Notes on the Troubleshooting and Repair of Audio Equipment and Other Miscellaneous Stuff


Chapter 15) Specific Problems and Repairs

  15.1) Classic ATT Touch Tone phone 'battlewagon' will not dial properly

Typical symptoms are: everything works fine except erratic or no dialing.  For
some buttons, dial tone would not go away.  For others, tones would be
accepted but will be erratic and result in incorrect digits.  Certain digits
may sound weak, wavery, or single frequency (rather than the proper DTMF
dual tones).

(Note that this is not the same as the situation where the phone does not dial
at all - there are no tones of any kind generated.  In this case, the wires
to the phone may simply be reversed - old ATT touch tone phones will not dial
out if they are but will work in all other respects.  Modern phones generally
don't care about phone line polarity.)

While the internal wiring 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.

There are two cores which each consist of two halves glued together.
Breaks seem to be a common problem due to both the age and the brittle
cement used on some revs of this model phone, and probably, as a result
of rough treatment when hanging up the handset, or dropping or throwing
of the desk phone.

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.  Momentarily depress the hook switches to silence the dial tone.
You will now have about 25 seconds before the nice polite operator recording
tells you how to make a call.  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 pitch 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).

These 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.

  15.2) ATT classic dial phone will not dial

I know, you haven't seen one of these in years, but I just had to throw
this in.

Most likely it was dropped - these phones simply do not seem to fail any
other way.  When dropped, assuming there is no obvious damage, a little
plastic stop inside the dial mechanism which is on a pivot flips the wrong
way.  This normally prevents dialing pulses from being generated when the
dial returns to its home position but when flipped, prevents dialing
totally.  It is real easy to flip it back into place.

  15.3) Jerrold 400 Cable Converter Problems

The most common symptoms for these cable boxes relate to their not staying
on or acting erratically when the buttons are pressed.  The causes are usually
quite simple:

1. Cold solder joints around the power supply regulator ICs (on chassis
   heat sink).

2. Dried up main filter capacitors - two large electrolytics in power supply
   on main board.

Be careful disassembling the main board from the chassis as at least one of
the regulator ICs clipped to the side of the chassis is insulated from this
heatsink and the insulation is easily damaged.

  15.4) Original Nintendo console erratic or dead

While the original Nintendo game machine is a couple of generations
out of date, many are still in use.  And, hey, kids usually don't care.

The most common problem with these units is a worn or dirty
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 sources listed at the end of this document.

First, try another game cartridge - the one that is not working may
just have dirty contacts or may be defective.  Clean the contacts
with a Qtip moistened with water followed by isopropyl alcohol.
(The water will remove the sugar from the candy that may have made its
way onto the connector.)

To get inside, you first remove the 6 screws on the bottom and then
about 12 screws which fasten the circuit board and shield to the bottom
of the case.  (Note: there are two screws which are longer and silver
colored - make sure they get back to their original location when you put
everything back together.)  Once all these screws are removed, the black
connector can be slid off the edge finger on the circuit board  Inspect
these connections - they just may be a bit corroded or dirty.  Use contact
cleaner and/or a pencil eraser and see if that makes any difference.
Use contact cleaner on the dual rows of fingers that connect to the
game cartridge as well.  A dental pick can be used to gently spread the
fingers apart ever so slightly and thus improve the connection when the
cartridge is inserted.

Even if this only makes 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
this cleaning will result in reliable operation for a long time to come.

  15.5) Repairing Texas Instruments TI-5X calculators

I have them up through TI-57 so I don't know if the following applies
to models higher than this (TI-58 and TI-59).

If it hasn't been used for a while (like 15 years?) then the NiCds are
likely deader than a door nail and will not accept a charge since they
are totally shorted.  Bad NiCds is very likely all that is wrong with
the calculator.

If your calculator has a pack that plugs in inside the back with 2 AA NiCds
and some circuitry, then it is the same.

First crack open the pack by using a butter knife or similar instrument
at the catches along the seam.  You will see a pair of AA NiCds and a
small circuit board.  This is a DC-DC convertor which boosts the 2.4 V
of the NiCds to about 10 V to operate the logic of the calculator.

Inspect the circuit board for corrosion and other obvious damage.  Unless
the calculator was stored in a damp area, it should be fine.  The batteries
will probably have crusty white stuff on the positive ends.  They are bad.
Don't even bother trying to zap them.

As a test, you can do either or both of the following:

1. Get a large electrolytic capacitor (e.g., 10,000 uF at 10 V) and put it
   in in place of the batteries.  Observe polarity.  Try out the calculator
   using the TI charger/adapter.  Operations will be a bit flakey but should
   basically work (the capacitor, no matter how large, apparently will not
   substitute for the NiCds).

