located and the Optical Deck including laser,
optics, and pickup worm drive mechanism.
The other parts include the upper plastic casting and metal shroud,
solenoid latch assembly, right and left guide rails, drawer assembly,
and front bezel, two springs, bottom plate, 6 screws.
There are only two electrical connectors inside: one flat printed cable
linking the PWB and optical deck and a two pin connector supplying power
to the eject solenoid. This is in pleasant contrast to some other CDROM
drives I have seen with a half dozen or more small connectors spread
all over the PWB making removal and testing very difficult and risky.
After about 10 minutes, I have the drive apart and can now reassemble the
major components on the bench outside the case to observe behavior.
I prop up the circuit board and reconnect the flexible cable - noting the
orientation marks. I can now run the drive with full visibility of
the mechanism and optics. With a CD in place, there is no danger
from the laser beam. I make sure the PWB cannot short to anything and
that the whole affair cannot tip over.
Having set up this contraption (you would have to see it to appreciate
appreciate this terminology), I am ready to continue testing.
Naturally, it now works perfectly.
No amount of abuse seems to phase it - wiggling cables, flexing the
circuit board, trying multiple CDs, all fail to reproduce the
original problems. Could it be the case? I can think of no reason
why it should make a difference? Is there anything else different?
I don't think so. Perhaps the sled was jammed somehow and disassembling
the drive fixed it. Who knows.
After reassembly, the drive continues to function perfectly.
Comments: How many times has someone brought you a 'broken' device which
has magically started working again on your bench. It certainly cannot
hurt your reputation. Admittedly, here, I had to actually do an exploratory
before rejuvenation to convince it that I meant business.
It has now been almost a year and the drive continues to function. I can
only guess that the cable may have been poorly seated or had some
dirt stuck in the contacts. Until it fails again, there isn't much more
to try. Unfortunately, the saying: "if it ain't broke, don't fix it" now
applies. I have no idea if the drive will ever again fail within its normal
life expectancy, but in the meantime, where did I put my Win95 CD?
(No comments, please, about choice of OS).
Patient: Late model Panasonic 17" color TV
Symptoms: Width slightly reduced. Slight evidence of 60 Hz hum bar,
brightness pulsating, raster shaking, somewhat channel dependent.
Testing: All of these symptoms were easily reproduced on the bench. The
60 Hz hum bar is the giveaway indicating a low voltage power
supply problem.
Rather than operating the TV off a Variac to confirm lack of regulation,
I decide to just try the most likely solution - a replacement main filter
capacitor. With power off and making sure the main filter capacitor is
discharged, I use a pair of clip leads to jumper my test cap across its
terminals.
The set now works perfectly.
Removing the old capacitor (not easy as the rivlets really do make nice
heat sinks), testing with my trusty Radio Shack DMM on its capacitance scale
reveals that the value has dropped by over 85% - pretty amazing that the
set worked at all!
One highly overpriced replacement filter capacitor (I used a local
distributor instead of my favorite mail order sources) and the deed is done.
No disasters on this one!
Comments: This capacitor was mounted next to a large heat sink - possibly
the power regulator. When replacing electrolytics, we often ignore one
very important specification - the temperature rating. Either the original
capacitor was defective or it was not rated for the thermal conditions inside
a compact TV. The TV was not that old - maybe 3 or 4 years at most. We all
can point to equipment we own that is still working after 20 or 30 years
going strong on the original filter capacitors.
Patient: Zenith 19" color TV, about 5 years old. This TV is owned by
a neighbor of mine who has small kids - more on this later.
Symptoms: Dead as a door nail. Only evidence that it is connected to
the line is a momentary flicker of lights when TV is turned on
indicating that the main filter capacitor is being charged.
Testing: This set has a pull-type on-off switch. There were no blown
fuses. Checking with a voltmeter shows 150 V on the main
filter capacitor with the switch in the on position. Ditto
for the collector of the HOT.
This would seem to indicate that there is a problem with the startup
drive to the Horizontal Output Transistor (HOT).
Off to the library for the SAMs....
