Notes On The Troubleshooting And Repair Of Television Sets


[Document Version: 2.62] [Last Updated: 05/25/1998]

Chapter 15) About the Author & Copyright

Notes On The Troubleshooting And Repair Of Television Sets

Author: Samuel M. Goldwasser
Corrections/suggestions: | Email

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:

  1. This notice is included in its entirety at the beginning.
  2. There is no charge except to cover the costs of copying.

Chapter 16) Introduction

  16.1) Television at the crossroads

Television in substantially its present form has been with us for nearly
50 years.  It is a tribute to the National Television Standards Committee
(NTSC) that the color television standards agreed upon in the early 1950s
have performed remarkably well making quite efficient use of valuable radio
spectrum space and the psychovisual characteristics of the human eye-brain
system.  However, HDTV (High Definition TV) will supplant and ultimately
replace the current standards.  We will all come to expect its superior
resolution, freedom from noise and ghosting, and pure CD sound.  Yet, the
perceived quality of TV broadcasts and cable will never likely be the major
issue with most consumers.  Content will continue to be the biggest problem.

It is likely that in roughly 15 years, HDTV - digitally processed and
transmitted as 1s and 0s - will completely replace the current system.
Acceptance in the marketplace is by no means assured but with the merging
of TV and computers - with the Internet as a driving force - it would seem
that the days of the stand-alone analog TV set are numbered.

  16.2) Television receiver fundamentals

The basic color television receiver must perform the same functions today as
40 years ago.  (Since B/W is a subset of the color standard, most references
in this document will be for color except as noted).  A studio video monitor 
includes all of the functions of a television receiver except the tuner
and IF (which rarely fail except for bad connections or perhaps lightning
strikes to the antenna or cable connection).  Therefore most
of the repair information in this document is applicable to both TVs and
studio monitors.  Modern computer monitors share many similarities with
TVs but the multisync and high scan rate deflection circuitry and more
sophisticated power supplies complicates their servicing.

As of this writing, all but the smallest TVs are based on the Cathode
Ray Tube (CRT) as the display device.  Tiny pocket sets, camcorder
viewfinders, and the like have started using LCD (Liquid Crystal Display)
panels but these are still inferior to the CRT for real time video.
There has always been talk of 'the picture on the wall' display.  While
we are closer than ever to this possibility, I believe that mass production
of an affordable wall mural TV screen is still decades away.  The reason
is simple economics - it is really hard to beat the simplicity of the
shadow mask CRT.  For example, a decent quality active matrix color LCD
panel may add $1000 to the cost of a notebook computer compared to $200
for a VGA monitor.  More of these panels go in the dumpster than make it
to product do to manufacturing imperfections.

Projection - large screen - TVs may, on the other hand, be able to take
advantage of a novel development in integrated micromachining - the
Texas Instruments Inc. Digital Micromirror Device (DMD).  This is basically
an integrated circuit with a tiltable micromirror for each pixel fabricated
on top of a static memory - RAM - cell.  This technology would
permit nearly any size projection display to be produced and would
therefore be applicable to HDTV.  Since it is a reflective device, the
light source can be as bright as needed.  However, this is still not
a commercial product but stay tuned.

  16.3) TV repair

Unlike VCRs or CD players where any disasters are likely to only affect
your pocketbook, TVs can be dangerous.  Read, understand, and follow the
set of safety guidelines provided later in this section whenever working
on TVs, monitors, or other similar high voltage equipment.

If you do go inside, beware: line voltage (on large caps) and high voltage
(on CRT) for long after the plug is pulled.  There is the added danger of
CRT implosion for carelessly dropped tools and often sharp sheetmetal
shields which can injure if you should have a reflex reaction upon touching
something you should not touch.  In inside of a TV or monitor is no place
for the careless or naive.

Having said that, a basic knowledge of how a TV set works and what can
go wrong can be of great value even if you do not attempt the repair yourself. 
It will enable you to intelligently deal with the service technician.  You
will be more likely to be able to recognize if you are being taken for a ride
by a dishonest or just plain incompetent repair center.  For example, a
faulty picture tube CANNOT be the cause of a color television only displaying
shows in black-and-white.  The majority of consumers probably do not know even
this simple fact.  Such a problem is usually due to a bad capacitor or other
10 cent part.

