Notes on the Troubleshooting and Repair of Computer and Video Monitors


  14.4) FCC ID Numbers of monitors

Only a few manufacturers actually produce the vast majority of computer
and video monitors.  For example, Radio Shack, Magnavox, and Emerson do not
make their own monitors (I can tell you are not really surprised!).  All
those house-brand monitors that come bundled with mail order or 'Mike and
Joe's Computerama' PCs are not actually put together in someone's garage!
Well, not that many, at least :-).

How do you determine the actual manufacturer?  For most types of consumer
electronics equipment, there is something called an 'FCC ID' or 'FCC number'.
Any type of equipment that may produce RF interference or be affected by
this is required to be registered with the FCC.  This number can be used
to identify the actual manufacturer of the equipment.

A cross reference and other links can be found at:


(Often only the first 3 characters of the FCC number will match. This is
sufficient to identify the manufacturer. - FMG)

  14.5) Parts information

I have found one of the most useful single sources for general
information on semiconductors to be the ECG Semiconductors Master
Replacement Guide, about $6 from your local Philips distributor.
STK, NTE, and others have similar manuals.  The ECG manual will
enable you to look up U.S., foreign, and manufacturer 'house' numbers
and identify device type, pinout, and other information.  Note that
I am not necessarily recommending using ECG (or other generic) replacements
if the original replacements are (1) readily available and (2) reasonably
priced.  However, the cross reference can save countless hours searching
through databooks or contacting the manufacturers.  Even if you have
a wall of databooks, this source is invaluable.  A couple of caveats:
(1) ECG crosses have been known to be incorrect - the specifications
of the ECG replacement part were inferior to the original. (2) Don't
assume that the specifications provided for the ECG part are identical
to the original - they may be better in some ways.  Thus, using the ECG
to determine the specifications of the parts in your junk bin can be

Other cross reference guides are available from the parts source
listed below.

  14.6) Monitor schematics and manuals

In some cases, these may be available from the manufacturer and even reasonably
priced (much less than other sources).  For example, a manual for a typical
CTX monitor is only $15 from CTX but around $50 elsewhere.  However, more often
than not, this will not be the case.

The following three companies have an extensive inventory of computer monitor
service manuals and schematics.  Typical prices are between $25 and $100.

* Computer Component Source (CCS), 1-800-356-1227

  CCS catalog 'centerfolds' hava had schematics for some common monitors like
  the IBM8513.  So, just asking for a catalog may get you some information.

* MI Technologies (http://www.mitechnologies.com/)

  However, there have been some complaints about the quality of some of their
  schematics.  In addition, apparently, unless you specifically request and
  sign a non-disclosure agreement, they will be printed on anti-copy red paper
  which is also impossible to read!  So, make sure you check out what you are
  buying.  They will accept returns.

* Electronix (http://www.electronix.com/schematics/)

The following may only be for IBM monitors (I don't know) and doesn't appear to
have a web site:

* Eagan Technical Sevices, Inc, 1380 Corporate Center Curve, Suite 115, Eagan,
  MN 55121, 612-688-0098.

  Eagan makes several schematics for IBM monitors.  I believe it includes the
  8503, 8512, 8513, 8514, 8518 and 8511.  Most are $50.  The 9517 schematic is
  an unbelievable $165.  You can order them directly from Eagan or through
  Sams' Photofacts (Howard Sams, http://www.hwsams.com/), same price.

And another:

* Chuntex, 1-800-888-2120.

  14.7) Information sources on the Internet

Many manufacturers are now providing extensive information via the
World Wide Web.  The answer to you question may be a mouse click
away.  Perform a net search or just try to guess the manufacturer's
home page address.  The most obvious is often correct.  It will usually
be of the form "http://www.xxx.com" where xxx is the manufacturers' name,
abbreviation, or acronym.  For example, Hewlett Packard is hp, Sun
Microsystems is sun, Western Digital Corp. is wdc.  NEC is, you
guessed it, nec.  It is amazing what is appearing freely accessible via
the WWW.  For example, monitor manufacturers often have complete information
including detailed specifications for all current and older products.
Electronic parts manufacturers often have detailed datasheets for their
product offerings.

