Notes On The Troubleshooting And Repair Of Television Sets


  25.33) Focus drift with warmup

This could be due to a problem with the focus voltage power supply, components
on the CRT neck board, or a tired worn CRT.

Focus is controlled by a voltage of 2-8 KV DC usually derived from the flyback
transformer and includes some resistors and capacitors.  One of these could
be changing value as it warms up. (assuming nothing else changes significantly
as the unit warms up - e.g., the brightness does not decrease.)

Focus voltage is derived from a subset of the high voltage winding on the
flyback using a resistive voltage divider which includes the focus pot.
These are extremely high value resistors - 200 M ohm is common - and so
leakage of any kind can reduce or increase the focus voltage.  All other
things being ok - i.e., the picture is otherwise fine - I would suspect this
type of failure rather than the CRT.

The connection to the CRT is usually a separate wire running from the flyback
or its neighborhood to the CRT neck board.  Look for components in this
general area.  Use cold spray or a heat gun to isolate the one that is
drifting.  If you have access to a high voltage meter, you should be able
to see the voltage change as the TV or monitor warms up - and when you cool
the faulty part.  If it is in the flyback, then sometimes the part with the
adjustments clips off and can be repaired or cleaned.  Most often, you will
need to replace the flyback as a unit.

* If the optimal adjustment point of the focus control doesn't change that
  much but the best focus is simply not as good as it should be, the CRT is
  probably the problem.  However, if the optimal point produces acceptable
  focus but it changes (and possibly moves off of one end of the adjustment
  knob range) as the unit warms up, the flyback or one of the components on
  the CRT neck board are likely drifting.

* If you have a high voltage meter, you can measure the focus voltage to
  determine if it is being changed by the focus pot and if it is in the
  ball park (2-8 KV typical).  Sometimes, the part of the flyback with the
  focus pot can be snapped off and cleaned or parts replaced but usually you
  need to replace the whole unit.  There may a capacitor or two on the PCB on
  the neck of the CRT that could have increased leakage as well thus reducing
  the focus voltage.

* To determine if the CRT is the problem, for sharp focus after the unit has
  warmed up.  Power-off for an hour or so and carefully pull the CRT neck board
  off of the CRT.  Then, power up the unit.  Let it run long enough such that
  there would have been a detectable focus drift.  Now, power-down, plug the
  CRT neck board back in, and power-up.  Watch the image as it appears on the

  - If the focus starts out fuzzy and sharpens up as the image appears and
    gradually becomes sharper as the CRT warms up the CRT is likely tired.

    The only catch here is that plugging the CRT neck board into the CRT
    results in an additional load on the flyback due to the picture beam
    current which heats it more as well.  Thus, if the problem takes a few
    minutes to appear, keep the brightness turned down except to check the
    appearance of the picture from time to time. 

    You can set the focus control for optimum when warmed up and just turn
    the TV on in well in advance of your favorite shows or add a user focus
    adjustment by drilling a hole in the plastic case for an *insulated*
    screwdriver or flyback focus knob extender :-).  The CRT may continue
    to function for quite a while so this is not impending doom.

  - If the focus is relatively stable as the image appears and increases
    in brightness *and* is about as sharp as it would be with the TV warmed
    up, the problem is most likely in the flyback.  However, also check for
    bad components or decayed (tan or brown) glue on the CRT neck board.  A
    drifting flyback will need to be replaced as it will probably get worse
    and fail completely.  Clean the surface of the circuit board and CRT
    socket in the vicinity of the focus and screen terminals and traces.
    Contamination or just dirt and grime can easily cause problems especially
    on humid days since the resistance of these circuits is extremely high
    (100s of M ohms).

  - If the focus is relatively stable as the image appears and increases
    in brightness *and* is similar to what it would be with the monitor cold,
    you have a very strange situation where some load on the high voltage
    power supply, perhaps, is causing a thermal problem.  This would be rare.

  25.34) Bad focus and adjustment changes brightness

This is the classic symptom of a short between the focus and screen
supplies - probably in focus/screen divider which is part of the flyback
or tripler.  If you have a high voltage meter, measuring the focus voltage
will show that (1) it is low and (2) it is affected by the SCREEN control
Similarly, the SCREEN voltage will be affected by the FOCUS control (which
is what is changing the brightness.

