Notes On The Troubleshooting And Repair Of Television Sets


  23.20) Deflection yoke testing

A faulty deflection yoke can affect the geometry (size and shape) of the
raster, result in insufficient high voltage and/or other auxiliary power
problems, and blow various components in the low voltage power supply or

* A simple test to determine if the yoke is at fault for a major geometry
  problem (e.g., a keystone shaped picture) is to interchange the connections
  to the yoke for the axis that is not affected (i.e., the vertical coils if
  the width is varying from top to bottom).  If the raster/picture flips
  (indicating that you swapped the proper connections) but the shape of the
  raster remains the same - the geometry is unchanged, the problem is almost
  certainly in the deflection yoke.

* Where high voltage (and other flyback derived voltages) are reduced and
  other problems have been ruled out, unplugging the deflection yoke (assuming
  no interlock) may reveal whether it is likely at fault.  If this results in
  high voltage and a relatively clean deflection waveform or returns the power
  supply or deflection chip load to something reasonable, a defective yoke is
  quite possible.

  CAUTION: powering a TV or monitor with a disconnected yoke must be done with
  care for several reasons:

  - The CRT electron beam(s) will not be deflected.  If it turns out that the
    yoke is the problem, this may result in a very bright spot in the center
    of the screen (which will turn into a very dark permanent spot quite
    quickly) :-(.  Disconnecting only the winding that is suspect is better.
    Then, the other direction will still scan resulting in a very bright line
    instead of a super bright spot.  In any case, make sure the brightness is
    turned all the way down (using the screen/G2 control on the flyback if
    necessary).  Keep an eye on the front of the screen ready to kill power at
    the first sign of a spot or line.  Disconnecting the CRT heater as an
    added precaution would be even better unless you need to determine if
    there is a beam.

  - Removing the yoke (which is effectively in parallel with the flyback)
    increases the inductance and the peak flyback voltage on the HOT.  In the
    extreme, this may blow the HOT if run at full line voltage/normal B+.  It
    is better to perform these tests using a Variac at reduced line voltage if

  - The deflection system will be detuned since the yoke inductance plays a
    very significant role in setting the resonance point in most designs.
    Don't expect to see totally normal behavior with respect to high voltage.
    However, it should be much better than with the faulty yoke.

* If possible, compare all measurements with a known good identical deflection
  yoke.  Of course, if you have one, swapping is the fastest surest test of
  all!  In many cases, even a not quite identical yoke will be close enough to
  provide useful information for testing.  However, it must be from a similar
  piece of equipment with similar specifications - size and scan range.  Don't
  expect a color TV yoke to work in a high performance SVGA monitor!

  Note: the substitute yoke doesn't have to be mounted on the CRT which would
  disturb purity and convergence adjustments but see the caution above about
  drilling holes in the CRT face plate!

The deflection yoke consists of the horizontal coils and vertical coils (wound
on a ferrite core), and mounting structure.  Little magnets or rubber/ferrite
strips may be glued in strategic locations.  DO NOT disturb them!  In rare
instances, there may be additional coils or other components mounted on the
same assembly.  The following deals only with the actual deflection coils
themselves - the other components (if any) can be tested in a similar manner.

Where the test procedure below requires removal of the yoke, see the section:
"Removing and replacing the deflection yoke" first.

* Horizontal - the horizontal section consists of an even number of windings
  hooked up in parallel/interleaved with half of the windings on each of the
  two ferrite core pieces.

  The horizontal windings will be oriented with the coil's axis vertical and
  mounted on the inside of the yoke (against the CRT neck/funnel).  It may be
  wound with thicker wire than that used for the vertical windings.

  - Resistance check - This may be possible without removing the yoke from
    the CRT if the terminal block is accessible.  Disconnect the individual
    windings from each another and determine if the resistances are nearly
    equal.  Check for shorts between windings and between the horizontal and
    vertical windings as well.

    Typical resistance of the intact windings (at the yoke connector assuming
    no other components): TV or NTSC/PAL monitor - a few ohms (3 ohms typical),
    SVGA monitor - less than an ohm (.5 ohms typical).