  14.8) Interchangeability of components

The question often arises: If I cannot obtain an exact replacement or
if I have a monitor, TV, or other equipment carcass gathering dust, 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.
Typical parts of this type include flameproof resistors, some types of
capacitors, and specific parts dealing with CRT high voltage regulation.
However, during testing, it is usually acceptable to substitute electrically
equivalent parts on a temporary basis.  For example, an ordinary 1 ohm
resistor can be substituted for an open 1 ohm flameproof resistor to determine
if there are other problems in the horizontal deflection circuits before
placing an order - as long as you don't get lazy and neglect to install the
proper type before buttoning up the monitor or TV.

For other components, whether a not quite identical substitute will work
reliably or at all depends on many factors.  Some deflection circuits are
so carefully matched to a specific horizontal output transistor that no
substitute will be reliable.

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.

2. Resistors, capacitors, inductors, diodes, switches, potentiometers,
   LEDs, and other common parts - except for those specifically marked as
   safety-critical - substitution as long as the replacement part fits
   and specifications 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. Rectifiers - many are of these are high efficiency and/or fast recovery
   types.  Replacements should have at equal or better  PRV, Imax, and Tr

4. Posistors - many of these are similar.  Unfortunately, the markings on
   the devices are generally pretty useless in determining their ratings.
   Note, however, that the prices for replacement posistors may be quite
   reasonable from the original manufacturer so it may not make sense to
   take the risk of using an unknown part.

   (From: Stefan Huebner (Stefan.Huebner@rookie.antar.com)).

   In most cases you can use a standard 3-terminal-device, the resistance of
   the temperature dependent resistors in it are nearly identical. Here is a
   list of possible replacement devices:

   380000-01, 24340521, 2199-603-1201, 163-024A, 163-035A, CO2200-N66, C8ROH,
   QX265P05503, 32112026, 4822-A1-11240148, 02199-003-120, 15-08-001A,
   5391560067, F400001.

5. Transistors and thyristors (except HOTs and SMPS 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 specifications are not exceeded.  However, performance may not
   be quite as good.  For power types, make sure to use a heatsink.

6. Horizontal output (or SMPS) transistors - exact replacement is generally
   best but except for very high performance monitors, generic HOTs that
   have specifications that are at least as good will work in many cases.
   Make sure the replacement transistor has an internal damper diode if
   the original had one.  For testing with a series light bulb, even a
   transistor that doesn't quite meet specifications should work well
   enough (and not blow up) to enable you to determine what else may be
   faulty.  The most critical parameters are Vceo/Vcbo, Ic, and Hfe which
   should all be at least equal to the original transistor.  I have often
   used by favorite BU208D as a temporary substitute for other HOTs in TVs
   and SMPS (chopper) transistors.  However, for high performance monitors,
   a BU2508D type is a better choice.  Make sure you use a heatsink (with
   insulating washer if applicable) and thermal grease in any case - even
   if you have to hang the assembly with a cable-tie to make it fit.

   However, using an HOT with much better specs may actually result in early
   failure due to excessive heating from insufficient and/or suboptimal base
   drive.  See the document: "TV and Monitor Deflections Systems" for more

   Also see the section: "Replacement power transistors while testing".

7. Deflection yokes - in the old days, particularly for TVs, all of these
   were quite similar.  It was common to just swap with one that fit
   physically and at most need to adjust or change a width coil.  With
   high performance auto-scan monitors, this is no longer the case.  Sometimes
   it will work but other times the power supply won't even be able to come
   up as a result of the impedance mismatch due different coils and pole
   piece configurations.  In addition, there may be other geometry correction
   coils associated with the yoke that could differ substantially.

   However, if you are really determined, see the section: "Swapping of deflection yokes".

