Notes On The Troubleshooting And Repair Of Television Sets


  25.15) Red, green, or blue full on - fog over picture

This could be a heater-cathode (H-K) short in the CRT or a failure
of a component in the chroma circuits or video output (driver board).

Don't panic - heater-cathode shorts in CRTs can often be worked around.

Note: before proceeding, it is a good idea to make sure that the screen is
degaussed - else you could be attempting to track down problems with the wrong

Some simple tests can confirm or rule out other possibilities.

* Compare the voltages for the video drive signals to the CRT on the little
  board on the neck of the CRT with the CRT both connected and unplugged.
  A schematic will help greatly in locating these signals.

  - If there is a significant difference especially on the bad color, then the
    CRT is a likely candidate.  Try tapping the neck of the CRT GENTLY (with
    it plugged in and while viewing a picture) to see if it is an intermittent

  - If there is no significant difference, you may have a bad driver or a
    problem in the chroma circuits.

* Look for bad connection/cold solder joints, probably on the little
  board on the neck of the CRT.  Use an insulated stick to gently prod
  the board and its components in an effort to induce/cure the problem.
  Look carefully for hairline cracks around the component leads.

* You can swap components between two colors and/or test with an ohmmeter
  on that driver board to determine what is bad.  The nice thing about
  color monitors and TVs is that there three copies of each of these
  components.  Swapping and/or comparisons between these is an excellent
  diagnostic technique.

* Another simple test: Disconnect the cathode for the full-on color from its
  drive.  If it is still full-on, there is probably an H-K short in the CRT
  since the only way to get each color on the screen is via the cathode
  connection to the CRT neck board.  If it is removed and there is still that
  color, the current must be taking another path inside the CRT.

* Alternatively, interchange the outputs of the bad color with a good one
  by jumpering on the video driver board (on the CRT neck).  If the bad
  color changes, then the problem is in the circuitry and not the CRT.

  Here is the procedure in more detail (example for red full on):

  (From: J. K. Emerine (jkemerine@aol.com)).

  To identify if the fault is in the crt or a control problem try this (WITH

  On the CRT board, lift the output end of the green cathode final resistor.
  Do the same with the offending red cathode's resistor.  Use short insulated
  jumpers to 'swap' drive signals - drive the red cathode with the green
  drive and the green cathode with red drive.  (Note that if this problem
  only occurs after a warmup period, color at turn on will be - well - wierd,
  but it is just a test.)

  - If the symptom returns = 'goes red' the CRT is shorting.  (See the section:
    "Providing isolation for a CRT H-K short". --- sam)

  - If instead the symptom becomes 'goes green' then the red drive leg has
    the fault and the CRT is probably good.  (In this case, there may be bad
    connections or a bad component on the CRT drive board or further back
    in the chroma circuitry. --- sam)

  25.16) Shorts in a CRT

Occasionally, small conductive flakes or whiskers present since the day of
manufacture manage to make their way into a location where they short out
adjacent elements in the CRT electron guns.  Symptoms may be intermittent or
only show up when the TV or monitor is cold or warm or in-between.  Some
possible locations are listed below:

* Heater to cathode (H-K).  The cathode for the affected gun will be pulled
  to the heater (filament) bias voltage - most often 0 V (signal ground).  In
  this case, one color will be full on with retrace lines.  Where the heater
  is biased at some other voltage, other symptoms are possible like reduced
  brightness and/or contrast for that color.  This is probably the most
  common location for a short to occur.

* Cathode to control grid (K-G1).  Since the G1 electrodes for all the guns
  are connected together, this will affect not only the color of the guilty
  cathode but the others as well.  The result may be a very bright overloaded
  *negative* picture with little, none, or messed up colors.

* Control grid to screen (G1-G2).  Depending on circuitry can result in any
  degree of washed out or dark picture.

* Screen to focus (G2-F).  Screen (G2) and focus voltage will be the same and
  the controls on the flyback will interact.  Result will be a fuzzy white
  raster with retrace lines and little or very low contrast picture.  Symptoms
  will be similar to those of a flyback with breakdown in the focus/screen
  divider network.