  - Inspection - Look for charring or other evidence of insulation breakdown
    due to arcing or overheating.  For the horizontal windings, this will
    require removing the yoke from the CRT since little if any of the windings
    are visible from the outside.  However, even then, most of the windings
    are hidden under layers of wire or behind the ferrite core.

  - Ring test.  See the document "Testing of flyback (LOPT) transformers".
    This deals with flyback transformers but the principles are the same.
    Disconnecting the windings may help isolate the location of a fault.
    However, for windings wound on the same core, the inductive coupling
    will result in a short anywhere on that core reducing the Q.

* Vertical - The vertical section is usually manufactured as a pair of windings
  wired in parallel (or maybe in series) though for high vertical scan rate
  monitors, multiple parallel/interleaved windings are also possible.

  The vertical windings will be oriented with the coil's axis horizontal and
  wound on the outside of the yoke.  The wire used for the vertical winding
  may be thinner than that used for the horizontal windings.

  - Resistance check - This may be possible without removing the yoke from
    the CRT if the terminal block is accessible.  Disconnect the individual
    windings from each other and determine if the resistances are nearly
    equal.  Check for shorts between windings and between the horizontal
    and vertical windings as well.

    Typical resistance of the intact windings (at the yoke connector assuming
    no other components): TV or NTSC/PAL monitor - more than 10 ohms (15 ohms
    typical), SVGA monitor - at least a few ohms (5 ohms typical).

  - Inspection - Look for charring or other evidence of insulation breakdown
    due to arcing or overheating.  The accessible portions of the vertical
    windings are mostly visible without removing the yoke from the CRT.
    However, most of the windings are hidden under layers of wire or behind
    the ferrite core.

  - Ring test - Since the vertical windings have significant resistance and
    very low Q, a ring test may be of limited value.

  23.21) Deflection yoke repair

So you found a big black charred area in/on one of the yoke windings.  What
can be done?  Is it possible to repair it?  What about using it for testing
to confirm that there are no other problems before ordering a new yoke?

If the damage is minor - only a few wires are involved, it may be possible to
separate them from each other and the rest of the winding, thoroughly clean
the area, and then insulate the wires with high temperature varnish.  Then,
check the resistances of each of the parallel/interleaved windings to make
sure that you caught all the damage.

Simple plastic electrical tape can probably be used for as insulation for
testing purposes - it has worked for me - but would not likely survive very
long as a permanent repair due to the possible high temperatures involved.
A new yoke will almost certainly be needed.

  23.22) Testing of flyback (LOPT) transformers

How and why do flyback transformers fail?

Flybacks fail in several ways:

1. Overheating leading to cracks in the plastic and external arcing.  These
   can often be fixed by cleaning and coating with multiple layers of high
   voltage sealer, corona dope, or even plastic electrical tape (as a 
   temporary repair in a pinch).

2. Cracked or otherwise damaged core will effect the flyback characteristics
   to the point where it may not work correctly or even blow the horizontal
   output transistor.

3. Internal shorts in the FOCUS/SCREEN divider network, if present.  One sign
   of this may be arcover of the FOCUS or SCREEN sparkgaps on the PCB on the
   neck of the CRT.

4. Internal short circuits in the windings.

5. Open windings.

More than one of these may apply in any given case.

First, perform a careful visual inspection with power off.  Look for cracks,
bulging or melted plastic, and discoloration,  Look for bad solder connections
at the pins of the flyback as well.  If the TV or monitor can be powered
safely, check for arcing or corona around the flyback and in its vicinity,

Next, perform ohmmeter tests for obvious short circuits between windings,
much reduced winding resistances, and open windings.

For the low voltage windings, service manuals may provide the expected
DC resistance (Sams' PhotoFact, for example).  Sometimes, this will change
enough to be detected - if you have an ohmmeter with a low enough scale.
These are usually a fraction of an ohm.  It is difficult or impossible to
measure the DC resistance of the HV winding since the rectifiers are usually
built in.  The value is not published either.

Caution: make sure you have the TV or monitor unplugged and confirm that
the main filter capacitor is discharged before touching anything!  If you
are going to remove or touch the CRT HV, focus, or screen wires, discharge
the HV first using a well insulated high value resistor (e.g., several
M ohms, 5 W) to the CRT ground strap (NOT signal ground.  See the section:
"Safe discharging of capacitors in TVs and video monitors".