This document will provide you with the knowledge to deal with a large
percentage of the problems you are likely to encounter with your TVs.
It will enable you to diagnose problems and in many cases, correct them
as well.  With minor exceptions, specific manufacturers and models
will not be covered as there are so many variations that such a treatment would
require a huge and very detailed text.  Rather, the most common problems
will be addressed and enough basic principles of operation will be provided
to enable you to narrow the problem down and likely determine a course of
action for repair.  In many cases, you will be able to do what is required
for a fraction of the cost that would be charged by a repair center.

Should you still not be able to find a solution, you will have learned a great
deal and be able to ask appropriate questions and supply relevant information
if you decide to post to sci.electronics.repair.  It will also be easier to do
further research using a repair text such as the ones listed at the end of
this document.  In any case, you will have the satisfaction of knowing you
did as much as you could before taking it in for professional repair.
With your new-found knowledge, you will have the upper hand and will not
easily be snowed by a dishonest or incompetent technician.

  16.4) Repair or replace

If you need to send or take the TV to a service center, the repair
could easily exceed half the cost of a new TV.  Service centers
may charge up to $50 or more for providing an initial estimate of repair
costs but this will usually be credited toward the total cost of the repair
(of course, they may just jack this up to compensate for their bench time).

TV prices have been dropping almost as fast as PC prices.  Therefore, paying
such prices for repair just may not make sense.  Except for picture tube
problems, most TV faults can be corrected without expensive parts, however.
Keeping a 5 year old TV alive may be well worthwhile as basic TV performance
and important features have not changed in a long time.

If you can do the repairs yourself, the equation changes dramatically as
your parts costs will be 1/2 to 1/4 of what a professional will charge
and of course your time is free.  The educational aspects may also be
appealing.  You will learn a lot in the process.  Thus, it may make sense
to repair that old clunker for your game room or beach house.  (I would
suggest the kid's room but most TV watching just rots the brain anyhow so
a broken TV may be more worthwhile educationally than one that works.)

Chapter 17) TV Receivers 101

  17.1) Subsystems of a television set

A TV set includes the following functional blocks:

1.  Low voltage power supply (some may also be part of (2)).  Most of the lower
    voltages used in the TV may be derived from the horizontal deflection
    circuits.  Sometimes, there is a separate switching power supply but
    this would be the exception.  Rectifier/filter capacitor/regulator from AC
    line provides the B+ to the switching power supply or horizontal
    deflection system.  Degauss operates off of the line whenever power is
    turned on (after having been off for a few minutes) to demagnetize the CRT.

2.  Horizontal deflection.  These circuits provide the waveforms needed to
    sweep the electron beam in the CRT across and back some 15,734 times
    per second (for NTSC).  The horizontal sync pulse from the sync separator
    locks the horizontal deflection to the video signal.

3.  Vertical deflection.  These circuits provide the waveforms needed to
    sweep the electron beam in the CRT from top to bottom and back 60 times
    per second (for NTSC).  The vertical sync pulse from the sync separator
    locks the vertical deflection to the video signal.

4.  CRT high voltage (also part of (2)).  A modern color CRT requires
    up to 30 KV for a crisp bright picture.  Rather than having a totally
    separate power supply, nearly every TV on the planet derives the HV
    (as well as many other voltages) from the horizontal deflection using
    a special transformer called a 'flyback' or 'Line OutPut Transformer (LOPT)
    for those of you on the other side of the lake.

5.  Tuner, IF, AGC, video and audio demodulators.  Input is the antenna or
    cable signal and output are baseband video and audio signals.  There is
    usually someplace inside the TV where line level video and audio are
    present but it may not be accessible from the outside of the cabinet
    unless you paid for the more expensive model with the A/V option. 
    Very often, the tuner is a shielded metal box positioned on the bottom
    right (as viewed from the front) separate from the main circuit board.
    Sometimes it is on the main circuit board.  The IF section may be in
    either place.

    On older or cheap TVs with a knob tuner, this is usually mounted to the
    front panel by itself.  There are usually separate boxes for the VHF and
    UHF tuners.

6.  Chroma demodulator.  Input is the baseband video signal.  Outputs are
    the individual signals for the red, green, and blue video to the CRT.

7.  Video drivers (RGB).  These are almost always located on a little
    circuit board plugged directly onto the neck of the CRT.  They boost
    the output of the chroma demodulator to the hundred volts or so needed
    to drive the cathodes of the CRT.