Many Zenith TVs use a simple multivibrator to generate a startup
signal to the horizontal driver transistor until the flyback
can generate the secondary voltages needed to operate the deflection
ICs. Once these voltages are present, the startup circuit is disabled.
Indeed, such a design is used for this TV.
Checking with a scope (powering the TV through my isolation transformer)
at the base of the HOT shows no drive signal.
Tracing back, there is no signal at the driver transistor or from the
output of the startup circuit. One of the two transistors in the startup
multivibrator is bad.
I do not have a suitable replacement - it is a high voltage low current
Zenith part similar to an MPSA43 - 200V. I will need to obtain one, or
better yet, two to replace both transistors in the multivibrator.
To confirm that the rest of the TV is operational, I use a common
technique to 'jump start' a TV where the startup circuit is defective.
This is to inject a signal of around 15-16 KHz directly into the
base of the HOT to substitute for the startup circuit.
With the TV turned on, momentarily touching the output of a pulse generator
set for 15 KHz and a couple of volts amplitude to the HOT base brings the TV
to life. Everything appears normal except that the TV does not start on its
own. Somehow, I don't think my neighbor would approve of this solution.
(Also, I am not giving up my pulse generator!).
Caution: jump starting a TV like this is risky. In addition to the dangers of
mucking with a live TV, injecting a signal with improper characteristics
into the HOT can destroy it and possible a lot of other circuitry - instantly.
For example, a single cycle with too long an ON time can blow the HOT from
overcurrent while driven on or overvoltage during flyback. Use this approach
with care.
Replacement of the multivibrator transistors with the exact Zenith parts
completes the repair successfully.
Comments: Examining the schematic of the startup circuit reveals that it
appears to be designed to fail - especially with kids about. While
the transistors are rated at 200 V (they are running on the 150 B+
from the line power supply), the transistor power rating is only .6 W.
Even though they are running in a switching mode, I believe that
repeated on/off cycles can stress these to the breaking point. Something
was mentioned about my neighbor's kids turning the TV on and off repeatedly.
I have not duplicated this experiment but suspect that such treatment
at least may contribute to premature failure. Fortunately, in this case, it
was only in the startup circuit.
Power-on is a stressful time for many types of equipment due to inrush
current, transient voltage, so many things changing quickly, etc.
In addition, designers may not study and characterize the behavior
during startup with the same amount of care that they presumably
(we hope) do for steady-state operation.
Patient: Samsung VCR, Model VR 2610.
Symptoms: Inserting a tape works fine - it plays, it records, it FFs, it
REWs. However, attempting to eject a cassette results in an
infinite loop - the VCR grabs the tape back just before it
pokes out of the slot. Sometimes, the tape can be grabbed in
time but usually the cassette does not exit far enough.
Testing: Symptoms confirmed. With the top off, it is easy to catch the
tape but I don't suppose this would be an acceptable solution.
In addition, the cassette carriages seems king of sloppy - loose
for want of a better term. This would indicate a mechanical
problem with the cassette basket - the mechanism which moves
the cassette into position inside the VCR.
First step: a close examination of the basket mechanism. Nothing obvious - no
broken parts visible.
Next step: attempt to remove the basket. With most VCRs, this is a simple
matter of 4 screws and perhaps a connector. Not here. There are 4 screws,
but once the screws are removed, only one side wants to come loose. The
left side, with most of the gears and whatsits, is firmly fixed to the
base of the tape deck. No doubt, there are critical timing relationships
that might be disturbed once removed. It stays for now.
Perhaps, removing the bottom cover will reveal something. 8 screws later,
bottom cover off. What's this? A spring!! So now, we know that something
is indeed broken and most likely in the basket somewhere. This sort of spring
is not the type to have just popped off - it is a close wound coil spring
with hooks at each end. And, guess what, there is also a tiny bit of white
nylon which was probably the tab onto which the spring was hooked at one end.
A close examination of the visible portions of the basket above and below
deck finally turns up something now that I know generally what I am looking
for. Thankfully, it is accessible and I hopefully don't need to pursue
removing the basket which almost certainly would not be a fun thing to do.