* Focus to high voltage (F-HV).  High voltage will be pulled down - probably
  arcing at the focus spark gaps/other protective devices.  Line fuse and/or
  HOT may blow.

* Other locations between electron gun elements as feed wires.

Replacing the CRT may be required but there are a variety of 'techniques' that
can often be used to salvage a TV that would otherwise end up in the dump
since replacing a CRT is rarely cost effective:

1. Isolation - this will usually work for H-K shorts as long as only one gun
   is involved.

2. Blowing out the short with a capacitor - depending on what is causing the
   short, this may be successful but will require some experimentation.

3. Placing the CRT (TV or monitor) face down on a soft blanket and *gently*
   tapping the neck to dislodge the contamination.  Depending on the location
   of the short, one side or the other might be better as well.  Sometimes,
   this can be done in-place while watching the picture.

A combination of (2) and (3) may be required for intermittent shorts which
don't appear until under power.  See the sections below for additional
details.  However, for shorts involving the focus and high voltage elements,
even a sharp edge can result in arcing even if there is no actual short.
There is no remedy for these types of faults.

  25.17) Providing isolation for a CRT H-K short

This procedure will substitute a winding of your own for the one that is
built in to the flyback to isolate the shorted filament from the ground
or voltage reference.  Note that if you have a schematic and can determine
where to disconnect the ground or voltage reference connection to the
filament winding, try this instead.

The flyback is the thing with the fat red wire coming out of it (and perhaps
a couple of others going to the CRT board or it is near this component
if your set has a separate tripler) and may have a couple of controls for
focus and screen.  It should have some exposed parts with a ferrite core
about 1/2-3/4" diameter.
The filament of the CRT is the internal heater for each gun - it is what
glows orange when the set is on.  What has happened is that a part of the
fine wire of the bad color's filament (assuming this is indeed your problem)
has shorted to the cathode - the part that actually emits the electrons.
Normally, the heater circuit is grounded or tied to a reference voltage
so when it shorts to the cathode, the cathode voltage level is pulled to
ground or this reference.

You will need some well insulated wire, fairly thick (say #18-22).  Find a
spot on the flyback where you can stick this around the core.  Wrap two
turns around the core and solder to the CRT filament pins after cutting the
connections to the original filament source (scribe the traces on the board
to break them).  Make sure you do not accidentally disconnect anything else.

This winding should cause the filaments to glow about the same brightness as
before but now isolated from ground.  If they are too dim, put another turn
on the flyback to boost the voltage as this will result in low emission,
blooming, and possible damage to the cathodes after awhile.  (Don't go
overboard as you may blow the filament totally if you put too many turns on
the core - you then toss the TV.)

Route the wires so that there is no chance of them getting near the high
voltage or any sharp metal edges etc.  Your picture quality may be a tad
lower than it was before because of the added stray capacitance of the
filament wiring being attached to the the (formerly bad) video signal, but
hey, something is better than nothing.

If you are not inclined to build your own isolation transformers, kits are

(From: Alan Harriman (capstv@sprynet.com)).

A company called KDTV/IWE carries kits (core, wire and tie) for $3.30 each.
It takes all of two minutes to wind.

Check out: http://www.seidata.com/~kdtv.

BTW, I am just a satisfied customer.

  25.18) Rescuing a shorted CRT

If the short is filament-cathode (H-K), you don't want to use the following
approach since you may blow out the filament in the process.  If this is the
case, you may be able to float the filament and live with the short (see the
section on: "Red, green, or blue full on - fog over picture".

Shorts in the CRT that are between directly accessible electrodes can
be dealt with in a more direct way than for H-K shorts.  At this point
you have nothing to loose.  A shorted CRT is not real useful.

If the short is between two directly accessible electrodes like cathode-grid,
then as a last resort, you might try zapping it with a charged capacitor.
Start with a relatively small capacitor - say a few uF at a couple hundred
volts.  Check to see if the short is blown after each zap - few may be needed.
Increase the capacitance if you fell lucky but have had little success with
the small capacitor.