8.  Sync separator.  Input is baseband video.  Output is horizontal and
    vertical sync pulses to control the deflection circuits.

9.  Audio amplifier/output.  The line level audio is amplified to drive
    a set of speakers.  If this is a stereo TV, then these circuits must
    also perform the stereo demultiplexing.

10. System control.  Most modern TVs actually use a microcontroller - a fixed
    program microcomputer to perform all user interface and control functions
    from the front panel and remote control.  These are becoming increasingly
    sophisticated.  However, they do not fail often.  Older TVs use a bunch
    of knobs and switches and these are prone to wear and dirt.

Most problems occur in the horizontal deflection and power supply sections.
These run at relatively high power levels and some components run hot.
The high voltage section is prone to breakdown and arcing as a result
of hairline cracks, humidity, dirt, etc.

The tuner components are usually quite reliable unless the antenna experiences
a lightning strike.  However, it seems that even after 20+ years of
solid state TVs, manufacturers still cannot reliably solder the tuner
connectors and shields so that bad solder connections in these areas are
common even in new sets.

  17.2) Why projection TVs are not just normal TVs in big boxes

In order to achieve the necessary brightness with a large display format,
three separate monochrome CRTs are used with optics to combine the three images
properly at the screen.  This creates an entire set of additional problems in

(From: Jeroen H. Stessen (Jeroen.Stessen@ehv.ce.philips.com)). 

The average projection TV has about twice as many parts as its direct-view
counterpart.  Some of the extra parts are essential for projection because CRT
projection tubes require dynamic convergence. The other extra parts have to do
with the fact that a more expensive TV also should have some extra features,
like Dolby ProLogic sound, a satellite tuner and such.

Generally, the electronics are based on a standard chassis that is also used
for direct-view CRT television. Even the deflection circuits require minor
adaptations at most. The high-voltage circuit is different because the EHT,
focus and G2 voltages must be distributed over 3 CRTs. So this requires a
special high-voltage part, which also includes an EHT capacitor and bleeder.

There will be 3 CRT panels with video amplifiers.  Because of the extremely
high brightness, projection tubes will burn the phosphor screen immediately in
fault conditions so a protection circuit is essential.

And last but certainly not least, there is the dynamic convergence panel. The
heart is a waveform generator IC, often of a Japanese brand but nowadays
there's also a digital variant by Philips. The old-fashioned way requires many
many potentiometers to program the waveforms. Then there's 5 or 6 convergence
amplifiers and a corresponding extra power supply. And usually this is where
the single deflection circuits are distributed to the 3 CRTs.  At the same
time the deflection currents are sensed for the protection circuits.

Designing a PTV from a DVTV requires several man-years of work. In the
factory, a special corner is devoted to the assembly. There you'll find
specially educated people and the speed of the assembly line is a lot lower
than usual. It requires many more adjustments, e.g. 3 optical and 3 electrical
focus adjustments and then convergence.

  17.3) For more information on TV technology

The books listed in the section: "Suggested references" include additional
information on the theory and implementation of the technology of television
standards and TV receivers.

  17.4) On-line tech-tips databases

A number of organizations have compiled databases covering thousands of common
problems with VCRs, TVs, computer monitors, and other electronics equipment.
Most charge for their information but a few, accessible via the Internet, are
either free or have a very minimal monthly or per-case fee.  In other cases, a
limited but still useful subset of the for-fee database is freely available.

A tech-tips database is a collection of problems and solutions accumulated by
the organization providing the information or other sources based on actual
repair experiences and case histories.  Since the identical failures often
occur at some point in a large percentage of a given model or product line,
checking out a tech-tips database may quickly identify your problem and

In that case, you can greatly simplify your troubleshooting or at least
confirm a diagnosis before ordering parts.  My only reservation with respect
to tech-tips databases in general - this has nothing to do with any one in
particular - is that symptoms can sometimes be deceiving and a solution that
works in one instance may not apply to your specific problem.  Therefore,
an understanding of the hows and whys of the equipment along with some good
old fashioned testing is highly desirable to minimize the risk of replacing
parts that turn out not to be bad.