The spring is supposed to be connecting two gear-type wheels in the EJECT
mechanism. With the spring sprung, these were free to rotate when they should
not have and their free play was sufficient to cause the EJECT operation
to screw up.
So, how to repair? There is no good way to glue nylon and even if there
was, the tab is so small that it would be impossible to provide a strong
enough bond to withstand the spring force. Replacement of the part with
the broken tab is a possibility though again not a pleasent one - it would
require removing the basket. Of course, replacing the entire basket is
another unpleasent and expensive options. Installing a metal post in place
of the tab is also a possibility - one that I do not really want to
contemplate.
Well, it appears as though there is nothing particularly critical about
the spring placement. Is there an alternative location to connect the
end with the broken tab? Yes, it would appear that it will be sufficient
to hook it around another large wire spring. However, then it is probably
stretched too long, so I make a link out of a piece of a paper clip and
this seems to be about right. (Paper clips, bailing wire, scotch tape
and chewing gum (well maybe not chewing gum) are among my favorite things).
Getting all this in place under spring tension between the edge of the case
and the basket plastic frame proves a bit of challenge - requring a dental
picks, needlenose pliers, patience, and few carefully chosen four letter
words - but I prevail. The EJECT operation now works perfectly.
While not pretty, I believe the newly designed spring attachment will be
much more robust than the original. I should write to Samsung!
Comments: This is a another case of poor design - there can be no other way
of describing it. The spring is rather large (you can visualize it, can't
you?) and the tab much too small. Another .0001 cent of plastic and it
would outlast the rest of the VCR. There was absolutely no excuse as
there is plenty of space to enlarge and reinforce the tab.
Patient: Nintendo original game system.
Symptoms: Power light blinks indicating that it is not able to run the
program contained in the game cartridge.
Testing: Tried multiple cartridges without success.
The most common problem with these units is a worn or dirty system unit
game cartridge connector. In this case, the red power/status light will
continue to flash even after the RESET button is pressed with a
game cartridge in place. Replacements are available for about $9
from the usual sources (MCM Electronics, etc.)
First, I try another game cartridge - the one that is not working may
just have dirty contacts or may be defective. This does not work.
So I need to get inside. Fortunately, unlike some other consumer stuff,
this is quite easy. Six screws underneath followed by about a dozen
to remove the metal shield and circuit board so the connector can be removed
and inspected.
Before removing the connector from the circuit board edge, I give the system
another chance to redeem itself. With the latching mechanism removed, it is
possible to press the cartridge down somewhat lower than normal increasing
the chances for good contact. Indeed when this is done, it is possible
to occasionally get a good reset and game startup on the TV. This
certainly confirms the original suspicion.
Now, can I revive the original connector or must it be replaced? There are
three kinds of problems that generally occur with these connectors:
1. Wear of the contacts to the game cartridge. Although full pressure
is not applied until the cartridge is latched, there is still wear
every time a game is inserted or removed. This take its toll.
It is often possible to use a dental pick or a bent paper clip (one
of my favorite tools!) to slightly spread the spring contacts so that
they grip the edge contacts of the game cartridge more tightly. On a
high mileage unit, however, they may be worn through to the point of
actually breaking in half resulting in replacement as the only option.
2. Corrosion at the connector to the circuit board. Cleaning with
a pencil eraser or at most some very fine sandpaper (600-1000 grit)
will usually restore to as-new condition unless some really corrosive
agent was at involved.
3. Kid grime on game cartridge edge connector transferred to Nintendo
cartridge connector. Favorites: Coke, sugar candy, and ice cream.
So, the first step is cleaning of both sets of connector contacts and
the main circuit board edge finger. You may need to use a variety of
solvents to completely remove all crud. Water may work better on sugar
syrup than normal contact cleaner or alcohol. For the edge finger, a
pencil eraser very useful.
Don't neglect the game cartridge connectors. These generally do not wear
but may collect all kinds of strange stuff. Rather than fight with the
security screws that you may find holding the case together, I usually
simply use a Qtip with water, contact cleaner, or alcohol - or one after
the other - to clean these contacts. Again, very fine sandpaper may be
needed in extreme cases.