If the fault is intermittent, you will, of course, need to catch the CRT
with the socket disconnected and the short still present.  Try some gentle
tapping if necessary.  If you do this with the charged capacitor across
the suspect electrode, you **will** know when the short occurs!

  25.19) Picture tube replacement

It is possible to replace the picture tube.  However, this is likely to
be both expensive and possibly time consuming with respect to adjustments
like purity and convergence.  When replacing:

* Discharge both the old and new tubes before you start to be sure you won't
  have any unpleasant surprises.

* Take extreme care when handling - at the very least, a slip can result in a
  broken neck and a bad and expensive day.

"The 25VCXP22 picture tube of my RCA Accutouch XL-100 CCU-942 TV start fading.
 Its 100% transistorized, everything still works perfectly after about 20
 years service.  but:

 * Can I still buy new RCA 25VCXP22 picture tube? What is the approximate cost?

 * Any equivalent tube for direct replacement?  Cost?

 * If no replacement picture tube is available, what is other option?"

(From: Chris Jardine (cjardine@wctc.net)).

What you have here is genericly referred to as the 25V as opposed to the 25A
picture tube. While there are minor differences with respect to the letters
after the V for the most part they are interchangeable. When I worked my way
through engineering college I worked at a TV repair shop and my job was mostly
changing picture tubes. Yeah, we did enough of them to keep a tech busy 4 to 5
hours a day changing them and I got pretty good and could change, color
balance, convergence, etc. the tube in about 45 minutes. We for the most part
used 3 major tubes, 1) 25A, 2) 25V, and 3) 21FJ (a little nostalgic for those
who remember this one). This was back when your TV would have been fairly new
(1981 to 82). These are available from many different sources - RCA, Channel
Master, Wisconsin Tube, etc. The price would vary depending on the quality of
the tube. I remember that we could get a 25A for about $35 at the time due to
our volume - one truck per month. The most expensive I've seen them has been
just over $200. This is quite a range and there are now many other types of
tubes including in-line, trinitron, etc. 

I hope this helps and thanks for the trip down memory lane!

(From: Chris Jardine (cjardine@wctc.net)).

The important thing here is that the tube begins with 25V. If it does it should
work in your set. The only thing you have to know is whether the tube has
'ears' attached permanently. The 25V comes both with and without these mounting
ears permanently attached. I know that you can still get one of these from any
of a number of suppliers. I know that Channel Master and RCA (Thompson,
whatever!) still make them available as well as any of a number of local CRT

  25.20) Dark picture

A TV or monitor with a picture that is too dark may have a fault or the CRT may
just be near the end of its useful life.

First, confirm that your video source - computer, camera, etc. - is producing
a proper signal.

Is the brightness at all erratic?  Does whacking the monitor have any effect?
If so, then you may have bad connections on the CRT driver card or elsewhere.
If the brightness tends to fade in and out over a 10 to 20 second period,
a bad filament connection is likely.  Check for the normal orange glow of
the filaments in the neck of the CRT.  There should be 3 orange glows.  If
they are excessively reddish, very dim, or fade in and out, you have located
a problem.  See the section: "Picture fades in and out".

Common causes of brightness problems:

0. Dirty CRT faceplate or safety glass.  Don't laugh.  It sounds obvious, but
   have you tried cleaning the screen with suitable screen cleaner?  It is
   amazing how dirty screens can get after a few years - especially around

   Wipe gently with a slightly dampened cloth - not soaking or you may end
   up with real problems when the water drips down inside and hits the
   electronics!  On TVs with a separate protective faceplate, clean both
   the front and rear surfaces of this plate as well as the CRT itself.

1. Old CRT.  The brightness of the CRT deteriorates with on-time.  It does not
   matter much how bright your run your TV.  An indication of a weak CRT would
   be that turning up the SCREEN (G2) or master brightness control only results
   in a not terribly bright gray raster before the retrace lines show up.
   There may be indications of poor focus and silvery highlights as well.  A
   CRT brightener may help.  See the section: "Brightening an old CRT".