Partially short circuited windings (perhaps, just a couple of turns)
and sometimes shorts in the focus/screen divider will drastically lower
the Q and increase the load the flyback puts on its driving source with
no outputs connected.  Commercial flyback testers measure the Q by
monitoring the decay time of a resonant circuit formed by a capacitor and
a winding on the flyback under test after it is excited by a pulse
waveform.  It is possible to easily construct testers that perform a
well.  See the companion document "Testing of flyback (LOPT) transformers"
for further information.

Chapter 24) High Voltage Power Supply Problems

  24.1) HV power supply fundamentals

Most, if not all, TVs derive the high voltage for the CRT second anode,
focus, and (sometimes) screen (G2) from the horizontal deflection system.
This technique was developed quite early in the history of commercial TV
and has stuck for a very simple reason - it is very cost effective.  A
side effect is that if the horizontal deflection fails and threatens to
burn a (vertical) line into the CRT phosphors, the high voltage dies as well.

Most TV high voltage supplies operate as follows:

1. Horizontal output transistor (HOT) turns on during scan.  Current increases
   linearly in primary of flyback transformer since it appears as an
   inductor.  Magnetic field also increases linearly.  Note: flyback is
   constructed with air gap in core.  This makes it behave more like an
   inductor as far as the primary drive is concerned.

2. HOT shuts off at end of scan.  Current decreases rapidly.  Magnetic field
   collapses inductively coupling to secondary and generates HV pulse.
   Inductance and capacitance of flyback, snubber capacitors, and parasitic
   capacitance of circuitry and yoke form a resonant circuit.  Ideally,
   voltage waveform across HOT during flyback (retrace) period will be a
   single half cycle and is clamped by damper diode across HOT to prevent

3. Secondary of flyback is either a single large HV winding with HV rectifiers
   built in (most often) or an intermediate voltage winding and a voltage
   multiplier built in or a separate unit (see the section: "What is a tripler?".)  The output will be DC HV pulses.  

4, The capacitance of the CRT envelope provides the needed filtering to
   adequately smooth the HV pulses into a DC voltage.

5, A high resistance voltage divider provides the several KV focus voltage
   and sometimes the several hundred volt screen (G2) voltage as well.
   Often, the adjustments for these voltages are built into the flyback.
   Sometimes they are mounted separately.  The focus and screen are
   generally the top and bottom knobs, respectively.

  24.2) What is a tripler?

In some TVs, the flyback transformer only generates about 6-10 KV AC which
is then boosted by a diode-capacitor ladder to the 18-30 KV needed for modern
color CRTs.  The unit that does this is commonly called a tripler since it
multiplies the flyback output by about 3 times.  Some TVs use a quadrupler
instead.  However, many TVs generate the required HV directly with a winding
with the required number of turns inside the flyback transformer.

Triplers use a diode-capacitor ladder to multiply the 6-10 KV AC to 18-30 KV
DC.  Many triplers are separate units, roughly cubical, and are not repairable.
Some triplers are built in to the flyback - it is probably cheaper to
manufacture the HV diodes and capacitors than to wind a direct high voltage
secondary on the flyback core.  In either case, failure requires replacement 
of the entire unit.

For external multipliers, the terminals are typically marked:

        IN - from flyback (6-10 KV AC).
        OUT - HV to CRT (20-30 KV DC).
        F - focus to CRT (2-8 KV).
        CTL - focus pot (many megohm to ground).
        G, GND, or COM - ground.

Symptoms of tripler failure are: lack of high voltage or insufficient high
voltage, arcing at focus protection spark gap, incorrect focus voltage, other
arcing, overload of HOT and/or flyback, or focus adjustment affecting
brightness (screen) setting or vice-versa.

  24.3) High voltage shutdown due to X-ray protection circuits

A TV that runs for a while or starts to come on but then shuts down may
have a problem with the X-ray protection circuitry correctly or incorrectly
determining that the high voltage (HV) is too great (risking excessive
X-ray emission) and shutting everything down.

A side effect of activation of this circuitry is that resetting may require
pulling the plug or turning off the real (hard) power switch.

Is there anything else unusual about the picture lately that would indicate
an actual problem with the HV?  If this is the case, then there may be
some problem with the HV regulation.  If not, the shutdown circuit may
be overly sensitive or one of its components may be defective - a bad
connection of leaky cap (or zener).