8. CRTs - aside from the issues of physical size and mounting, many factors
   need to be considered.  These include deflection angle, neck diameter,
   base pinout, focus and screen voltage requirements, purity and convergence
   magnets, etc.  Color CRT replacement from scratch (not using a CRT and
   yoke/convergence/purity assembly from another monitor) is rarely worth the
   effort in any case.  But, trying to substitute a different CRT is really
   asking for frustration.

   For monochrome CRTs, there is less variation and this may be worth a try.

The following are usually custom parts and substitution of something from
your junk box is unlikely to be successful even for testing: flyback (LOPT)
and SMPS transformers, interstage coils or transformers, microcontrollers,
and other custom programmed chips.

Substituting mainboards and other modules from identical models is, of course,
possible but some realignment may be needed.  Even a monitor from the same
manufacturer that is not quite identical may use the same subsystems, perhaps
depopulated or jumpered differently.

  14.9) Horizontal output transistor pinouts

You will nearly always find one of two types of horizontal output transistors
in TVs and monitors:

* Metal can - TO3 package:
             / O \         View from bottom (pin side)
           / o   o \
          (  B   E  )      B = Base, E = Emitter, C = Collector
           \       /
             \ O / C       The metal case is the Collector.

* Plastic tab - TO3Pn (n = several suffixes) package:

            /     \
           |   O   |      View from front (label side)
           |       |
           |       |      B = Base, E = Emitter, C = Collector
             | | |        If there is an exposed metal tab, this is the
             | | |         Collector as well.
             B C E

Some other transistor types use the same pinout (TO66 for metal can, TO218
and TO220 for plastic tab) but not all.  However, for horizontal output
transistors, these pinouts should be valid.

Note that those with a built in damper diode may read around 50 ohms between
B and E (near 0 on the diode test range) - this is normal as long as the
resistance is not really low like under 10 ohms.

  14.10) How do you locate the HOT

Well, it is usually the LARGEST transistor in the set near the LARGEST
transformer in the set (flyback - the thing with the FAT red wire connecting
to the picture tube) on the LARGEST heat sink in the set.

Got that? :-)

Or, in the good old days - oops - but that was before computer monitors...

(From: Don Wall (d.wall@nunet.neu.edu)).

Sure, it's usually the largest tube in the set, has a top cap, runs very hot,
and is often a 6BQ6G or some such.  (tongue firmly in cheek) Actually, back in
the days of yore, the Horizontal Output Tube was frequently referred to as the
HOT; guess some things don't change!

  14.11) Replacement power transistors while testing

During testing of horizontal deflection circuits or switchmode power supplies,
particularly where the original failure resulted in the death of the HOT or
chopper, overstress on replacement transistors is always a possibility if all
defective components have not be identified.

Therefore, using a part with better specifications may save you in the long
run by reducing the number of expensive blown parts.  Once all other problems
have been located and repaired, the proper part can be installed.

However, this is not always going to work.  In a TV and especially a high
performance monitor, the HOT may be closely matched to the drive and output
components of the deflection circuits.  Putting in one with higher Vce, I,
or P specifications may result in overheating and failure due to lower Hfe.

Where possible, a series load like a light bulb can be used limit the maximum
current to the device and will allow you to power the equipment while checking
for other faults.  Some designs, unfortunately, will not start up under these
conditions.  In such cases, substituting a 'better' device may be the best
choice for testing.

(From: Glenn Allen (glenn@manawatu.gen.nz)).

I been repairing SMPS of all types but when I started on those using MOSFETs
I was blowning a few of them when replaced because something else was faulty.

Ever since I have been using a BUZ355 on a heat sink I haven't blown it.  It
is rated at 800 V, 6 A, and 220 W. it is a TO218 case bigger than a T0220.
It seems the higher ratings allows you to do repair where as a something like 
a 2SK1117 or MTP6N60 will just blow.

  14.12) Testing of replacement HOTs

The following is useful both to confirm that a substitute replacement HOT is
suitable and that no other circuit problems are still present.  However,
single scan line anomalies (particularly when changing channels and/or where
reception is poor with a TV or when switching scan rates and/or when no or
incorrect sync is present with a monitor) resulting in excessive voltage
across the HOT and instant failure are still possible and will not result
in an HOT running excessively hot.