2. Bad component in filament circuit or bad connection reducing filament
   voltage.  This should be easy to check - there are only a few parts
   involved.  If it is erratic, bad connections are likely.

3. Brightness control faulty - bad pot, bad connections, or problem with its
   power supply.  Depending on specific problem, control may or may not have
   any effect.  If digitally adjusted, there could be a problem with the
   logic or control chip.  If the button or menu item has no effect at all,
   then a logic or control problem is likely.

4. Improperly set SCREEN (G2) voltage (usually on flyback) or faulty divider
   network.  See the section: "Adjustment of the internal SCREEN and color controls".

5. Improperly set video bias (background) levels or fault in video drive
   circuitry.  See the sections starting with: "Optimal procedure for setting brightness/background and screen adjustments".

6. Fault in video amplifiers.  With all three color affected equally, this
   would most likely be a power supply problem.  A video amplifier problem
   is likely if turning up the SCREEN (G2) or master brightenss control
   results in a very bright raster before the retrace lines appear.  Cheack
   signals out of the video/chroma(IC.

7. Fault in beam or brightness limiter.  Many TVs and monitors measure the
   beam current (possibly indirectly) and limit the maximum to a safe value.
   The purpose of this may be to protect the CRT phosphors, and/or to assure
   that the power supply does not go out of regulation, and/or to limit X-ray
   emission.  If this circuit screws up, a dark picture may result.  Checking
   the signals and voltages at the CRT socket should determine if this is the

8. High voltage is low.  However, this would likely result in other symptoms
   as well with focus, size, and geometry.

  25.21) Brightening an old CRT

If performing adjustments of the internal background and/or screen controls
still results in a dark picture even after a long warmup period, the CRT
may simply be near the end of its useful life.  In the old days of TVs
with short lived CRTs, the CRT brightener was a common item (sold in every
corner drugstore, it seemed!).

You can try a similar approach.  Caution: this may shorten the life of
the CRT - possibly quite dramatically (like it will blow in a couple of
seconds or minutes).  However, if the monitor or TV is otherwise destined
for the scrap heap, it is worth a try.

The approach is simple: you are going to increase the voltage to the
filaments of the electron guns making them run hotter.  Hopefully, just
hotter enough to increase the brightness without blowing them out.

Voltage for the CRT filament is usually obtained from a couple of turns
on the flyback transformer.  It is usually easy to add an extra turn or two
which will increase the voltage and thus the current making the filaments
run hotter.  This will also shorten the CRT life - perhaps rather drastically.
However, if the TV or monitor was headed for the dumpster anyhow, you have
nothing to lose.

  25.22) Picture tube brightener

(From: Kevin Carney (carneyke@mhv.net)).

Try a CRT brightener from MCM Electronics about $20. It boosts the
filament voltage a volt or two. I have used them before and they help.
You can also try running a power supply on the filament with the
monitor OFF. Set the supply at the filament voltage and slowly bring
the voltage up. If the filament is 6.3 volt bring it up gradually to
10 -12 volts for about a half hour. This will brighten it up some.
Be careful because too much voltage can open the filament !

Before doing this did you check the screen voltage setting and the
RGB settings for drive and background ?

There are also commercial CRT rejuvenators that supposedly zap the
cathodes of the electron guns.  A TV repair shop may be able to
provide this service, though it is, at best, a short term fix.

  25.23) More drastic measures to brighten CRT

(From: LEE (leep@mailhub.scf.lmsc.lockheed.com)).