2. Unplug the TI battery pack and set it aside.  Find a 9 V power supply or
   a 9V battery.  Connect this to the red and black wires coming from the
   logic board connector which went to the battery pack.  NOTE: the wire
   color coding is backwards on at least some of these.  Black is positive
   for some reason.  However, nothing disasterous happens if you connect it
   backwards as far as I can tell since I was testing it backwards for quite
   a while until I caught on.  And, I thought TI was a real company! 

If these tests are successful, the calculator is likely fine and you just
need a new set of AA NiCds with solder tabs to make it as good as new.

Or, if you don't need the authenticity of a genuine TI form-and-function
rechargeable battery pack, use a 9V AC adapater, 9V Alkaline, or 9V NiCd
battery and charge it externally.

Chapter 16) Service Information

  16.1) Determining belt type and size

Belts are normally specified by their cross section - square, flat, round,
and their inside circumference (IC).  The IC is used since it is virtually
impossible to accurately measure the diameter of a belt.

Assuming you cannot locate an actual part number, determine the type of
belt; square, flat, or round.  If you do not have the old belt, this is
usually obvious from the pulleys.  Most small belts (as opposed to  V-belts
on 1 HP shop motors!) used in consumer electronic equipment are of square
cross section though flat types are sometimes found in the main drives of
VCRs, cassette/tape decks, and turntables (remember those?).  Measure or
estimate the thickness.

The IC is always specified with the belt fully relaxed.  This can be
measured by hooking the old belt on one end of a ruler and pulling it
just tight enough so that it more or less flattens out.  Read off the
length, then double it for the IC.  Get a new belt that is 5% or so smaller
to account for the old one be somewhat stretched out.  Of course, if the
belt broke, measurement is real easy.  Or, if you do not care about
the old belt, just cut it and measure the total length.

If the old belt decomposed into a slimy glob of jellatinous black goop or is
missing, you will need to use a string or fine wire around the appropriate
pulleys to determine the IC.  Reduce this by 10-25% for the replacement.
Very often the match does not need to be exact in either thickness or
length - particularly for long thin belts.  A common rubber band may in
fact work just as well for something like a tape counter!

However, there are cases where an exact match is critical - some
VCRs and belt driven turntables or tape decks do require an exact
replacement for certain drive belts but this is rare.

Some parts suppliers make determining replacement belts very easy with
the PRB system in which the part number fully codes the shape, size,
and thickness.

Making custom length rubber belts: 

The following will probably work for most drive belts except for those which
are critical for accurate speed control in devices like cassette decks and

(From: Melissa & Jim (mgkepner@facstaff.wisc.edu)).

3M and Eastman make cyanoacrylate adhesives (super glue) that are
specially made for making custom O-rings from linear stock.  This
seems to be exactly the same problem you are approaching.  These glues
work very well and produce a joint as strong as the base material, but
without the need for the needle and thread.  The joint can be made
almost invisible.  The only hard part is holding the pieces aligned
while the glue cures, but in this case that is only seconds.

I have used a machinists steel V-block for this, but one of the O-ring
manufacturers sells a plastic tool for exactly this purpose.  In the
US, I would check at a bearing supply house; they often carry O-ring
supplies as well.

  16.2) Rubber or elastic bands as drive belts?

It is 3 AM, you have finally removed the last of the 38 screws to access the
tape transport in your Suprex Never-Forget model X4123 answering machine and
what do you fine?  A broken belt, of course!  What to do?

As a test at least, a common elastic band may work.  The recordings will likely
have terrible wow and flutter but this will at least confirm that there is
nothing else broken.  In a pinch, this free solution can be left in place until
a proper replacement arrives.  This should work for many types of devices - CD
players, VCRs, tape decks, etc. - where grooved pulleys are used and the belt
is not called on to provide a great deal of power.

  16.3) Interchangeability of components

The question often arises: If I cannot obtain an exact replacement or
if I have a VCR, tape deck, or other equipment carcass gathering dust, or
I just have some extra parts left over from a previous project, can I
substitute a part that is not a precise match?  Sometimes, this is simply
desired to confirm a diagnosis and avoid the risk of ordering an expensive
replacement and/or having to wait until it arrives.

For safety related items, the answer is generally NO - an exact replacement
part is needed to maintain the specifications within acceptable limits with
respect to line isolation, X-ray protection and to minimize fire hazards.
However, these components are not very common in audio equipment or other
consumer devices (other than TVs, monitors, and microwave ovens) except for
possibly in their power supply.

For other components, whether a not quite identical substitute will work
reliably or at all depends on many factors.  Some designs are so carefully
optimized for a particular part's specifications that an identical
replacement is the way to return performance to factory new levels.