Even if these procedures only make a slight improvement - you can press down
on the cartridge and the machine will respond to the RESET button - you have
confirmed that the connector is indeed the problem. In many cases, just
cleaning will result in reliable operation for a long time to come.
In the case of this particular system, all three problems were present.
However, for the time being at least, the system has responded well to
treatment.
Comments: While the original Nintendo game machine is a couple of generations
out of date, many are still in use. And, hey, young kids usually don't care.
OK, you don't have to admit to being the one who cannot resist just a couple
rounds of 'Super Mario III'!
Old Nintendos can usually be picked up for $5-20 at garage sales sometimes
complete with a selection of games, sometimes bare. The games go for
$1-$5 depending on the barganing skills of the kid selling the stuff.
However, a bad connector is almost a sure bet with a secondhand system.
Consider that most electronic connectors are typically rated in terms
of hundreds of insertion/removal cycles. A Nintendo machine must endure
thousands of not necessarily gentle cycles over its lifetime. The connector
was not designed for that. Furthermore, you are likely to find all kinds
of muck inside, mostly unidentified, and often difficult to remove.
Nonetheless, these things are remarkably robust, electronic failures
are infrequent, and they can usually be revived without much difficulty.
Patient: Sharp Model VC7864U VCR in generally good condition, garage
sale acquisition, $1.
Symptoms: Former owner complained about difficulty in ejecting.
Testing: Tried playing multiple cassettes (not all at once!). For the
most part, the VCR behaved normally. Maybe just a bit sluggish
loading but no other obvious problems. Why did he dump it?
I did my usual cleaning - rubber parts did not look to bad, leave them
for now. Even the idler tire appears to be in decent condition. I will
use the VCR and see if any problems appear.
The first sign of trouble appears once when attempting to use REVIEW
mode - the VCR abruptly stopped and attempted to unload the tape. The
loading motor was spinning but nothing was happening (I think it was turning
in the wrong direction and the belt was slipping - I am not sure). Oh boy,
time to leave the cover off. Manually giving the motor shaft (fortunately
it is accessible from above the deck) a couple of turns convinces the VCR
to complete a correct unload cycle.
Well, this sounds like the classic 'if it is an erratic Sharp VCR, the mode
switch must be dirty or bad' problem.
(2 years pass as I am in no mood to bother with this repair at the moment.)
OK, now I have a need for a reasonably decent VCR to replace my cousin's
Mitsubishi HS328U which is finally dying. So, I dig the Sharp out of the
closet and see about its condition. Now, it doesn't even want to play a
tape at all. Well, I know I have to deal with the mode switch, so first
things first.
The mode switch on this model is sandwiched between the loading gears
and a mounting plate - all parts of what I will call the 'loading gear
assembly'. To access the mode switch, this entire unit needs to be
removed and partially disassembled. The gears operate the roller guide
loading mechanism, and a couple of cam operated levers which are conveniently
hidden when it is removed or reinstalled. It is driven by the loading
motor via a couple of idler gears.
Timing marks: In the unloaded position, there is a hole in one gear
that appears to line up with a slot. So, with the roller guides retracted
(and the gears which operate this linkage have timing marks which also
line up), this hole should be centered in the slot. Fine. This appears
to be the only critical relationship with respect to removing the loading
gear assembly.
I unsolder the 4 connections to the mode switch, remove 3 screws, and -
sproing! What was that? OK, one or both cams still had a lever with
spring pressure applied. Hopefully, it will be possible to extend these
these when replacement time comes along.
With the loading gear assembly removed, it is still not possible to access
the mode switch. Now to disassemble it. There are two fancy cam gears which
obviously must be timed correctly - in one position there appear to be
an arrow and triangular hole which line up. I add a couple of marks of
my own for good measure with a felt tip pen. A simple split washer
holds the gear I need to remove onto its shaft. (Note: these split
washers are not designed to be reused but with care in removal, they
can usually be replaced without any long term problems. Of course,
a professional would have an assortment of replacement sizes handy.)
Removing the gear carefully, there don't appear to be any flat washers
or spacers to worry about.