If the horizontal frequency is not correct (probably low) due to a faulty
horizontal oscillator or sync circuit or bad horizontal hold control (should
one exist!), HV may increase and trigger shutdown.  Of course, the picture
won't be worth much either!

One symptom of excessive HV (but not required) is an overly bright picture
of reduced size.

The HV shutdown circuit usually monitors a winding off of the flyback
for voltage exceeding some reference and then sets a flip flop shutting
the horizontal drive off.

On some Sony models, a HV resistive divider performs this function and these
do fail - quite often.  The big red Hstat block is a common cause of immediate
or delayed shutdown on certain Sony monitors and TVs.  See the section: "Sony TVs/monitors and Hstat".

  24.4) Low or no high voltage

Most of these problems are due to faults in the horizontal deflection
system - shorted HOT, shorted windings or HV rectifiers in the flyback,
defective tripler, or other bad parts on the primary side of the flyback.

However, if you discover an inch layer of filth inside the TV, the HV
could simply be shorting out - clean it first.

In most cases, these sorts of faults will put an excessive load on the
horizontal output circuits so there may be excessive heating of the HOT
or other components.  You may hear an audible arcing or sizzling sound from
internal shorts in the flyback or tripler.  Either of these may bet hot,
crack, bulge, or exhibit visible damage if left on with the fault present.

Most modern TVs do not regulate HV directly but rather set it via
control of the low voltage power supply to the HOT (B+), by snubber
capacitors across the HOT, and the turns ratio of the flyback.  The
HV is directly related to the B+ so if this is low, the HV will be low
as well.  Faulty snubber capacitors will generally do the opposite - increase
the HV and the X-ray protection circuits may kick in.  However, low HV
is also a possibility.  The only way the turns ratio of the flyback can
change is from a short which will manifest its presence in other ways as
well - excessive heating and load on the horizontal output circuits.

While a shorted second anode connection to the CRT is theoretically
possible, this is quite unlikely (except, as noted, due to dirt).

  24.5) Excessive high voltage

Any significant increase in HV should cause the X-ray protection circuits
to kick in and either shut down the set or modify the deflection in such
a way as to render it harmless.

Symptoms include arcing/sparking of HV, smaller than normal picture, and
under certain scenarios, possible excessive brightness.

Causes of the HV being too high are:

1. Excess B+ voltage to the HOT.  The likely cause is to a low voltage
   regulator failure.

2. Open snubber capacitors across the HOT.  These are under a lot of
   stress and are located near hot components so failure is possible.

3. Incorrect excessively long scan drive to HOT caused by failure of
   horizontal oscillator/sync circuits.  However, other things like the
   HOT will probably blow up first.  The picture will definitely be
   messed up.

4. Failure of HV regulator (tube sets and a few solid state sets - actual
   HV regulators are relatively uncommon today.)  This may result in an
   underscanned (smaller than normal) picture.

  24.6) Snaps, crackles, and other HV breakdown

Various problems can result in occasional or sustained sparking or arcing
sounds from inside the monitor.  Note that a static electricity buildup
is common on the front of the screen.  It is harmless and there iss nothing
you can do about it anyhow.

The following may result in occasional or sustained sounds not commonly
associated with a properly working TV or monitor.  There may or may not be
flashes or blanking of the screen at the same time as the audible noise.
See the same-named sections that follow for details.

* Arcing, sparking, or corona from CRT HV anode (red wire/suction cup).

* Arcing at CRT sparkgaps.

* Arcing from flyback or vicinity.

* Arcing due to bad connections to or disconnected CRT return.

* Flashovers inside the CRT.

  24.7) Arcing, sparking, or corona from CRT HV anode (red wire/suction cup)

Symptoms could include a sizzling corona or more likely, an occasional
or rapid series of sharp snaps - possibly quite loud and quite visible - from
the anode connection (at the suction cup) on the CRT to the grounded coating
on the outside of the CRT or a chassis ground point (or any other conductor
nearby).  Corona is a high resistance leakage through the air without total
breakdown.  The snapping is caused by the sudden and nearly complete discharge
of the CRT anode capacitance through a low resistance ionized path similar to

There are two likely causes:

1. Dirt, dust, grime, around and under the suction cup on the CRT are
   providing a discharge path.  This may be more severe in humid weather.
   Safely discharge the HV and then remove and thoroughly clean the HV
   suction cup and the area under it and on the CRT for several inches
   around the HV connection.  Make sure there are no loose wires or other
   possible places for the HV to discharge to in the vicinity.