The other disadvantage - at least from one point of view - is that you do not
learn much by just following a procedure developed by others.  There is no
explanation of how the original diagnosis was determined or what may have
caused the failure in the first place.  Nor is there likely to be any list
of other components that may have been affected by overstress and may fail
in the future.  Replacing Q701 and C725 may get your equipment going again
but this will not help you to repair a different model in the future.

Having said that, here are three tech-tips sites for computer monitors, TVs,
and VCRs:

* http://www.anatekcorp.com/techforum.htm            (currently free).
* http://www.repairworld.com/                        ($8/month).
* http://elmswood.guernsey.net/                      (Free, very limited).
* http://ramiga.rnet.cgl.com/electronics/info.html   (free large text files).

This one has quite a bit of info for just TVs (at present):

* http://home.inreach.com/ba501/Tech_Tip_Page.htm

These types of sites seem to come and go so it is worth checking them out from
time-to-time even if you don't have a pressing need.  If possible, download
and archive any useful information for use on a rainy day in the future.

Chapter 18) CRT Basics

Note: Most of the information on TV and monitor CRT construction, operation,
interference and other problems. has been moved to the document: "TV and
Monitor CRT (Picture Tube) Information".  The following is just a brief
introduction with instructions on degaussing.

  18.1) Color CRTs - shadow masks and aperture grills

All color CRTs utilize a shadow mask or aperture grill a fraction of an
inch (1/2" typical) behind the phosphor screen to direct the electron beams 
for the red, green, and blue video signals to the proper phosphor dots.
Since the electron beams for the R, G, and B phosphors originate from
slightly different positions (individual electron guns for each)
and thus arrive at slightly different angles, only the proper phosphors
are excited when the purity is properly adjusted and the necessary
magnetic field free region is maintained inside the CRT.  Note that
purity determines that the correct video signal excites the
proper color while convergence determines the geometric
alignment of the 3 colors.  Both are affected by magnetic fields.
Bad purity results in mottled or incorrect colors.  Bad convergence
results in color fringing at edges of characters or graphics.

The shadow mask consists of a thin steel or InVar (a ferrous alloy)
with a fine array of holes - one for each trio of phosphor
dots - positioned about 1/2 inch behind the surface of the phosphor
screen.  With most CRTs, the phosphors are arranged in triangular
formations called triads with each of the color dots at the apex
of the triangle.  With many TVs and some monitors, they are
arranged as vertical slots with the phosphors for the 3 colors
next to one another.

An aperture grille, used exclusively in Sony Trinitrons (and now
their clones as well), replaces the shadow mask with an array of finely
tensioned vertical wires.  Along with other characteristics of the
aperture grille approach, this permits a somewhat higher possible
brightness to be achieved and is more immune to other problems like
line induced moire and purity changes due to local heating causing
distortion of the shadow mask.

However, there are some disadvantages of the aperture grille design:

* weight - a heavy support structure must be provided for the tensioned
  wires (like a piano frame).

* price (proportional to weight).

* always a cylindrical screen (this may be considered an advantage
  depending on your preference.

* visible stabilizing wires which may be objectionable or unacceptable
  for certain applications.

Apparently, there is no known way around the need to keep the fine
wires from vibrating or changing position due to mechanical shock
in high resolution tubes and thus all Trinitron monitors require
1, 2, or 3 stabilizing wires (depending on tube size) across the 
screen which can be see as very fine lines on bright images.  Some
people find these wires to be objectionable and for some critical
applications, they may be unacceptable (e.g., medical diagnosis).

  18.2) Degaussing (demagnetizing) a CRT

Degaussing may be required if there are color purity problems with the
display.  On rare occasions, there may be geometric distortion caused
by magnetic fields as well without color problems.  The CRT can get

* if the TV or monitor is moved or even just rotated.

* if there has been a lightning strike nearby.  A friend of mine
  had a lightning strike near his house which produced all of the
  effects of the EMP from a nuclear bomb.

* If a permanent magnet was brought near the screen (e.g., kid's
  magnet or megawatt stereo speakers).

* If some piece of electrical or electronic equipment with unshielded
  magnetic fields is in the vicinity of the TV or monitor.  

Degaussing should be the first thing attempted whenever color
purity problems are detected.  As noted below, first try the
internal degauss circuits of the TV or monitor by power cycling a few
times (on for a minute, off for 30 minutes, on for a minute, etc.)
If this does not help or does not completely cure the problem,
then you can try manually degaussing.