There is a slight possibility that this may be in the CRT as well.  Measure
the FOCUS and SCREEN voltage with a high voltage meter.  If they are identical
pull the plug on the CRT.  If they are now their normal values, then a
shorted CRT is a distinct possibility - see the section: "Rescuing a shorted CRT".

  25.35) Charlie's comments on focus problems

(From: Charles Godard (cgodard@iamerica.net)).

Most true focus problems that I have encountered (when the IHVT is ok) are
related to leaks or resistance on the focus output.  The diming of the screen
when the focus pot is adjusted leads me to think in terms of a leaky socket.
I'd remove the ground from the crt socket to the tube dag and see if it
sparks.  If so there may be a leak in the socket to ground.  It could also be
leaking to another pin, such as the screen grid. A rhetorical question: What
happens to the screen voltage when the focus pot is adjusted?

I have seen sockets that had no arching or other telltale signs, leak through
the plastic housing to ground out the focus voltage.

Look closely at the screen.  If the blurring is in the form of small circles,
then you have an open or hi-resistance focus electrode inside the tube.  The
circles may vary in visibility with brightness.

If you still haven't found the problem, try to confirm that this is truly a
focus problem.  Remove the crt socket and observe the hi-voltage.  If it
climbs more than about 1k, say all the way up to 25kv, then you may have a
beam current problem rather than a focus problem.  In that case re-check all
crt board voltages.

If you have done all of the above and removing the socket makes no change in
the hi-voltage, then try to determine why the hi-voltage is low.

Watch the screen as the brightness, contrast, or screen control are adjusted.
See if you can observe any signs of blooming.  When the IHVT doesn't provide
enough current to satisfy the demands of the tube for current, the the picture
tends to appear to expand like a balloon. i.e., bloom.  This can be caused by
not enough drive to the IHVT. Carefully monitor the b+ to the horizontal drive
stages to see that is is stable and correct.

  25.36) Blank picture, good channel tuning and sound

Since the tuner and sound are ok, horizontal deflection which usually
generates power for most of the set is also working.

Does 'blank picture' means a totally black screen with the brightness and 
contrast controls having no effect whatsoever?  Or, is there is no picture
but there is a raster - light on the screen?  The direction in which
troubleshooting should proceed differ significantly depending the answer.

Here are some questions:

1. As above, is there any light on the screen at any settings of the brightness
   and contrast controls, and/or when switching channels.  Can you see any
   raster scanning lines?

2. Can you hear the high pitched (15735 Hz) of the horizontal deflection?

3. Looking in the back of the set, can you see the glow of the CRT filament?

4. Do you get that static on the front of the tube that would indicate that
   there is high voltage?  Any cracking or other normal or abnormal sounds
   or smells?

Possible causes of no raster:

* No or low high voltage (low voltage, deflection, or high voltage power
  supply failure).

* Fault with other voltages like G1 or screen (G2) to CRT.

* Filament to CRT not getting powered.

* Drive to CRT bad/shut off as a result of fault elsewhere.  For example,
  failure of the vertical deflection may disable HV or blank the screem to
  protect the CRT from burn-in due to the very bright horizontal line that
  would result.  With some sets, it is possible that the X-ray protection
  circuitry will blank the screen without affecting tuning or audio.

Possible causes of no video (but a good raster): Problem in video IF, video
amplifiers, video output, cutoff due to other fault.

It could be as simple as a bad connection - try gently prodding the boards
with an insulated stick while watching the screen.  Check for loose connectors
and reseat all internal connectors.

  25.37) Purple blob - or worse

Have you tried demagnetizing it?  Try powering it off for a half hour, then
on.  Repeat a couple of times.  This should activate the internal degausser.
See the section: "Degaussing (demagnetizing) a CRT".

Is there any chance that someone waved a magnet hear the tube?  Remove it
and/or move any items like monster speakers away from the set.

Was your kid experimenting with nuclear explosives - an EMP would magnetize
the CRT.  Nearby lightning strikes may have a similar effect.