As a start, I crank the brightness control all of the way up. I then turn the
color control all of the way up. I let the set run with a bright screen for
around 15 min. This procedure cleans up the cathode surfaces so that they can
emit more electrons. Now turn the controls back to normal and see if any 
improvement took place.  If not, Wrap 2 or 3 turns of around 18 gauge insulated
wire around the flyback and add this extra power in series with existing
filament leads from flyback. You can experiment with the number of turns etc.
to get brighter filaments. do not run the filaments white - just a brightened
yellow. This will probably turn out to be around 8-9v in most cases. I had to
do this on two different Sanyo replacement flybacks as they had low filament
voltage from the factory. (flakey replacement parts).  I`ve been running one of
these Sanyos for around 4 years now with a nice bright picture (13")

  25.24) Left portion of screen is dark or faded

"I've got an old TV where the left 1/3 of the screen is 'faded'.  It is
 especially noticable when a dark picture is showing (like a night time

This is normally caused by a bad filter capacitor on the power supply line
(typically 200 V) that feeds the RGB output transistors. It is usually a scan
derived voltage off of the flyback.  Look for an electrolytic capacitor of
around 4.7 to 10 uF, 160 to 250 V fed from a rectifier diode on this supply. 

  25.25) Color balance changes across screen from left to right

The characteristics are that a solid white screen will tend to be blue tinted
on one side and red tinted on the other.  This is usually a subtle effect and
may be unavoidable with some designs.

There are several possibilities:

1. Purity - this means the beams are landing on the wrong phosphor dots.
   This is what would be affected by moving from one location to another
   or even rotating the TV on its base without degaussing.  If the problem
   just appeared, degaussing may be needed.

   What do you have near the TV or monitor?  Loudspeakers or other devices
   which generate magnetic fields can easily cause all sorts of color purity
   problems.  Relocate the offending device(s) or the TV or monitor and then
   degauss it.

   See the section: "Degaussing (demagnetizing) a CRT".

   If the problem still persists, purity adjustment may be needed.  However,
   this isn't likely to  have changed so look for other causes before tackling
   these adjustments.

2. Unequal electron gun to shadowmask/screen distance - the electron beams for
   the red and blue video travel slightly different distances on the left and
   right sides of the screen so their intensity (due to focus not being optimal
   and other factors) in each case may differ slightly affecting color balance.

3. Doming - This would only happen in very bright areas and causes the
   shadow mask to expand and distort.  (Doming should not be a problem with
   Trinitron CRTs which use tensioned wires in their aperture grill.)  This
   would also not really affect left-right color balance in particular.

I don't really know how much of a problem (2) is in practice or whether some
manufacturers compensate for it.

  25.26) Bleeding highlights

On very bright areas of the picture, one or more colors may bleed to
the right resulting in a trail of those colors.  The difference between
this problem and the section: "Trailing lines in one or more colors" is
that in this case, only highlights are affected.

One cause of this is that the color gain, contrast, or intensity controls
(whatever they are called on your set) are set too high.  See the section
on: "Color balance adjustment".  Check the settings of any brightness limiter
controls as well.

  25.27) Trailing lines in one or more colors

Assuming this is not a form of ghosting resulting from poor reception
conditions, then it could be any of the following:

* Poor decoupling in the power supplies for the video drive circuits - probably
  on the CRT neck board.  Check for bad (low uF or high ESR) filter capacitors
  (electrolytic mostly) on this board or the power supplies feeding it.

* Insufficient CRT filament voltage.  This could be a result of bad connections
  or a bad component in the filament power supply (probably from the flyback).
  Check to see if the filaments are glowing bright orange and check the voltage
  if possible (though this can be tricky since it is often fed from a winding
  on the flyback and is a pulse waveform, not DC or a sinusoid.  The service
  manual (or Sams' Photofact) will probably have info and waveforms.

* Bad CRT (more likely if only one color is affected).  A weak electron gun can
  result in this behavior.  Swap it with one that work properly.  If the same
  color is still bad, that CRT gun is weak.  The CRT will need rejuvenation or
  need to be replaced (more likely, the entire TV will be tossed into the

  25.28) Brightness changes from left-to-right across screen

Slight variations in brightness across the face of the CRT are not unusual.
In fact, if you used a photometer to actually measure the brightness, you
might be amazed at the actual variance even with the best TV - you just
don't notice it.  However, a major variation - usually a decay from left to
right but could be the other way indicate a component failure.  Of course,
make sure the face of the screen is clean!