Here are some guidelines:

1.  Fuses - exact same current rating and at least equal voltage rating.
    I have often soldered a normal 3AG size fuse onto a smaller blown 20 mm
    long fuse as a substitute.  Also, they should be the same type - slow
    blow only if originally specified.  A fuse with a faster response
    time may be used but it may blow when no faults actually exist.

2.  Resistors, capacitors, inductors, diodes, switches, trimpots, lamps and
    LEDs, and other common parts - except for those specifically marked as
    safety-critical - substitution as long as the replacement part fits
    and specifications are met should be fine.  It is best to use the same 
    type - metal film resistor, for example.  But for testing, even this is
    not a hard and fast rule and a carbon resistor should work just fine.

3.  Potentiometers - user knobs usually control one or more of these.  There
    are four considerations in locating a suitable replacement: resistance,
    and taper, power rating, configuration, and mechanical fit.  Configuration
    refers to the number of ganged pots, concentric knobs, etc.  Matching
    this from your junk box may prove to be the toughest challenge!  Many
    of the controls for audio equipment use what is known as an 'audio taper'.
    This means that the resistance change with knob rotation is not linear
    but is designed to produce a uniform incremental change in perceived
    volume, for example.  Replacement with a linear taper pot will squish
    all of the effect towards one end of the range but it will still work.
    If measuring the resistance of a (good) potentiometer with its wiper
    set in the middle results in significantly different readings from center
    to each end, it is most likely an audio taper pot (though some other
    weird taper or other peculiarity is possible).

4.  Rectifiers - many are of these are high efficiency and/or fast recovery
    types.  Replacements should have equal or better PRV, If, and Tr
    specifications.  For line rectifiers, 1N400x types can usually be used.

5.  Transistors and thyristors (except power supply choppers) - substitutes
    will generally work as long as their specifications meet or exceed those
    of the original.  For testing, it is usually ok to use types that do not
    quite meet all of these as long as the breakdown voltage and maximum
    current ratings are not exceeded.  However, performance may not be quite
    as good.  For power types, make sure to use a heatsink.

6.  Switching power supply transistors - exact replacement is generally
    best but switchmode transistors that have specifications that are at
    least as good will work in many cases.  See the documents:  "Notes on
    the Troubleshooting and Repair of Television Sets", "Notes on the
    Troubleshooting and Repair of Computer and Video Monitors", and "Notes
    on the Troubleshooting and Repair of Small Switchmode Power Supplies"
    for more info.

7.  Audio and erase heads - may be possible if the mountings are reasonably
    compatible.  However, there could be other unknowns like coil impedance
    and drive requirements.  The connectors are not likely to be similar
    either.  There are usually significant differences in head configuration
    and mounting arrangement between 2 head, 3 head, and autoreverse cassette
    or open reel tape decks.

8.  Motors - small PM motors may be substituted if they fit physically.
    Make sure you install for the correct direction of rotation (determined
    by polarity).  Capstan motors - especially the direct drive type - are
    probably not interchangeable.  However, generic speed regulated
    cassette drive motors are available.

9.  Sensors - many are sufficiently similar to permit substitution.

10. Power transformers - in some cases, these may be sufficiently similar
    that a substitute will work.  However, make sure you test for compatible
    output voltages to avoid damage to the regulator(s) and rest of the
    circuitry.  Transformer current ratings as well as the current
    requirements of the equipment are often unknown, however.

11. Belts, tires, and pinch rollers - a close match may be good enough at
    least to confirm a problem or to use until the replacements arrives.

12. Mechanical parts like screws, flat and split washers, C- and E-clips,
    and springs - these can often be salvaged from another unit.

The following are usually custom parts and substitution of something from
your junk box is unlikely to be successful even for testing: SMPS (power
supply) transformers, interstage coils or transformers, microcontrollers,
other custom programmed chips, display modules, and entire power supplies
unless identical.

  16.4) Why are there parts missing from my equipment?

It is not uncommon for parts to be missing from production equipment due
to design changes or field mods.  Thus, it may not mean anything.  Inspect
the solder pads - if they look the same as all the others, it was probably
never installed in the first place.  Of course, that could have been a
manufacturing omission as well.  Parts just don' jump ship without leaving
evidence behind!

Don't be tempted to add a part just because there is an empty spot.  In
some cases, like the RCA TV that would tend to blow HOTs if the power failed,
that would be a really bad idea and complicate your troubleshooting.

Whole blocks of circuitry are often left unpopulated on lower priced models.
You didn't pay for those features.  Sometimes, this can work to your advantage
enabling you to upgrade to a fancier model for the cost of the parts.

