Notes on the Troubleshooting and Repair of Computer and Video Monitors


  13.42) How can I determine monitor specifications or whether it supports SVGA?

There is no easy way to tell by just examining the monitor visually.  Even
those with only a 9 pin rather than a 15 pin connector are sometimes SVGA
(e.g., Mitsubisthi AUM1381 and NEC Multisync II which will do 800x600 at
56 Hz V non-interlaced and 1024x768 interlaced at 43 Hz V).

You cannot even safety test scan rates on all monitors - some (mostly older
ones) will blow up or be damaged by being driven with incorrect video.

For a monitor that you already have, posting the model number or looking
it up is really the only way to be sure of its capabilities.

Quicky tests:

1. Check the video connector.  If it has a high density (VGA) 15 pin connector
   then there is a greater likelihood of SVGA but not always.

2. Check the manufacturing date on the back.  If it has a manufacturing date
   of 1991 or later, the likelihood of it supporting SVGA is higher as
   demand for VGA-only monitors was rapidly declining by this point.

3. Check the dot pitch on the CRT by examining the screen with a magnifier.
   If it is really coarse, the monitor probably cannot do anything beyond VGA.

4. Become familier with the major manufacturers and models so that you will
   recognize the common SVGA models.

While not conclusive, positive results on the first 3 of these tests definitely
increases the likelihood that it supports at least some SVGA modes.  Of course,
if you recognize a model number, you have dramatically increased your odds
of success - assuming it works!

The following URLs provide quick access to the general specifications of
many common PC and MAC compatible video monitors:

 http://www.nashville.net/~griffin/monitor.html http://www.hercules.com/monitors/mdb.htm http://hawks.ha.md.us/hardware/monitor.html http://www.mindspring.com/~nunez/info/monitors/

(From: Adrian Kwong (a.kwong@ieee.ca)).

Most new monitors employ frequency protection.  The symptom that you will
typically see is, a complete lack of video.  Most monitors with multicolored
power LED's, usually change color to indicate an error.  Some monitors like
Nokia's, will flash the screen on and off (black and white) to indicate that
the over-frequency protection circuits have been activated.

I have blown a few monitors by setting the video resolutions either too
high, or setting the vertical refresh to something that puts the horizontal
frequency waaay above the rated specifications.

I actually have no idea how some of these monitors actually received a UL or
CSA approval stamp, as I have seen some of these monitors catch on fire.  Most
of the 'blow outs', were just capacitors that exploded and about a room full
of smoke fills the vicinity.

All of the monitors that I blew up, were really old monitors with no frequency

  13.43) CRT replacement worth it?

The sad fact is that even if you can obtain a new CRT you won't have the proper
set up for getting proper alignment and convergence. They generally use various
permanent magnet glued to the perimeter of the yoke to set the geometry of the 
raster. It takes a special factory jig to do this step or really great
persistence and patience.  However, if you have the time and will resist
punching a hole in the new CRT before you finish, by all means.

Also, consider the cost of a new CRT may be more than half the cost of the
monitor when it was new.

Replacing a monochrome CRT is a snap in comparison.

A better (or at least less stressful) approach is to locate a monitor that died
due to a circuit problem and salvage the CRT including the yoke and all the
other magical magnets and coils.

(From: Andy Cuffe (baltimora@psu.edu)).

I have found that most 15" monitors use compatible CRTs.  I just put the CRT
from an old Gateway2000 with analog controls into a nice 2 year old monitor.
As long as the yokes and CRT sockets are similar it should work fine.  Don't
try to swap the yokes or you will never get it converged.

  13.44) An informal history of X-ray protection

(The following is from: Marty).

Most of the old tube type color TV sets used a shunt HV regulator tube,
usually a 6BK4. If it failed, or some component in the HV circuit failed, the
high voltage, normally 25KV, could go up to 35KV or more, causing some X-Ray 
leakage from the CRT. In the early 70s when news of this radiation scare was 
first announced, there was a public outcry to immediately fix the problem. The 
Feds hastily imposed a requirement on manufacturers of TV sets to somehow 
render a TV set "unwatchable" if the HV exceeded rated limits.

The manufacturers first response was to follow the letter of the law and the 
first "HEW" circuit simply blanked the video when the HV exceeded a setpoint 
to make the set "unwatchable".

It was quickly noticed that the HV was not turned off with this circuit and 
the CRT still could emit some radiation. Many TV sets with this feature were
left on  so the consumer could listen to the sound, so the feds tightened the 

By this time new TV sets were all solid state and some manufacturers 
experimented with HV shutdown circuits, but most of these circuits were poorly 
designed and not reliable.