If demagnetizing does not help, then it is possible that something shifted
on the CRT - there are a variety of little magnets that are stuck on at the
time of manufacture to adjust purity.  There are also service adjustments
but it is unlikely (though not impossible) that these would have shifted
suddenly.  This may be a task for a service shop but you can try your
hand at it if you get the Sams' Photofact or service manual - don't attempt
purity adjustments without one.

If the set was dropped, then the internal shadow mask of the CRT may have
become distorted or popped loose and you now have a hundred pound paper
weight.  If the discoloration is slight, some carefully placed 'refrigerator'
magnets around the periphery of the tube might help.  See the section:
"Magnet fix for purity problems - if duct tape works, use it!"

It is even possible that this is a 'feature' complements of the manufacturer.
If certain components like transformers and loudspeakers are of inferior
design and/or are located too close to the CRT, they could have an effect
on purity.  Even if you did not notice the problem when the set was new,
it might always have been marginal and now a discoloration is visible due
to slight changes or movement of components over time.

  25.38) Color rings - bullseye pattern

This probably means the degaussing circuitry is terminating suddenly instead
of gradually as it should.  The most likely cause is a bad solder connection
to the degauss thermistor or posistor or something feeding it.

You can confirm this by manually degaussing the screen with the TV or monitor
turned on.  If the problem disappears, the above diagnosis is probably valid.
Check for bad solder connections in the vicinity of the degauss components
and AC line input.

  25.39) Magnet fix for purity problems - if duct tape works, use it!

The approach below will work for slight discoloration that cannot be eliminated
through degaussing.  However, following the procedures in the section: "CRT purity adjustment" would be the preferred solution.  On the other hand, the
magnets may be quick and easy.  And, where CRT has suffered internal distortion
or dislocation of the shadowmask, adjustments may not be enough.

In any case, first, relocate those megablaster loudspeakers and that MRI
scanner with the superconducting magnets.

The addition of some moderate strength magnets carefully placed to reduce or
eliminate purity problems due to a distorted or dislocated shadowmask may be
enough to make the TV usable - if not perfect.  The type of magnets you want
are sold as 'refrigerator magnets' and the like for sticking up notes on steel
surfaces.  These will be made of ferrite material (without any steel) and will
be disks, rectangles, flexible strips.  Experiment with placement using
masking tape to hold them in place temporarily.  Degauss periodically to
evaluate the status of your efforts.  Then, make the 'repair' permanent using
duct tape or silicone sealer or other household adhesive.

Depending on the severity of the purity problem, you may need quite a few
magnets!  However, don't get carried away and use BIG speaker or magnetron
magnets - you will make the problems worse.

Also note that unless the magnets are placed near the front of the CRT, very
significant geometric distortion of the picture will occur - which may be a
cure worse than the disease.

WARNING: Don't get carried away while positioning the magnets - you will be
near some pretty nasty voltages!

(From: Mr. Caldwell (jcaldwel@iquest.net)).

I ended up with the old 'stuck on a desert island trick':

I duck taped 2 Radio Shack magnets on the case, in such a way
as to pull the beam back.!!!!

A $2 solution to a $200 problem.  My friend is happy as heck.

RCA sells magnets to correct corner convergence, they are shaped like chevrons 
and you stick them in the 'right' spot on the rear of the CRT.

(From: Tom Sedlemyer (wesvid@gte.net)).

First set purity as best you can.

Obtain some pieces of refrigerator door magnet strips from an appliance
repair shop (they usually have some lying around).

Cut the strips into 1 inch pieces.  Place a strip as on the bell of the
picture tube as close to the yoke as possible and in line with the corner that
has the purity error.   Rotate the magnet until you correct the purity error
and tape it in place. Multiple magnet strips can be used and you may
experiment with the size of the strips for best effect.  It is very important
that the strips are positioned close to the yoke or the effect will not hold.
The only drawback to this method is some very slight distortion of the
geometry of the raster, but it beats hell out of paying for a new CRT.

  25.40) Color TV only displays one color

I assume that now you have no other colors at all - no picture and no
raster.  Let us say it is red - R.

It is probably not the CRT.  Do you have a scope?  Check for the R, G,
and B video signals at the CRT.  You will probably find no signals
for the defective colors.