* A fault in the power supplies to the video amplifier and/or video output
  circuits.  Most likely, an electrolytic capacitor has dried up and is not
  adequately filtering the power derived from the flyback which then has
  ripple at the horizontal scan rate and thus locked to the screen.  The
  voltage decays from left-to-right between horizontal flyback pulses.

  The most likely location for these capacitors is in the vicinity of the
  flyback transformer on the mainboard or on the CRT neck board.  Check the
  capacitors with capacitor tester or ESR meter and/or take a look at the
  power right at the video amplifier and video output drivers.

* Horizontal linearity is bad - this may actually be a horizontal geometry
  problem and not a brightness problem.

  See if objects on left side of the screen are stretched compared to those on
  the right (or vice-versa).  If they are, the problem is in the horizontal
  deflection circuits - possibly a bad S correction capacitor or linearity

* Inoperative degauss circuit, TV moved or rotated without degaussing, or
  magnetic field from some other device (like a permanent magnet) is affecting
  CRT - slight amounts of magnetization may reduce brightness (by moving the
  beams into the black space between phosphor dots) before affecting color
  purity (where the beams land on the wrong phosphor dots).

  Try deguassing manually.  See the section: "Degaussing (demagnetizing) a CRT".

  25.29) Picture fades in and out

If the picture faded away on the order of 10-20 seconds (and if it comes
back, also comes up to full brightness in same time frame - possibly
with the persuasion of some careful whacking) AND with NO other
significant changes such as size, focus, etc., then take a look in the back of
the tube for the filament to be lit - the orange glow near the CRT socket.  If
there is none, then you probably have a bad solder connection on the circuit
board on the neck of the CRT.  Look for fine cracks around pins on that board.
Try prodding it with an insulating stick to see if the picture comes back.
Resolder if necessary.  It is probably not a bad CRT as the filaments
are usually wired in parallel and all would not go bad at the same time.

However, if only a single color fades in and out, then a bad connection
inside the CRT is a distinct possibility - look for only one of the
filament's glow to be coming and going.  This is probably not worth fixing.

If the picture faded away with other symptoms, then there is probably
a fault in the video amplifier/output one of its power supplies -
still probably a loose connection if you are able to get it back by

  25.30) Occasional brightness flashes

These may last only a fraction of a scan line or much much longer.

This could mean an intermittent fault in a variety of places including
the video circuitry and SCREEN power supply:

* Brightness circuitry - SCREEN, master background or its power supply.
  Could be in or around flyback or focus/screen divider.  Could perhaps
  be in the CRT, but probably less likely.

* Video amp before or at chroma demodulator - since after this point, you would
  most likely get colored flashes since only one of the RGB signals would
  likely be effected.

If you get it from all sources, then tuner/IF is ruled out.

Suppose you just have no signal to a direct video input.  What do you
get?  If you still get flashes, it should be real easy to monitor either
the video outputs or SCREEN supply (with a HV divider on your scope) for
noise.  Then trace back to power or noise source.

  25.31) Excessive brightness and/or washed out picture

There are a number of possibilities including incorrect screen (G2) or bias
(G1) voltages, or a problem in the video or blanking circuitry.  Any of these
could be the result of bad connections as well.  A short in the CRT can also
result in these symptoms.

* Excessive brightness/washed out picture is often an indication of a
  problem with the screen (G2) supply to the CRT.  May be a bad capacitor
  or resistor divider often in the flyback transformer assembly or on 
  the board on the neck of the CRT.

* If the excessive brightness just developed over time, then a simple
  adjustment of the screen or background brightness controls may keep
  it (and you) happy for a long time.