Commercial CRT Degaussers are available from parts distributors
like MCM Electronics and consist of a hundred or so turns of magnet wire
in a 6-12 inch coil.  They include a line cord and momentary switch. You 
flip on the switch, and bring the coil to within several inches of the 
screen face. Then you slowly draw the center of the coil toward one edge 
of the screen and trace the perimeter of the screen face. Then return to 
the original position of the coil being flat against the center of the 
screen.  Next, slowly decrease the field to zero by backing straight up 
across the room as you hold the coil. When you are farther than 5 feet 
away you can release the line switch. 

The key word here is ** slow **.  Go too fast and you will freeze the
instantaneous intensity of the 50/60 Hz AC magnetic field variation
into the ferrous components of the CRT and may make the problem worse.

It looks really cool to do this while the CRT is powered.  The kids will
love the color effects.

Bulk tape erasers, tape head degaussers, open frame transformers, and the
"ass-end" of a weller soldering gun can be used as CRT demagnetizers but
it just takes a little longer. (Be careful not to scratch the screen
face with anything sharp.) It is imperative to have the CRT running when
using these whimpier approaches, so that you can see where there are 
still impurities. Never release the power switch until you're 4 or 5 
feet away from the screen or you'll have to start over.

I've never known of anything being damaged by excess manual degaussing
though I would recommend keeping really powerful bulk tape erasers turned
degaussers a couple of inches from the CRT.

If an AC degaussing coil or substitute is unavailable, I have even done
degaussed with a permanent magnet but this is not recommended since it is more
likely to make the problem worse than better.  However, if the display
is unusable as is, then using a small magnet can do no harm. (Don't use
a 20 pound speaker or magnetron magnet as you may rip the shadow mask right
out of the CRT - well at least distort it beyond repair.  What I have in
mind is something about as powerful as a refrigerator magnet.)

Keep degaussing fields away from magnetic media.  It is a good idea to
avoid degaussing in a room with floppies or back-up tapes.  When removing
media from a room  remember to check desk drawers and manuals for stray
floppies, too. 

It is unlikely that you could actually affect magnetic media but better
safe than sorry.  Of the devices mentioned above, only a bulk eraser or
strong permanent magnet are likely to have any effect - and then only when
at extremely close range (direct contact with media container).

All color CRTs include a built-in degaussing coil wrapped around the 
perimeter of the CRT face. These are activated each time the CRT is 
powered up cold by a 3 terminal thermister device or other control
circuitry.  This is why it is often suggested that color purity problems
may go away "in a few days".  It isn't a matter of time; it's the number
of cold power ups that causes it.  It takes about 15 minutes of the power
being off for each cool down cycle. These built-in coils with thermal
control are never as effective as external coils.

See the document: " TV and Monitor CRT (Picture Tube) Information" for
some additional discussion of degaussing tools, techniques, and cautions.

Chapter 19) TV Placement And Preventive Maintenance

  19.1) General TV placement considerations

Proper care of a TV does not require much.  Following the recommendations
below will assure long life and minimize repairs:

* Subdued lighting is preferred for best viewing conditions  but I will not
  attempt to tell you how to arrange your room!

* Locate the TV away from extremes of hot and cold.  Avoid damp or dusty
  locations if possible.  (Right you say, keep dreaming!)

* Allow adequate ventilation - TVs use more power than any of your other
  A/V components.  Heat buildup takes its toll on electronic components.
  Leave at least 3 inches on top and sides for air circulation if the
  entertainment center does not have a wide open back panel.  Do not
  pile other components like VCRs on top of the TV if possible (see below).

* Do not put anything on top of the TV that might block the ventilation
  grill in the rear or top of the cover.  This is the major avenue for
  the convection needed to cool internal components.

* If possible, locate the VCR away from the TV.  Some VCRs are particularly
  sensitive to interference from the TV's circuitry and while this won't
  usually damage anything, it may make for less than optimal performance
  due to RF interference.  The reverse is sometimes true as well.

  In addition, modern VCRs are NOT built like the Brooklyn Bridge!  The
  weight of a TV or stereo components could affect the VCR mechanically,
  messing up tape path alignment or worse.

* If possible, locate your computer monitor away from the TV.  Interaction
  of the electromagnetic fields of the deflection systems may result in
  one or both displays jiggling, wiggling, or vibrating.