This is almost certainly a chroma circuit problem as any failure of the
CRT or a video driver would cause it to lose a single color - the other
two would be ok.  Therefore, it is probably NOT the CRT or a driver on
the little board on the neck of the CRT.

Try turning up the SCREEN control to see if you can get a G and B raster
just to confirm that the CRT is ok.

Locate the video drive from the mainboard for the good and a bad color.
Interchange them and see if the problem moves.  If so, then there is
a video signal problem.  If not, it is on the little CRT board.

It could be a defective chroma IC or something else in the chroma decoder.

  25.41) Disappearing Red (or other color)

Problem: I have been given an old colour TV.  The reception is good, but very
often, when the contrast and brightness of the TV image is low (e.g. when
a night scene is shown), the red colour slowly disappears, leaving behind
the green and blue image and many red lines.

The remaining red retrace are the giveaway that this is most likely not
a CRT problem.

(If there were no red lines, it could be the filament for the red gun
of the CRT going on and off due to a bad connection inside the CRT - bad

How is a black and white picture?  (Turn down the color control).

If B/W picture is good, then the problem is somewhere back in the chroma
decoder circuitry.

Check the video input to the CRT video driver board and signals on that board.
If B/W picture is also bad, then you can compare red and green signals
to determine where they are becoming different.  The red lines in your
description sounds like the red video output circuit is drifting and messing
up the background level, blanking, screen, or other setting.  Could be a
capacitor or other component.

  25.42) The wandering black blob on old Sony

"I had a Sony KV1920 TV (very old) that suddenly started to displayed
 a black blob on the screen.  The blob was anywhere from 1" around to
 almost the size of the entire screen.  It had a sharp, not fuzzy,
 outline, and it would shrink in size as the TV warmed up, usually
 disappearing completely in 30 minutes.  It shrank in sudden jumps,
 not gradually.  Sometimes the blob would be stationary, other times
 it would tumble around rapidly all over the screen."

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

Measure the regulated +130 volt line... I think you'll find it has drifted
upwards just enough to trigger the "protective" blanking circuit. In those
sets, if the B+ (and consequently the HV) went up, the screen was
automatically blanked so you couldn't use the set. It was before HV
shutdown. Older RCA TV sets used to throw the horizontal out of sync. The
low voltage regulator is an analog type with a pass transistor that is
probably leaky, causing the high B+. Changes in line voltage and loading
(with brightness changes) cause the partially blanked picture to change
(the black blob moves around, sometimes blanking the entire screen). When
you replace the defective component, reset the 130 volts with the pot, and
you're back in business. The pot itself may have a bad spot... just move it
one way or the other to get off that spot.

  25.43) Vertical brightness or color bars

These are typically more or less equally spaced possibly more evident at
the left side of the screen.  They result only in brightness or color
variations, not deflection speed.  Diagonal lines are straight and not

Note that the appearance of these bars differs from those caused by ringing
in the deflection circuits where diagonal lines will show a squiggling
stair-step appearance.

The most likely cause is a dried up electrolytic capacitor in the scan derived
power supply for the video or chroma circuits or video output.  Check for this
ripple with a scope or test/replace any suspect capacitors.

Chapter 26) Tuner, AGC, and Sync Problems

  26.1) No reception from antenna or cable

Make sure your source is providing a signal and that the cable connectors
are good (center pin not broken or bent).  Try another TV if possible.

Make sure you source select switch or mode is set correctly.  Someone
may have accidentally set it to direct video or AUX input.

Are all bands affected?  If so, the tuner or IF is faulty.  If there is
a lot of snow, then it is probably toward the front (circuitry wise) of
the tuner.  If it is just a black screen, then it could be in the IF or
video amplifier.

If only certain bands are bad - channels 2-6 for example, then certain
parts of the tuner circuitry are faulty.  However, make sure the CATV
mode is set correctly as this affects reception on a band-by-band basis.

The problems may be due to bad solder connections of the tuner shields,
connectors, coils, and other components.  Try prodding the tuner to see
if you can make the problem come and go or at least change.

  26.2) Picture is overloaded, washed out, or noisy

This indicates an Automatic Gain Control (AGC) problem often caused by
a dried up capacitor.  You will probably need a schematic to go much
further.  This could be a problem in the tuner, IF, or video amplifiers.