Once the gear is removed - making a note as to which side is up though
this is pretty obvious - the mode switch is exposed. Squeezing the
center of the split shaft enables the cover to be popped off and the
interior appears. I almost lost the springy wiper as it is not fixed to the
plastic cover but popped free when first removed. A frantic search was
needed to locate it on the floor. The wiper fingers and encoder contact
traces seem to be in good condition but whatever was used as a lubricant
is a little gummy and might be the problem. A simple cleaning seems to
take care of that. I also bend the wiper fingers a bit to increase the
contact force very slightly.
Now, to get everything back together. First, the wiper is replaced and
the mode switch cover is snapped back in place. Free rotation is confirmed.
Then, the gear that was removed is returned to its shaft along with a
cam follower lever that was under it. The split washer is replaced.
To install the entire loading gear assembly means that the original
gear timing relationships must be re-established. In addition, care must
be taken to make sure those two cam follower levers I mentioned previously
are properly positioned. This takes a bit of work but eventually, I
am convinced that everything works as it should. The screws are tightened
and then free movement of all the parts is confirmed by manually cycling the
loading mechanism. The 4 mode switch connections are then resoldered.
Now for the test. Since this was not a hard failure to begin with, there
is no guarantee that any problems will be detected.
The tape seems to load correctly but then the VCR unloads and shuts down.
What is wrong? It would appear that the takeup reel is not turning. Hum,
probably that rubber wasn't as great as I had assumed a couple of years back.
I now do a more complete cleaning and, in particular, remove the idler tire
and inspect it. It appears to be ok but as a test, I turn it inside-out.
Now, everything works as expected. Testing with a cassette cheater (shell),
there appears to be adequate takeup torque. I clean the idler tire again and
reinstall it in the normal configuration. All modes appear functional even
when testing with a full takeup reel - requiring the most takeup torque.
I will order new rubber anyhow and replace it at a convenient time or if
problems reappear.
This VCR now appears to operate reliably and consistently. I have seen
no evidence of the original erratic behavior. Only time will tell for sure.
Comments: I cannot overemphasize the importance of making careful notes as
well as adding timing marks of your own when removing any parts of a VCR which
could conceivably have critical timing relationships. Not doing this can
really mess up your day. Err on the side of excess - it won't cost you
anything.
Sharp VCRs seem to be particularly prone to mode switch problems: Of the
3 Sharp VCRs under my control, 3 of them have developed dirty mode switches
resulting in a variety of erratic symptoms including, as noted, going into
the wrong mode as well as aborting the tape loading operation for no good
reason.
It would seem that a VCR design using an optical mode switch instead of one
with sliding contacts would be much more reliable at only modest additional
cost. After all, VCRs already use a number of optical sensors and cheap
computer mice use optical encoders not very different in design from a mode
switch. At least, it would be nice if mode switches were readily accessible.
Some are visible as soon as the bottom cover is removed. Others require
substantial disassembly with associated risks of incorrect reassembly resulting
in mechanical timing problems or even damage when the unit is cycled.
Patient: Approximately 10 year old BIG Magnavox color console (when
furniture meant something).
Symptoms: Horizontal deflection jittery, possible vertical collapse, arcing
flyback - all in one set! This info from Dave whose friend
owns the set. Dave is a tech at work who is now doing more
software than hardware (not necessarily by choice).
Testing: I did not actually see the original problems, nor did I have
access to the entire set as Dave came in one morning with the
guts of this set under his arm (more like both arms). We actually
attempted to power it without the yoke or CRT but there was
absolutely no evidence of anything. Surprise surprise.
Since the original description of the problems is somewhat incomplete,
a visual inspection is made and the HOT is tested for shorts just to be
sure. There were none. However, the visual inspection did confirm that
the flyback had a narrow but rather long (maybe a couple of inches) crack
in its housing, There was no conclusive evidence of arcing but this is
one area where the original symptoms were fairly definitive as the owner
stated that there was arcing around the flyback. (He probably knew just
enough to be dangerous, but hopefully has not done anything we will regret.)
This would explain the jittery horizontal but what about the vertical problems?
Were there really vertical problems. I never did get a good answer to this
question - at least not until later.