* Locate loudspeakers and other sources of magnetic fields at least a couple
  of feet from the TV.  This will minimize the possibility of color purity
  or geometry problems.

* Make sure all input-output video and audio connections are tight and
  secure to minimize intermittent or noisy pictures and sound.  Use proper
  high quality cable only long enough to make connections conveniently.

* Finally, store video cassettes well away from all electronic equipment
  including and especially loudspeakers.  Heat and magnetic fields will
  rapidly turn your priceless video collection into so much trash.  The
  operation of the TV depends on magnetic fields for beam deflection. 
  Enough said.

  19.2) Preventive maintenance

Preventive maintenance for a TV is pretty simple - just keep the case clean
and free of obstructions.  Clean the screen with a soft cloth just dampened
with water and at most, mild detergent.  DO NOT use anything so wet that
liquid may seep inside of the set around the edge of the picture tube - you
could end up with a very expensive repair bill when the liquid shorts out
the main circuit board lurking just below.  If the set has a protective
flat glass faceplate, there is usually an easy way (on newer sets with this
type of protection) of removing it to get at the inner face of the CRT.  Clean
both the CRT and the protective glass with a soft damp cloth and dry
thoroughly.  If you have not cleaned the screen for quite a while, you will 
be amazed at the amount of black grime that collects due to the static
buildup from the high voltage CRT supply.

In really dusty situations, periodically vacuuming inside the case and the use
of contact cleaner for the controls might be a good idea but realistically,
you will not do this so don't worry about it.

  19.3) Warning about using a TV as a computer or video game display

"I remember a while back (about 10 years) most home computers used to hook up
 to televisions. I seem to remember them having some effect on the TV though.
 I think they made the TV go blurry after a while. I was just wondering what
 these computers used to do to the televisions to mess them up like that.  I
 thought a TV signal was a TV signal."

The problem was screen burn.  Since computers of that ear were mostly text
and video games tended to use fixed patterns for scenery, patterns tended
to be burned into the phosphor such that they were noticeably darker and
less sensitive in those areas.  This was exacerbated by the tendency to
run them devices at very high brightness levels.

Modern computers and video games should not be nearly as much of a risk since
the displays are so much more varied and dynamic.  Nevertheless, setting the
brightness at a moderate level would be prudent.

Chapter 20) TV Troubleshooting

  20.1) SAFETY

TVs and computer or video monitors are among the more dangerous of consumer
electronics equipment when it comes to servicing.  (Microwave ovens are
probably the most hazardous due to high voltage at high power.)

There are two areas which have particularly nasty electrical dangers: the
non-isolated line power supply and the CRT high voltage.

Major parts of nearly all modern TVs and many computer monitors are directly
connected to the AC line - there is no power transformer to provide the
essential barrier for safety and to minimize the risk of equipment damage.
In the majority of designs, the live parts of the TV or monitor are limited
to the AC input and line filter, degauss circuit, bridge rectifier and main
filter capacitor(s), low voltage (B+) regulator (if any), horizontal output
transistor and primary side of the flyback (LOPT) transformer, and parts
of the startup circuit and standby power supply.  The flyback generates most
of the other voltages used in the unit and provides an isolation barrier so
that the signal circuits are not line connected and safer.

Since a bridge rectifier is generally used in the power supply, both
directions of the polarized plug result in dangerous conditions and an
isolation transformer really should be used - to protect you, your test
equipment, and the TV, from serious damage.  Some TVs do not have any
isolation barrier whatsoever - the entire chassis is live.  These are
particularly nasty.

The high voltage to the CRT, while 200 times greater than the line input,
is not nearly as dangerous for several reasons.  First, it is present in a
very limited area of the TV or monitor - from the output of the flyback
to the CRT anode via the fat red wire and suction cup connector.  If you
don't need to remove the mainboard or replace the flyback or CRT, then
leave it alone and it should not bite.  Furthermore, while the shock from
the HV can be quite painful due to the capacitance of the CRT envelope, it
is not nearly as likely to be lethal since the current available from the
line connected power supply is much greater.

  20.2) Safety guidelines

These guidelines are to protect you from potentially deadly electrical shock
hazards as well as the equipment from accidental damage.

Note that the danger to you is not only in your body providing a conducting
path, particularly through your heart.  Any involuntary muscle contractions
caused by a shock, while perhaps harmless in themselves, may cause collateral
damage - there are many sharp edges inside this type of equipment as well as
other electrically live parts you may contact accidentally.