The following assumes you are sure the signal source is strong - try a VCR or
other local one (channel 3/4, not the RCA jacks).

(From: Glenn Watkins (blueribb@mail.comcat.com)).
Substitute a variable voltage source for the tuner's AGC voltage. Most of the
time the range of AGC is from 1 to 7 volts.  If you can get a decent snow
free picture with an external AGC source, then the tuner is probably OK.

  26.3) Jumping picture on white scenes

This could be an AGC problem if the picture appears overloaded.  However, if
the picture is normal except unstable, the sync separate is the place to

(From: Jack Schidt (jack@wintel.net)).

White screens are a worst case video pattern for sync separators, and will
cause an erratic shift in the vertical multivibrator trigger level unless
the horizontal and video information is filtered out [integrated] prior to
driving the vertical sync input of the processor IC.

This will show up with a scope as high frequency noise going into the
vertical sync input.

Look for a small electrolytic [in fact, all of them], around 1-10 uF or so
near the deflection/sync processor IC.  Often simply increasing the value of
this cap will help.

  26.4) Interference when using VCR RF connection

(Some of these comments also apply to use of LaserDisc players, satellite
receivers, video games, or other sources with RF modulator (Channel 3/4)

This may consist of patterns or lines in the picture.

If this only happens on the antenna or cable, it may be a problem
with these sources or the tuner in the VCR rather than the TV.
As a test, try the connecting the TV directly to the antenna or cable.

If it only happens on cable, there may be a (temporary) problem with
cable transmission - contact your cable company.

If it happens on playback of good quality (commercial) recordings, then
it could be a compatibility problem between the VCR and TV.

Make sure your patch cable connections are secure and that the cables
are not damaged - in particular that the center pin is intact.

Try fine tuning if your TV has this capability.  If this does not
help, try switching the channel 3/4 selector on the VCR to the opposite
position and try that channel, sometimes one will be better than the other
particularly if one of these or an adjacent channel is active in your area.

If you have RCA baseband video inputs on your TV, try this connection to
the VCR.  These should work better in any case.

Confirm that it is not actually a problem with the VCR - try another TV
if possible.

If you just changed your component placement, the VCR or TV  may be
picking up interference from another component.  Turn off everything but
the VCR and TV and see if that identifies the culprit.  Move the TV
away from the VCR so see if they are interfering with each other - the
TV may be introducing interference into the VCR.

Occasionally, the particular patch cable or its length may affect
reception quality - try another one.

If none of this helps, you VCR's RF modulator may be bad or slightly
weak.  Alternatively, the tuner in the TV may be faulty.  If reception
is generally noisy on all sources, AGC or RF/IF alignment may need
adjusting.  However, not all tuners are created equal.  Your TV may
simply be making the best of a marginal situation.

A light dimmer on the same circuit as the TV may result in similar
symptoms.  If you are tuning up your motorcycle (or automobile) in the
same room, this may be spark ignition interference.

  26.5) RF Interference on TV

"I have interference lines on my TV - they are particularly heavy on 2, not so
 prominent on 9 - one TV is on Radio Shack Color Supreme 100 (souped up
 rabbit ears), other is on a roof top antenna, both have coax from antenna
 to TV.  I have HAM operator two doors away.  Is there an FAQ on
 interference - if so where.  How do I need to describe the interference
 pattern in order to seek help.  Thanks."

The FCC has an online interference handbook, with color photos showing how
different types of interference affect a TV's picture:

     * http://www.fcc.gov/Bureaus/Compliance/WWW/tvibook.html

(From: Andrew Mitchell (amitchell@sympatico.ca)).

Probably the easiest solution is to visit your ham neighbor and
describe your difficulty.  Amateur radio operators are licensed by
federal governments (FCC) and are required as part of their examination
to demonstrate a knowledge of this type of interference.  It may well be
that the ham is not the source of the problem and even if this is the
case I'm sure he or she will be of assistance.

(From: Alan N. Alan, WDBJ-TV, KM4IG (alann@intrlink.com)).