(From: Raymond Carlsen (rrcc@u.washington.edu)).

After installing a replacement HOT in a TV set or monitor, I like to check the
temperature for awhile to make sure the substitute is a good match and that
there are no other problems such as a weak H drive signal. The input current
is just not a good enough indicator.  I have been using a WCF (well calibrated
finger) for years. For me, the rule of thumb, quite literally, is: if you can
not hold your finger on it, it's running too hot, and will probably fail
prematurely. Touching the case of the transistor or heat sink is tricky....

Metal case transistors will be connected to the collector and have a healthy
pulse (>1,200 V peak!) and even with plastic case tab transistors, the tab will
be at this potential.  It is best to do this only after the power is off and
the B+ has discharged.  In addition, the HOT may be hot enough to burn you. 

A better method is the use of an indoor/outdoor thermometer. I bought one
recently from Radio Shack for about $15 (63-1009). It has a plastic 'probe' on
the end of a 10' cable as the outdoor sensor. With a large alligator clip, I
just clamp the sensor to the heat sink near the transistor and set up the
digital display near the TV set to monitor the temperature. The last TV I used
it on was a 27" Sanyo that had a shorted H.  output and an open B+ resistor.
Replacement parts brought the set back to life and the flyback pulse looked
OK, but the transistor was getting hot within 5 minutes... up to 130 degrees
before I shut it down and started looking for the cause. I found a 1 uF 160
volt cap in the driver circuit that was open. After replacing the cap, I
fired up the set again and monitored the heat sink as before. This time, the
temperature slowly rose to about 115 degrees and stayed there. I ran the set
all day and noticed little variation in the measurement. Test equipment doesn't
have to cost a fortune. 

  14.13) Removing and replacing the deflection yoke

Should you need to remove the deflection yoke on a color CRT, some basic
considerations are advised both to minimize the needed purity and convergence
adjustments after replacement as well as to prevent an unfortunate accident.

The position and orientation of the yoke (including pitch and yaw) and magnet
assembly (purity and static convergence rings, if used) are critical.  Use
paint or White-Out(tm) to put a stripe across all of the magnet rings so you
will know their exact positions should they accidentally shift later.  If there
are rubber wedges between the yoke and the funnel of the tube, assure that they
are secure.  Tape them to be doubly sure as adhesive on old tape dries up with
age and heat and becomes useless.  This will avoid the need for unecessary
dynamic convergence adjustments after reassembly.

The neck is the most fragile part of the CRT so do not apply any serious
side-ways force and take care not to bend any of the pins when removing and
replacing the CRT socket.

The yoke and purity/static convergence assemblies will be clamped and possibly
glued as well.  However, the adhesive will probably be easily accessible - big
globs of stuff like hot melt glue and/or RTV silicone.  Carefully free the
adhesive from the glass neck of the CRT.  Loosen the clamps and gently wiggle
the magnets and yoke off the neck.  They may appear stuck from age and heat
but should yield with gently persuasion.

Once the yoke is replaced, some fine adjustments of the picture rotation,
purity, and static and dynamic convergence may be needed but hopefully with
your most excellent diagrams, these will be minimal.

Similar comments apply for monochrome CRTs but there are far fewer issues as
the yoke is positioned firmly against the funnel of the CRT and rotation and
centering are usually the only adjustments.  However, there may be magnets
located on swivels or glued to strategic locations on the CRT envelope to
correct for geometric distortion.

  14.14) Swapping of deflection yokes

This should work with identical TVs or monitors.  Your mileage will vary if
you are attempting a swap between monitors with similar specifications.
Chances of success for monitors with widely different screen sizes or scan
rate specifications is close to zero.

One indication of compatibility problems would be major differences in
resistance readings for the corresponding yoke windings, CRT HV and other
bias levels, etc.

Before you do the transplant, see the section: "Removing and replacing the deflection yoke" for procedures and precautions to minimize problems in

Make a precise diagram of everything you do.