2. The high voltage has gone through the roof.  Usually, the X-ray protection
   circuitry should kick in but it can fail.  If cleaning does not help,
   this is a likely possibility.  See the sections: "High voltage shutdown due to X-ray protection circuits" and "Excessive high voltage".

  24.8) Arcing from flyback or vicinity

Arcing may be visible or audible and result in readily detectable levels
of ozone.  Note that very slight traces of ozone may not indicate anything
significant but if the TV smells like an office copier, there is probably
some discharge taking place.

WARNING: It is possible for arcing to develop as a result of excessive high
voltage.  Symptoms might be a smaller than normal excessively bright picture
but this may not be able to be confirmed until the flyback is repaired or
replaced.  See the section: "Excessive high voltage".

* On the HV output, it will probably be a loud snapping sound (due to the
  capacitance of the CRT) with associated blue/white sparks up to an inch or
  more in length.  If the arc length is short enough, this may turn into a
  nearly continuous sizzling sound with yellow/orange arc and melting/burning

* Prior to the HV rectifier, it will likely be a continuous sizzle with
  orange/yellow/white arc and melting/burning plastic or circuit board

* Internal arcing in the flyback may be audible and eventually result in
  a bulging and/or cracked case (if some other component doesn't fail first
  as this would take some time to develop).

* A corona discharge without actual sparks or a visible well defined arc
  is also possible.  This may be visible in a totally dark room, possibly
  more likely when the humidity is high.  A thorough cleaning to remove all
  dust and grime may be all that is needed in this case.

* If the arc is coming from a specific point on the flyback - a crack or
  pinhole - this may be patched well enough to confirm that the rest of the
  TV is operational and a new flyback is worth the money.  Otherwise, there
  is no way of knowing if the arcing may have damaged other circuitry until
  a replacement flyback - possibly money wasted - arrives.

  To attempt a repair, scrape off any dirt or carbon that is present along the
  path of the arcing and its vicinity.  Then, clean the area thoroughly with
  alcohol and dry completely.  Otherwise, the dirt and carbon will just act as
  a good conductor and the arcing will continue under your repair!  Several
  layers of plastic electrical tape may be adequate for testing.  Multiple
  coats of high voltage sealer or non-corroding RTV silicone (if it smells like
  viniger - acetic acid - as it cures, this may get in and affect the windings)
  would be better if the objective is an actual repair.  This may prove to be
  a permanent fix although starting the search for a source for a new flyback
  would not hurt just in case.  The arc most likely did damage the insulation
  internally which may or may not be a problem in the future.

  Also see the section: "Dave's complete procedure for repair of an arcing flyback".

* In some cases, the pinhole or crack is an indication of a more serious
  problem - overheating due to shorted windings in the flyback or excessive
  secondary load. 

* If the arc is from one of the sparkgaps around the CRT or the CRT socket,
  this could also be a flyback problem indicating internal shorts in the
  focus/screen network.

* If the arcing is inside the CRT, this could indicate a bad CRT or a problem
  with the flyback focus/screen network and no or inadequate sparkgap

Where repair seems possible, first, clean the areas around the arc thoroughly
and then try several layers of plastic electrical tape.  If the TV works
normally for say, an hour, then there is probably nothing else wrong and you
can try for a proper sealing job or hope that tape holds out (put a few more
layers on - each is good for about 8-10 KV theoretically).

However, replacement of the flyback really is the best alternative to minimize
risk of future problems.  This is the only option where there could be a
potential issue of liability should subsequent failure result in a fire.

Once I had a TV where the main problem was a cracked flyback arcing
but this took out one of the fusable resistors for the power supply to
the *vertical* output so the symptoms included a single horizontal line.
Don't ask me to explain - replacing that resistor and the flyback (the
flyback tested good, but this was for someone else) fixed the TV.