Zenith thought they had the answer by regulating the HV with a bank of 5 
capacitors across the horizontal output transistor to "hold down" the HV to 
25KV. If one capacitor opened, the HV would only rise about 2KV, not a 
dangerous situation. This wasn't good enough for the feds.

The "fix" that Zenith finally came out with, was a "4 legged capacitor. Two 
legs were the emitter return for the horizontal output transistor, & two legs 
were the HV holddown capacitor (the equivalent value of the bank of 5 caps). 
This "fix" was accepted by HEW and millions of TVs were produced. It worked 
so well, that other manufacturers soon followed the lead (Magnavox, GE, etc.).

Then the worst happened! The 4 legged monsters started failing in a large 
numbers. Not opening completely & not shorting out. They sometimes allowed the 
HV to skyrocket to over 50KV. Some of them even cut the necks off of the CRTs.

Zenith issued a recall on those models with the problem (more than one entire 
model year). After several "improved" versions of the capacitor, the 
problem was fixed but that recall almost bankrupted the company. Other 
companies had failures too, but usually not as dramatic as Zenith's.

Magnavox used the HV holddown capacitor, both single & 4 leg version in 
several 70s era TV sets and is a good candidate for fireworks as well.

  13.45) Turning a TV (or monitor) into an oscilloscope?

This question comes up so often and it does sound like a neat project to
give a defunct TV a second life.  Don't expect to end up with a Tek 465
on the cheap when you are done.  However, it could be a fun learning

CAUTION: See the safety recommendations below.

You will be severely limited in the performance of such a scope.  TVs and
monitors are designed to operate at a very narrow range of horizontal scan
rates and the high voltage is usually derived from the horizontal deflection.
So, you would need to retain the original deflection system for this purpose
at least.

1. You will need to disconnect the defection yoke from the horizontal and
   vertical deflection circuits of the TV or monitor without killing the HV.
   (also, doing all this without killing yourself as well).  Depending on
   the design, this may be as simple as unplugging the yoke connector.  More
   than likely, you will need to substitute a load for the horizontal
   deflection coil. A coil from another sacrificial similar TV or monitor
   would probably suffice.

Warning: at this point you have a really bright spot in the middle of the
screen which will turn to a really black spot if the brightness is not turned
way down really really quickly.

2. For the horizontal, you need a ramped current source.  You are driving
   a non-ideal inductor (the deflection coil) so it has both inductance and
   resistance.  Thus the waveform is a trapezoid - a voltage ramp (for the
   resistive part) superimposed on a voltage step (for the inductive part).
   This should not be too difficult.  Don't expect to be able to achieve
   really fast sweep.  Even running at normal TV rates is non-trivial.

3. Similarly, for the vertical you need to drive with a voltage (your signal)
   controlled current source. However, if you just screwing around, then the
   linearity etc. for the vertical may not be that important.  In this case,
   one way is to put a current sensing resistor in series with the deflection
   coil and use this in a power op amp type of feedback arrangement.  (You
   could do this for (2) as well.

4. There is a good chance that the original brightness control will work
   as an intensity adjustment.  However, with some TVs and monitors, this
   depends on receiving a valid video signal.  You may need to improvise.
   If you do want to control the intensity from a signal source, you
   should be able to tap into the drive signals going to the little board
   on the neck of the CRT.

5. Don't expect high bandwidth, uniform response, or any of the other
   things you take for granted with a decent scope.  That takes work.
   However, as a fun project, this certainly qualifies.  Interchanging
   the functions of the horizontal and vertical deflection yoke (and
   rotating it 90 degrees) may provide a better match of horizontal
   and vertical bandwidth to your intended applications or experiments.

6. With a color TV or monitor, these experiments could be quite interesting
   and educational but there may be color fringing effects since you are not
   compensating for certain aspects of dynamic convergence at all.

7. SAFETY: Once you disconnect the deflection yoke from the TV or monitor's
   circuits, move the original circuits out of the way and put a barrier
   between between you and the rest of the TV or monitor. All you will need are
   connections to the deflection yoke on the CRT (unless you want to do
   intensity modulation in which case you will need to drive the video
   output(s) to the CRT cathodes.  I would recommend against doing this
   if your unit is one of those with a totally 'live' chassis as there would
   be additional safety hazards and circuit complications).

  13.46) Displaying a video signal as a picture on an oscilloscope

I am not sure why anyone would really want to do this other than as an
experiment - it would be interesting one.