The purpose of this set of guidelines is not to frighten you but rather to
make you aware of the appropriate precautions.  Repair of TVs, monitors,
microwave ovens, and other consumer and industrial equipment can be both
rewarding and economical.  Just be sure that it is also safe!

* Don't work alone - in the event of an emergency another person's presence
  may be essential.

* Always keep one hand in your pocket when anywhere around a powered
  line-connected or high voltage system.

* Wear rubber bottom shoes or sneakers.

* Don't wear any jewelry or other articles that could accidentally contact
  circuitry and conduct current, or get caught in moving parts.

* Set up your work area away from possible grounds that you may accidentally

* Know your equipment: TVs and monitors may use parts of the metal chassis
  as ground return yet the chassis may be electrically live with respect to the
  earth ground of the AC line.  Microwave ovens use the chassis as ground
  return for the high voltage.  In addition, do not assume that the chassis
  is a suitable ground for your test equipment!

* If circuit boards need to be removed from their mountings, put insulating
  material between the boards and anything they may short to.  Hold them in
  place with string or electrical tape.  Prop them up with insulation sticks -
  plastic or wood.

* If you need to probe, solder, or otherwise touch circuits with power off,
  discharge (across) large power supply filter capacitors with a 2 W or greater
  resistor of 100 to 500 ohms/V approximate value (e.g., for a 200 V capacitor,
  use a 20K to 100K ohm resistor).  Monitor while discharging and verify that
  there is no residual charge with a suitable voltmeter.  In a TV or monitor,
  if you are removing the high voltage connection to the CRT (to replace the
  flyback transformer for example) first discharge the CRT contact (under the
  insulating cup at the end of the fat red wire).  Use a 1M to 10M ohm 5 W or
  greater wattage (for its voltage holdoff capability, not power dissipation)
  resistor on the end of an insulating stick or the probe of a high voltage
  meter.  Discharge to the metal frame which is connected to the outside of
  the CRT.

* For TVs and monitors in particular, there is the additional danger of
  CRT implosion - take care not to bang the CRT envelope with your tools.
  An implosion will scatter shards of glass at high velocity in every
  direction.  There are several tons of force attempting to crush the typical
  CRT.  While implosion is not really likely even with modest abuse, why take
  chances?  However, the CRT neck is relatively thin and fragile and breaking
  it would be very embarrassing and costly.  Always wear eye protection when
  working around the back side of a CRT.

* Connect/disconnect any test leads with the equipment unpowered and
  unplugged. Use clip leads or solder temporary wires to reach cramped
  locations or difficult to access locations.

* If you must probe live, put electrical tape over all but the last 1/16"
  of the test probes to avoid the possibility of an accidental short which
  could cause damage to various components.  Clip the reference end of the
  meter or scope to the appropriate ground return so that you need to only
  probe with one hand.

* Perform as many tests as possible with power off and the equipment unplugged.
  For example, the semiconductors in the power supply section of a TV or
  monitor can be tested for short circuits with an ohmmeter.

* Use an isolation transformer if there is any chance of contacting line
  connected circuits.  A Variac(tm) is not an isolation transformer!
  The use of a GFCI (Ground Fault Circuit Interrupter) protected outlet is a
  good idea but will not protect you from shock from many points in a line
  connected TV or monitor, or the high voltage side of a microwave oven, for
  example.  (Note however, that, a GFCI may nuisanse trip at power-on or at
  other random times due to leakage paths (like your scope probe ground) or
  the highly capacitive or inductive input characteristics of line powered
  equipment.)  A fuse or circuit breaker is too slow and insensitive to provide
  any protection for you or in many cases, your equipment.  However, these
  devices may save your scope probe ground wire should you accidentally connect
  it to a live chassis.

* Don't attempt repair work when you are tired.  Not only will you be more
  careless, but your primary diagnostic tool - deductive reasoning - will
  not be operating at full capacity.

* Finally, never assume anything without checking it out for yourself!
  Don't take shortcuts!

Go to [Next] segment

Go to [Table 'O Contents]

Written by Samuel M. Goldwasser. | [mailto]. The most recent version is available on the WWW server http://www.repairfaq.org/ [Copyright] [Disclaimer]