In another case, a pinhole developed in the flyback casing probably
due to poor plastic molding at the time of manufacture.  This resulted in
a most spectacular case of sparking to a nearby bracket.  A few layers of
electrical tape was all that was needed to affect a permanent repair.

  24.9) Dave's complete procedure for repair of an arcing flyback

(From: Dave Moore (penguin@datastar.net).

First I clean the afflicted area with Electromotive spray from Autozone. It's
for cleaning alternators.  On Z-line I remove the focus control and wash with
the alternator cleaner and a tooth brush until all dirt and carbon deposits
are removed. Then I take an xacto knife and carve out the carbonized hole
where the arcing broke through. Then take your soldering iron and close the
hole by melting adjacent plastic into it. (clean any solder off your iron with
solder-wick first). Then cut some plastic off of some other part off the
flyback where it wont be needed and use this to plastic weld (with your iron)
a hump of a patch into and over the arc hole. Smooth and seal with iron. Next
apply as thick a layer of silicone rubber as you can and let dry overnight.

  24.10) Arcing at spark gaps and gas discharge tubes on CRT neck board or elsewhere

These are protective devices intended to breakdown and divert excessive voltage
away from the CRT (usually).

This is rarely due to a defective sparkgap or gas discharge tube but rather is
a safety mechanism like a fuse designed to protect the internal electrodes of
the CRT if the focus or screen voltage should become excessive.  The sparkgap
breaks down first and prevents internal arcing in the CRT.  These sparkgaps
may be built into the CRT socket as well.

Arcing at a sparkgap or a glowing or flashing discharge tube may be accompanied
by total loss of picture or bad focus, brightness or focus fluctuations, or
any of a number of similar symptoms.  A common cause is a breakdown inside the
focus divider (usually part of the flyback or tripler) but could also be due to
excessive uncontrolled high voltage due to a failure of the B+ regulator or HOT
snubber capacitor, or (ironically) even a short inside the CRT.

* Spark gaps may be actual two or three pin devices with seemingly no insides,
  part of the CRT socket, or printed on the circuit board itself.

* Gas discharge tubes look like small neon lamps (e.g., NE2) but could be
  filled with some other gas mixture to provide a controlled higher breakdown

Therefore, like a fuse, don't just replace or disable these devices, locate and
correct underlying problem.  The CRT makes an expensive fuse!

  24.11) Arcing due to bad connections to or disconnected CRT return

The Aquadag coating on the outside of the CRT is the negative plate of the HV
filter capacitor.  If this is not solidly connected to the HV return, you will
have your 25 KV+ trying to go where it should not be.  There should be a wire
solidly attached to the CRT neck board or chassis.  Without this, voltage will
build up until it is able to take some other path - possibly resulting in
damage to sensitive solid state components in the process.  Therefore, is is
important to rectify the situation.

Warning: If you find this disconnected, don't just attach it anywhere.  You
may instantly kill ICs or other solid state components.  It must be connected
to the proper return point on the CRT neck board or chassis.

  24.12) Flashovers inside the CRT

Due to sharp edges on the electron gun electrodes, impurities, and other
manufacturing defects, there can be occasional arcing internal to the
CRT.  Properly designed HV, deflection, and power supply circuits can
deal with these without failing but not all monitors are designed well.

There is nothing you can do about flashovers assuming your HV is not
excessive (see the section: "Excessive high voltage".  If these persist
and/or become more frequent, a new CRT or new TV will be needed.

  24.13) Ozone smell and/or smoke from TV

Smoking is just as bad for TVs as for people and usually more quickly

White acrid smoke may indicate a failed electrolytic capacitor in the
power supply probably in conjunction with a shorted rectifier.  Needless to
say, pull the plug at once.

A visual inspection should be able to easily confirm the bad capacitor as it
will probably be bulging and have condensed residue nearby.  Check the
rectifier diodes or bridge rectifier with an ohmmeter.  Resistance across
any pair of leads should be more than a few ohms in at least one direction.
Remove from the circuit to confirm.  Both the faulty diode(s) and capacitor
should be replaced (though the capacitor may work well enough to test
with new diode(s).

If a visual inspection fails to identify the smoking part, you can probably
plug the set in for a few seconds until the source of the smoke is obvious
but be prepared to pull the plug in a real hurry.