  When good, a typical value would be in the 200 to 600 VDC at the CRT.  The
  screen (it may also be called master brightness, bias, or background) control
  should vary this voltage.  However, it may be difficult to measure as the
  resistors in the voltage divider network may be quite large - hundreds of M
  ohms.  If your unit has an external screen control (less likely these days)
  and it has no effect, trace out the circuitry in the immediate vicinity and
  check the resistors and potentiometer for opens, look for bad connections,
  etc.  If it is built into the flyback transformer and is sealed, the entire
  flyback will need to be replaced unless the actual problem turns out to be a
  bad connection or bad component external to the flyback.

* Where the brightness control has no effect, suspect a missing bias supply to
  the G1 (control grid) electrodes of the CRT.  This is usually derived from
  the flyback with a simple rectifier/filter capacitor power supply.  Parts
  may have failed (though not likely the flyback itself).  Adjusting the user
  brightness control should vary this voltage over a typical range of 0 to -50
  V with respect to signal ground.

* It could also be a problem with biasing of the video output transistors.
  There may individual controls for background brightness on the little board
  on the neck of the CRT.  However, we are looking for a common problem since
  all colors are wrong in the same way.  This is likely to be a missing voltage
  from a secondary supply from the flyback.

* A short between electrodes inside the CRT can result in brightness problems.
  It may be possible to check this with an ohmmeter with the power off and the
  CRT socket removed.  Test between G1, G2, and F where all colors are
  affected though a short between F and G2 will result in the focus control
  changing brightness and vice-versa - a classic symptom.

  However, in some cases, it only shows up when operating and one must deduce
  the presense and location of the short from its affect on voltages and bias

  See the section: "Rescuing a shorted CRT" and other related topics.

First, check for bad connections/cold solder joints by gently prodding
with an insulating stick.  Check voltages and bias levels.

  25.32) Bad focus (fuzzy picture)

Focus voltage on the CRT is usually in the range of 2-8 KV DC and should
be controllable over a fairly wide range by the focus pot - usually located
on the flyback or a little panel in its vicinity:

* If adjusting the pot results in a position of acceptable focus, you may be
  done.  It is not unusual for the focus setting to drift a over time.

* If the setting is already as good as possible but not really good enough,
  the CRT may be tired.  Alternatively, the filament voltage may be too low.
  Check for bad connections in the filament circuit.

* If the optimal setting is out of range of the focus pot, the problem is
  likely leakage in the focus divider in the flyback or one of the components
  on the CRT neck board.

Also see the sections: "Focus adjustment" and "Focus drifts with warmup".

The focus wire usually comes from the flyback or if the general area or from a
terminal on a voltage multiplier module in some cases.  It is usually a wire
by itself going to the little board on the neck of the CRT.

If a sparkgap (a little 2 terminal device with a 1/8" gap in the middle)
is arcing with power on, then the resistive divider has shorted inside
the flyback, focus board, or HV multiplier - whatever you TV has - and
the this unit will need to be replaced.  Ditto if the SCREEN control affects
focus and/or vice-versa.

Using a suitable high voltage meter (range at least 10 KVDC, 1000 M ohm or
greater input impedance), you should be able to measure it connected and
disconnected.  The ground return will be the outside coating of the CRT which
may or may not be the same as the metal chassis parts.  If the voltage is very
low (less than 2 KV) and the pot has little effect:

* When measured right off of the source disconnected from the CRT neck board,
  then the problem is probably in the focus network in the flyback (or wherever
  it originates).  Sometimes these can be disassembled and cleaned or repaired
  but usually requires replacement of the entire flyback or voltage multiplier.
  Note: you may need to add a HV (10 KV) capacitor between the focus wire and
  DAG ground to provide filtering so you get a DC level for your meter.

* When measured with the focus wire attached to the CRT neck board with the
  CRT connected but reasonable with the CRT unplugged, there is probably a
  short between the focus and another electrode inside the CRT.  See the
  section: "Rescuing a shorted CRT".

* When measured with the focus wire attached to the CRT neck board with the
  CRT unplugged, there is likely a component on the CRT neck board that is
  leaky or breaking down.  Also, check for decayed (tan or brown) glue which
  may turn leaky with age.

Go to [Next] segment
Go to [Previous] 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]