Keep the purity/static convergence magnet assembly with the original CRT if
possible and install it in the same or as nearly the same position as possible
when you replace it.

Once you are sure of the connections, power it up carefully - there is no
assurance that your yokes are compatible.  A yoke with a much lower resistance
or inductance than the original may overstress components in the power supply.

You will then need to go through all the adjustments starting with purity
and convergence.

  14.15) Swapping of non-identical CRTs

Given the problems of just replacing a CRT with an identical new one, it isn't
surprising that attempting to substitute a CRT which is not the same type will
result in difficulties - to say the least.  Obviously, the closer in size,
scan rate (for monitors), and deflection angle, the more likely the chances
of success.  Where the alternative is to junk the TV or monitor, it may be
worth a shot - and you may get lucky!

It may be best to transfer as much as possible with the CRT - yoke and purity
and convergence magnets.  The connectors to the yoke may need to be changed
but this may be the least of your problems.  Difference in yoke impedance and
other characteristics may result in anything from incorrect size to a truly
spectacular melt-down!  The latter is much more likely with SVGA monitors
compared to similar size/deflection angle TVs.

Where the neck size is the same, the yoke can be moved from one CRT to the
other but you will have to do a complete purity and convergence set up and
even then you may have uncorrectable convergence errors.  See the section:
"Swapping of deflection yokes".

(From: J. G. Simpson (ccjgs@cse.bris.ac.uk)).

Monitors are generally designed by choosing a CRT, then the EHT, then designing
a yoke to scan the CRT, then designing a driver circuit to drive the yoke.

In a CRT test lab it's common to have variable supplies for EHT and other
voltages, a small selection of yokes, and variable amplitude drive circuits.

EHT affects scan sensitivity, brightness, spot size.  You can't get high
brightness and small spot size on a large monitor with 3 KV of EHT.  Virtually
every variable has some effect on convergence.  Spot size is important, in as
much as you want most of it on the phosphor and not the shadow mask.

Provided the neck size is the same you can swap tubes in yokes but don't expect
it to work very well.  Different tube manufacturers may use radically different
gun structures.  A given yoke and its driver may give underscan or overscan and
it's pretty well certain that convergence will be way off.

The military spends a small fortune on trying to get the drop into the yoke and
it flies with no adjustment or convergence CRT.  For the rest of us swapping a
CRT is a pain in the butt.

  14.16) Decayed glue in electronic equipment

Larger components like electrolytic capacitors are often secured to the
circuit board with some sort of adhesive.  Originally, it is white and
inert.  However, with heat and age, some types decay to a brown, conductive
and/or corrosive material which can cause all sorts of problems including
the creation of high leakage paths or dead shorts and eating away at nearby
wiring traces.

The bottom line: Most of the time, this stuff serves no essential purpose
anyhow and should be removed.  A non-corrosive RTV or hot-melt glue can be
used in its place if structural support is needed.

  14.17) Repair parts sources

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 modern 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 flyback transformers or
degauss Posistors.

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.           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.

  14.18) Sources for adapters and cables

* Office and computer supply companies like Inmac and Global may have some very
  common types like VGA switch boxes and extension cables - of unknown quality.

However, there are companies specializing in cables for computers, video, and
communications.  For example:

* Black Box Corporation, phone: 412-746-5500, Fax: 412-746-0746,
  Web: http://www.blackbox.com/.

  Check out their on-line catalog and other information.

  14.19) Monitor replacement cables

Computer Component Source (see the section: "Repair parts sources" is one
supplier of replacement cables.

A&G Computerware claims to have replacement cables for over 800 models covering
119 monitor manufacturers.  They also offer to replicate an existing cable if
they don't have what you need.  I have no idea of pricing.

* A+G Computerware
  D10 Commericlal Avenue, Sanley Green Industrial Estate
  Cheadle Hulme, Cheshire SK8 6QH, United Kingdom
  Phone:+44 0161 486 0032
  Fax: +44 0161 486 1197
  Web: http://www.monitor-cables.com/

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