While it is likely that the flyback could be patched up at least temporarily,
it was decided to order a new one. The owner was willing to spend up to
$150 to repair the set - I have no idea why. No match from places like
MCM Electronics - must go directly to Magnavox (Philips, actually). $71,
ouch. Admittedly, this is one of the spiffiest flybacks I have seen lately
(at least since that A-line Zenith with the cool ribbed plastic coil form). It
has a detachable CRT anode wire - wire and suction cup sold separately!
Well, for $71, you cannot expect everything.
Although we have agreed to order the flyback, I decide to test the old one
anyhow, so next day I bring in my flyback testing widget (12 V chopper, see
document on flyback testing). This is the first time Dave has seen this
tester and Ed (our chief digital design engineer) is also curious but stands
at a safe distance, having a great deal of respect for a few puny KV. Ed
always stands at a safe distance when anything higher than 5 V is involved!
First step - locate the HV return. In this case, it is obvious because (1)
a separate bare wire is brought out to a pin and (2) this wire is connected
to no other pins on the flyback. (With a built in HV rectifier, it is
not possible to use a normal DMM to locate this wire.)
Next step - wrap a ten turn coil around the core of the flyback and connect
this to the chopper.
Apply power - a nice healthy arc can be drawn from the HV lead of the flyback
to the return connection, current draw on power supply is low. Flyback is
quite functional. This does not test for breakdown at full voltage but does
rule out hard shorted turns. (Ed can be overheard mumbling something about
sticking with 5 V logic.)
Result is fine by me, owner wants new flyback and this one will make
a great HV supply for a plasma globe or something - someday.
So, we pile the chassis and all its attachments onto a table in the corner
of the testing lab awaiting our shiny new flyback (minus the red wire, some
assembly required). It looks kind of pathetic there but no one else dares
go anywhere near let alone touch it after an off-hand comment about charged
capacitors!
Approximately 5 days later, our new flyback arrives and is soldered into place.
Next morning: I see Dave pulling up in his Chevy wagon. Guess what is
in the back? The entire huge, heavy TV, belly down. Oops. We quickly find
a place for it somewhat out of the way in a back room.
Apparently, there is no arcing and the horizontal deflection is stable.
But, there is absolutely no vertical at all - flat lined. OK, so the rumors
about vertical collapse were not exaggerated.
A little more visual inspection reveals a couple of interesting observations.
First, all the deflection circuitry - both horizontal and vertical - is
clustered in a small area near the flyback. In addition, the crack in the
crack in the original flyback is adjacent to some of the *vertical* output
circuitry. So, perhaps, the arcing was making its way to something in the
vertical deflection. What kind of output chip is it? Ah, my favorite - a
TDA3654. Fortunately, I have a bunch of them to keep one of my tough dogs
fed. So, I am well prepared if need be.
A quick measurement of power to the TDA3654 reveals that there is none.
Maybe this won't be so bad after all. Tracing back with an ohmmeter and
what do I find? An open fusable resistor! And, in exactly the right place
to be killing power to the chip. Could it be this easy? Actually - yes
in this case. I install a normal 1 ohm 1/4 watt resistor (only for testing).
I also use the ohmmeter to confirm that the rectifiers in the vicinity are
healthy. We are set!
Power! At first there is nothing on the screen but then snow gradually
appears - and it is full screen. There is no antenna. Of course, reception
inside our building is nearly non-existent due to all the computer RF
interference and steel beam construction. However, we quickly locate
a pair of rabbit ears (or maybe it was just a couple of feet of hookup
wire) and tune one of the few channels that is viewable at all - which happens
to be broadcasting the morning cartoons. But that is just fine. Everything
appears normal and I remind Dave to replace that resistor with a proper
flameproof variety.
Dave cannot believe it.
Ed is nowhere to be found.
Everyone loves the cartoons.
Comments: The mechanism for the vertical failure still remains obscure.
Apparently, the arc caused a momentary but not fatal short circuit in
some part in the vertical output circuitry which blew the resistor.
We always hear how sensitive ICs are to static - here we have 25KV of raw
power discharging nearby with apparently no permanent damage except to a
25 cent resistor.