If the smell/smoke is coming from the flyback, then it has probably gone
belly up.  You may be able to see a crack or bulge in the case.  While
the flyback will definitely need to be replaced, it is likely that nothing
else is wrong.  However, it might be prudent to use a Variac when performing
initial testing with the replacement just in case there is a secondary
short circuit or excess HV problem.

  24.14) X-ray and other EM emission from my monitor?

X-ray radiation is produced when a high velocity electron beam strikes
a target containing heavy metals.  In a modern monitor, this can only
take place at the shadow mask/aperture grille and phosphor screen of the CRT.

For X-rays, the amount of radiation (if any) will be proportional to
brightness.  The energy (determined by the CRT high voltage, called KVP
in the medical imaging field) is not affected.  This is one reason many
monitors and TVs are designed with brightness limiting circuits.

In any case, there will be virtually no X-ray emissions from the front of
the CRT as the glass is greater than an inch thick and probably contains
some lead for added shielding.  Also see the section: "Should I be worried about X-ray exposure while servicing a TV or monitor?".

Electromagnetic radiation (EM) is produced mostly from the deflection yoke
and to a lesser extent from some of the other magnetic components like
transformers and inductors.  Depending on monitor design (some are
specifically designed to reduce this), EM emissions can vary quite a bit.
Frequencies range from the 50/60 Hz of the power line or vertical scan rate
to several hundred KHz in the AM broadcast band.  The intensity and spectral
distribution will vary depending on horizontal and vertical scan rate.

A totally black screen will reduce X-ray emission to zero.  It will not
affect EM emissions significantly as most of this comes from the magnetic
parts, particularly the deflection yoke.

There is no measurable microwave, IR, or UV radiation.

I refuse to get into the discussion of what, if any, health problems result
from low level EM emissions.  There is simply not enough data.

  24.15) Should I be worried about X-ray exposure while servicing a TV or monitor?

The only source of X-rays in a modern TV or monitor is from the CRT.  X-rays
are generated when a high velocity electron beam strikes a heavy metal target.
For anything you are likely to encounter, this can only happen in a vacuum -
thus inside the CRT.  The higher the voltage, the greater the velocity and
potential danger.  Really old TVs (prior to around 1975) may still have HV
rectifier and regulator tubes - other sources of X-rays.  However, modern TVs
and monitors implement these functions with solid state components.

The thick front CRT faceplate protects users adequately but there may be some
emission from the thinner sides.  At 25-30 KV (quite low as X-ray energies go)
X-rays will be stopped by almost any metal so what you have to worry about
is where there are no shields.

However, realistically, there is very little danger.  I would not worry about
exposure unless you plan to be sitting for hours on the sides, behind, or
under the TV or monitor - with a picture (there will be none if the screen is

It is interesting that even those 1.5" Watchman and .5" camcorder viewfinder
CRTs have X-ray warning labels even though the high voltage used with these
isn't anywhere near high enough to be of any concern!

  24.16) Flyback shot by 4 year old with water pistol

Your 4 year old son shot the Sony in the flyback transformer. Smoke and sparks
everywhere.  Great aim!

Who says these FAQs cannot be funny?

Needless to say, unplug the set immediately.  Inspect around the target
area for obviously blown or damaged components.  Test fuses and fusable
resistors.  Repair burnt solder connections and circuit board traces.
Once the set is entirely dried out, power it up - preferably through a
series light bulb and/or Variac until you are sure nothing else will
let loose.  Look, listen, and smell for any unusual behavior.  If it
now works, then consider yourself lucky.  If not, there may be damage
to transistors, ICs, or other components.

(From: Richard Symonds (edison@nelson.planet.org.nz)).

We're seeing another 'hazard' these days, people cleaning their television
screens with window cleaner - no problem in the days of separate chassis but
with the entire circuit board jammed under the tube on most TVs these days
just a few drips and its all over.  Some have just corroded the switch banks
(had one recently just got into the A/V switch - when you walked around the
room the set changed to A/V and back by itself!) but a few have got around the
microprocessors and surface mount components and resulted in complete
write-offs.  I suppose the damage is the opposite of electroplating as the
microprocessors have constant voltage to them.  Never mind, they'll be a good
source of parts for future use.

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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]