OK, as a HAM myself, I can understand this. Channel 2 is the lowest TV channel,
right above the six meter band, 50-54 Mhz.  Channel 9 is well into VHF above
175 Mhz. It is possible that your neighbor operates 6 meters.

I would talk to him.  First,  the chances are it is YOUR equipment, and his
is legal and meets FCC specs.  But I would be willing to bet he will be very
cooperative to help you solve your interference.

The thing to do is talk to him, calmly, and tell him about your problem.  Then,
schedule a time where he can transmit his gear and see if your problem exists
along with his transmissions.. If it does, you can go from there.  Many ham
clubs have many engineers and radio and TV people in their memberships that
will jump in and help you solve your problem.  Again, he is probably legal,
and consumer equipment is not known for it's RF resistance.  Consumer
manufacturers cut corners wherever they can.  This includes filtering and

  26.6) Problems with ground loops and video hum bars

"I am having a problem isolating where my ground loop problem is coming
 from.  The symptoms I see are Bars on my TV which scroll up the screen. 
 The problem is these bars come and go, and when they are present they
 vary in intensity.  I have verified that the cable ground is connected
 to the earth ground on the outside of the house, but the problem still
 remains.  This problem is also screwing up any attempts to do video
 electronics experimentation.  I am really tired of seeing these bars and
 any help you could give would be appreciated."

(From: Paul Grohe (grohe@galaxy.nsc.com)).

1. Do these bars show up on other TV's connected to the same cable?

2. Is your TV connected to anything else? A/V receiver? VCR?

   If so, unplug *all* the equipment and plug it in one-at-a-time until the
   hum appears. If you have an AV receiver in the system, try running a jumper
   wire from the incoming CATV ground at the TV to the receivers chassis
   ground (usually the "phono ground screw").

   If you have any devices with un-polarized plugs, unplug them and rotate
   them 180 degrees, and plug them back in.

3. If you connect a temporary antenna and view "off-the-air" signals, are the
   bars still there?

If you still cannot eliminate the hum, try building a simple "ground isolator"
out of two 75-300 ohm baluns, as described in the link below:

* http://www.hut.fi/Misc/Electronics/docs/groundloop/antenna_isolator_building.html

Place it as close to the TV as possible.

(From: Charles Godard (cgodard@iamerica.net)).

This seems like a cable company problem, but you need to prove it to the cable
guy before he will start climbing pole's and changing amps and couplers
looking for an intermittent amp. (And I don't blame him.)

At the main cable line to the house and remove all couplers and put a single
line from the cable direct to the rf input on a single tv, then watch it for a
few of days.  If the problem re-occurs call the cable guy and show him what
you have done and explain the problem again.

Put yourself in the shoes of the cable guy.  He comes into a house with VCR's
and all the gadgets we all have hooked up to our TV's with lines running all
over the house, and can't get to the back of the TV to see what's there, and
he's not a TV repair guy anyway and nobody else in the neighborhood is
complaining and this problem may happen when it rains but it may not. mmmhhh

If it does not show up on the single TV, then the problem is probably yours.
Add one device at a time until you find the trouble maker.  Start with the
your Cable AMPLIFIER.

(From: 4real (alan69@iname.com)).

You eliminate all of the other junk attached between your main cable input to
your house and your TV to be sure it isn't the cause.

You will definitely want to suspect a problem with the amplifier you have
installed.  Especially if it is one of those cheap ones.  Usually when the
filter capacitor in an amplifier goes bad it will cause the hum bars you are
describing, and they can be intermittent.  Another problem may be that you
have too much signal going into the amplifier.  Amps are rated to handle a
certain amount of input signal (measured in db) depending on the number of
channels you wish to amplify, and the gain provided by the amp.  If you try to
feed an amplifier with too much signal it will overdrive it and cause a
venetian blind, or herringbone effect.  It could also be possible that the
cable company is supplying a signal with reverse tilt.  That means more signal
on the lower channels and less on the higher ones.  The lower channels might
be the ones overdriving the amplifier.  The only way to tell for sure is to
measure it with a signal level meter.  (very expensive unless you happen to be
in the business and have one handy) If this is the case (too much signal going
in) you probably don't need the amplifier to feed only 2 TVs.