Patient: Hewlett Packard AN/USM281A Oscilloscope. This is the militarized
version of the HP180 lab scope. PL1186 dual channel 50 Mhz vertical
plug-in, PL1187 delayed sweep timebase plug-in.
Symptoms: Horizontal position shifted almost off the screen; delayed sweep
and B timebase inoperative. Alternate triggering erratic at low
intensities(??).
Testing: How does one test a scope? Well, put it through its paces with
reasonable input signals - a 10 MHz clock oscillator provides a
nice test signal. Maybe another scope would be handy?
Prologue: (You can tell right off that this will be a feature length saga.)
I bought this scope at a garage sale. Now, understand, garage and tag sales
around the Philadelphia area where I live are usually of the "Aunt Minnie's
old silver plate" variety. Electronic equipment is usually limited to comatose
VCRs and color TVs that play in B/W (not that I complain about this sort of
stuff for the right price - as little as possible). However, one little
ad catches my eye: one item amongst all the bric-a-brac is 'test equipment'.
BTW, I never go to flea markets with any serious intention of buying anything.
It is clear where their stuff generally originates. All the junk I turn
down at garage sales ends up with hugely inflated prices at flea markets!
I get to THE sale relatively early (I am not quite the garage sale addict
type who gets up at 5 AM to be first in line). All that is visible are
a couple of pathetic old signal generators - one audio, the other RF. Well,
$10 for an RF signal generator isn't too bad. I could probably have
bargained him down to $5 but first the all important question: Anything else?
(Not that I expected any sort of affirmative response given the assortment
of hat boxes, deflated basketballs, and old Christmas decorations.) However,
surprise surprise! "There is one other item." So he crawls under a table
and drags out an HP AN/USM281A - a real oscilloscope! "Well, I have this,
um, oscilloscope. It is solid state, dual channel, 50 Mhz, etc." Now,
I am paying really close attention (but of course, not wanting to show it).
The only oscilloscopes I had seen at garage sales until this time (beside
my $3 Tek 321, but that is another story) are usually the really beaten up
Eico variety). He is actually doing a pretty fair sell job. So, how much
are you asking? "I would like to get $100 for it." Very interesting. Can I
try it? "Sure." So, he props 'my' scope on top of a rickety old bar stool
(I would have been quite upset if the thing had gone crashing to the floor
but still didn't want to act interested enough to suggest he find a more
stable spot.) I figure that if it appears to work at all, $100 is a good
price even if I have do some repair and calibration. I fiddle with the
controls, also noting that it comes with two nice looking 1X/10X probes.
Suddenly, the scope really cooperates - it must really want a new home
being so lonely stuck in the back of that garage. The trace scoots off to
the right of the screen. None of the front panel controls have enough range
to bring it back. I mumble: I cannot get the trace back. He says "Oh, um,
uh..." Before he can get too far, how about $50. "Sure, ok." I didn't do
enough testing to find out that the delayed sweep was also dead. For that
matter, even with the 10 turn delay time pot in plain view, the existence of
a delayed sweep mode did not register. I only found that out later. No
amount of fiddling would produce any difference between the A timebase and
Mixed A+B. The B timebase was totally dead.
So, how to go about tackling this? I have no service manual, no schematics,
and looking at circuit boards, the semiconductors are HP house numbers.
I didn't have an ECG manual. Fortunately, the component side of the
circuit boards are readily accessible. However, tracing the wiring is a
real treat with HP's love affair with multicolored striped bundled wiring
harnesses. I could try to buy a manual (this was somewhat before the days
of sci.electronics.repair). Nah, that would be cheating (and probably
expensive). I did try our local HP sales rep when we were looking into
their logic analyzers but he did the usual salesperson thing and lost interest
once we signed on the dotted line.
So, I had to repair it the old fashioned way - ohmmeter, circuit tracing,
seat of the pants, etc.
Objective #1: find the horizontal position problem. Even if the delayed
sweep remains broken, a dual trace 50 MHz scope is very useful. Fortunately,
this problem appears solid now (and not intermittent as was the case
previously) so ohmmeter tests of the horizontal sweep board components
should be possible.