  16.5) Web resources

Tandy (Radio Shack) has a nice web resource and fax-back service.  This is
mostly for their equipment but some of it applies to other brands and there
are diagrams which may be useful for other manufacturers' VCRs, TVs, CD
players, camcorders, remote controls, and other devices.

 http://support.tandy.com/                          (Tandy homepage) http://support.tandy.com/audio.html                (Audio products) http://support.tandy.com/video.html                (Video products)

Since Tandy does not manufacture its own equipment - they are other brands
with Realistic, Optimus, or other Radio Shack logos - your model may actually
be covered.  It may just take a little searching to find it.

  16.6) Some general references

Here are some suggested titles that might be found in your local public
library or a technical bookstore.

1.  Troubleshooting and Repairing Electronic Circuits
    Robert L. Goodman
    Second Edition
    TAB Books, Inc., 1990 
    Blue Ridge Summit, PA 17294-0214

2.  Small Electric Motors
    Rex Miller and Mark Richard Miller
    Second Edition, 1992
    MacMillan Publishing Company
    866 Third Avenue
    New York, NY 10022

3.  Repairing Quartz Watches
    Henry B. Fried
    American Watchmakers Institute Press, 1988
    Cincinati, OH
    ISBN 0-918845-06-8

4.  Readers Digest Fix It Yourself Manual
    The Readers Digest Association, 1996
    Pleasantville, New York/Montreal
    ISBN 0-89577-871-8

5.  The Complete Guide to Digital Audio Tape Recorders
      including Troubleshooting TIps
    Erik S. Schetina
    P.T.R. Prentice Hall,
    Englewood Cliffs, NJ 07632
    ISBN 0-13-213448-9

6.  DAT - The Complete Guide to Digital Audio Tape
    Delton T. Horn
    TAB Books, Inc., 1991
    Blue Ridge Summit, PA 17294-0214
    ISBN 0-8306-7670-8 (hardcover), ISBN 0-8306-3670-6 (paperback)

7.  Troubleshooting and Repairing FAX Machines
    Gordon McComb
    Tab Books, a division of McGraw-Hill, Inc., 1992
    Blue Ridge Summit, PA 17214
    ISBN 0-8306-7778-X (hardcover), 0-8306-3778-8 (paperback)

8.  Complete Guide to Home Entertainment Equipment - Troubleshooting and Repair
    John D. Lenk
    Prentice Hall, Inc., a division of Simon and Schuster, 1989
    ISBN 0-13-161001-5

9.  Understanding Telephone Electronics
    Fike and Friend

10. Installing Telephones
    Radio Shack
    Catalog number: 62-1060

11. All Thumbs Guide to Telephones and Answering Machines
    Gene B. Williams
    TAB Books, Inc., 1993
    Blue Ridge Summit, PA 17294-0214
    ISBN 0-8306-4435-0 (paperback)

    This one is very basic but does cover the most common problems and has
    illustrated instructions for general telephone wiring, adding extensions,
    answering machine cleaning, rubber parts, simple electronic problems, etc.

And, for that older audio equipment (including record changers):

12. Repairing Home Audio Systems
    E. Eugene Eckland
    McGraw-Hill Book Company, 1962
    Library of congress catalog number: 61-18021

  16.7) Recommended parts suppliers

For general electronic components like resistors and capacitors, most
electronics distributors will have a sufficient variety at reasonable
cost.  Even Radio Shack can be considered in a pinch.

However, for consumer electronics equipment repairs, places like Digikey,
Allied, and Newark do not have the a variety of Japanese semiconductors
like ICs and transistors or any components like tape heads or belts.

The following are good sources for consumer electronics replacement parts,
especially for VCRs, TVs, and other audio and video equipment:

* MCM Electronics                 (VCR parts, Japanese semiconductors,
  U.S. Voice: 1-800-543-4330.      tools, test equipment, audio, consumer
  U.S. Fax: 1-513-434-6959.        electronics including microwave oven parts
                                   and electric range elements, etc.)
  Web: http://www.mcmelectronics.com/

* Dalbani                         (Excellent Japanese semiconductor source,
  U.S. Voice: 1-800-325-2264.      VCR parts, other consumer electronics,
  U.S. Fax: 1-305-594-6588.        Xenon flash tubes, car stereo, CATV).
  Int. Voice: 1-305-716-0947.
  Int. Fax: 1-305-716-9719.
  Web: http://www.dalbani.com/

* Premium Parts                   (Very complete VCR parts, some tools,
  U.S. Voice: 1-800-558-9572.      adapter, cables, other replacement parts.)
  U.S. Fax: 1-800-887-2727.

* Computer Component Source       (Mostly computer monitor replacement parts,
  U.S. Voice: 1-800-356-1227.      also, some electronic components including
  U.S. Fax: 1-800-926-2062.        semiconductors.)
  Int. Voice: 1-516-496-8780.
  Int. Fax: 1-516-496-8784.

Also see the documents: "Troubleshooting of Consumer Electronic Equipment" and
"Electronics Mail Order List" for additional parts sources.

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