The last thing I can think of and the cable guy should have checked this: They
use 60 VAC on their main trunk lines to power their line amplifiers.  The taps
which feed the individual houses are supposed to prevent this ac from going to
the individual lines.  Occasionally one of these devices fails or a line guy
forgets to pull a fuse and hence the ac gets sent to your tv.  It won't
necessarily fry your tv but can cause problems.  It may even damage the tv
tuners that are connected to that feed.  In most cases if you touch the center
conductor of the cable and a good ground you can feel the ac.  It isn't enough
to hurt you but you will definitely know it's there.  To be on the safe side
you should test it with a volt meter.

(From: Cliff R. (craeihl1@nycap.rr.com)).

My guess would be your cable amplifier. The fact that you see TWO bars on your
screen tells me that it's 120 Hz interference - the frequency caused by ailing
full wave power supplies used in these amps. Take the amp out of the line for
a few days. If you don't have "snow" in the picture with it out, s...can it!
If you find it was bad and can't live without it, you might try making sure
all your internal cable, splitters, and connections are good quality & in good
shape. Radio Shack stuff......well, it stinks!  You can purchase primo
splitters & cable from your cable company and its not that
expensive. Certainly cheaper than an amp (which you might not need if the
cabling were up to snuff).

You could also cry to the cable company for more signal into your house.  This
may or may not work but it's worth a shot.

I would put an amp in line only as a LAST resort. Most of the inexpensive
amps sold are......cheap. They can easily cause more trouble than they cure.
If you must, go with a primo unit from Blonder-Tongue or Jerrold.

(From: Charles Hope (charles.hope@argonet.co.uk)).

It sounds very like a problem that I had and solved.  

Cause: Modern tv sets antenna connector does not have true earth on the
screen but is at a potential of half mains supply.  It is possible to draw
about 30 micro-amps from this.  

Hum bars are induced in the amplifier because there is a small resistance in
the earth path between output and input giving about 1 volt drop of this
stray mains signal.  Worse when raining because the cable ground is better

Solution:  Either ground the antenna screen or fit a "braid breaker" in the

  26.7) Missing or noisy channel or block of channels

If you are unable to receive certain channels or blocks of channels,
this is a tuner problem - could be as simple as bad connections - or
even simpler:.

First, check to see that the tuning mode is correct - TV, CATV, as
this is the most common cause of channels 'disappearing'.

TV channels are assigned frequencies ranging from 72 to almost 800 MHz
depending on broadcast or cable channel assignment.  To tune over such a
wide range requires splitting it up into various bands even if these are
not actually defined.  If you have a varactor tuned set, then you already
know about the Vl, Vh, and U bands which may use separate front-end
components.  Even modern quartz PLL synthesized tuners need to allocate
circuitry depending on frequency range.  Therefore, if a block channels is
not working, it could be due to a failure of some component related to that
frequency range.  Aside from looking for bad connections, resoldering the
shields and connector pins, prodding, pressing, praying, etc. you will need
a schematic to have any chance of finding such a fault.

There is another slight possibility.  Some TVs have a parental lockout
capability (pre V-chip) to prevent kids or other unauthorized access to
selected channels.  The channel selections may have been accidentally
altered.  Check your user manual for instructions on programming this
feature.  Even on models without this option, the same internal circuitry
could be present but not normally accessible.  A power surge or stray cosmic
ray could have put the set in a screwy mode.  Unplugging power for a minute or 
probably a much longer time might possibly reset such an anomaly.

  26.8) Loss of Channel after Warmup

If there is a general loss of picture and sound but there is light on the
screen, then most likely the tuner or IF stage is pooping out.

With both no sound and no picture but a raster and static, it is most
likely a problem in the tuner, power to the tuner, or its controller
(if non-knob type).

If it recovers after being off for a while, then you need to try a cold
spray in the tuner/controller to identify the component that is failing.
Take appropriate safety precautions while working in there!

If it stays broken, then most likely some component in the tuner, its
controller, or its power supply as failed. There is a slight chance that
it could be a bad solder connection - I have seen these in the tuner modules
of RCAs on several occasions (and many other manufacturers - apparently
not a solved manufacturing problem even after 40+ years!

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