If a composite video signal is the input, you will need a sync separator.
For VGA, the sync signals are already available.

You will have to construct a vertical deflection voltage ramp generator
which can be locked to your vertical sync signal.

The horizontal timebase of the scope will be fine for the horizontal
deflection and should easily lock to your horizontal sync pulse or
(if the scope has a TV trigger mode) directly to the video signal.

A video amplifier will be needed if your Z axis does not have an internal
amplifier (you need .7 V p-p to be full brightness range.)  Unless you provide
automatic gain control, this will need to include offset (brightness)
and gain (contrast) adjustments.  Even if there is an internal amplifier, it
may  not have the required bandwidth for the video signal.

However, the overall brightness may be disappointing - a scope is not designed
for overall high brightness.  The beam focus will not be as good as that on
a little TV either.

  13.47) Could a monitor be modified for 3D (stereo) display?

The whole idea of stereo 3-D vision to put the left and right views to the
appropriate eyeball.  There are two common ways of doing this:

1. Use different colors for the two views with color filters in from of
   each eye to separate the views.  This is what were often used for
   the really bad (content wise) sci-fi movies of the '50s.

2. Display alternate views on the same monitor screen but use LCD shutter
   glasses to allow each eye to only see the appropriate view.  This requires
   increasing the refresh rate to avoid unacceptable flicker.

The first approach can be used with any TV and a pair of monochrome
video cameras.  Of course, true color cannot be used since pure colored
images are needed to separate the stereo views.

Alternating views with synchronized LCD glasses is a possibility but
and has been used commercially but requires special hardware to
synchronize to the computer's video card.  Best results are obtained
with refresh rates of at least 120 Hz permitting 60 full left-right
frames per second.  If you try to this with a regular TV or CGA monitor,
the resulting refresh rate would be 30 Hz with a 50% duty cycle which is
likely to be useful only as a short experiment - else your viewers will
likely develop splitting headaches.

  13.48) Should I use a VGA to BNC cable if my monitor has BNC connectors?

(The following assumes a normal video card with a mini-DB15 VGA/SVGA
connector - if yours has BNC connectors, the improvement may be even

The answer is an unqualified maybe.  In principle, the BNC cable should have
higher bandwidth and better transmission line characteristics (impedance,
termination) and result in sharper crisper images with less ghosting,
ringing, and other artifacts.  However, this will only likely be significant
at higher refresh rates (1024x768 at 75 Hz and beyond) and depending on your
monitor and video card, you may see no change - or it may even get worse.  It
is best to purchase a good quality VGA to 5-BNC cable with a return privilage
and try it.  I suggest a 5-BNC cable even if you only need 3 or 4 connectors
so that it will be compatible with any monitor or video card you might have
in the future.  Cost should be in the $25 to $70 range.

Potential advantages of using the BNC connector inputs on your monitor
with a good quality cable are:

     * higher video bandwidth -> sharper display.

     * proper connectors (at one end, at least) and correct
       termination -> less ghosting and ringing.

For a good monitor with a high quality video card, the difference can be
dramatic - as is the case with my ATI GPT and NEC 5FG.

(From Bob Myers (myers@fc.hp.com)).

However, one should also note that connecting via BNCs generally disables
monitor "plug 'n' play" features, since these are based on ID information
conveyed on dedicated pins (using the VESA DDC & EDID standards) on the
15-pin "VGA" connector. 

As of last year, a new connector standard - the VESA Enhanced Video
Connector, or EVC - has been released, which will provide both greatly
improved video signal performance AND support for DDC and a number of
other features.

Most current monitors comply with the VESA Display Data Channel (DDC) standard
which provides a path and protocol for getting some basic ID information
(model, manufacturer, supported timings, chromaticites, etc.) back from the
monitor.  Under that standard, the following new signals have been added to
the DB-15 connector:

        Pin 9:  +5 VDC from host
        Pin 12: Serial data 
        Pin 15: Data clock 

Pin 10 (the old sync return pin) now does double duty as the return/reference
for DDC.  The DDC system uses the I2C spec for one level of implementation,
although a base level is also provided in which the data is clocked back
from the display by the vertical sync pulse.

The old 4-line ID scheme using pins 4, 11, 12, & 15 is obsolete.  I can't 
think of too many hosts, or ANY monitors, still using it.

Additional information on the EVC standard is available from the VESA web site,

And one manufacturer's way around the preceeding:

(From: Russ Smith (smith@ur-guh.com)).

The Nanao F2-21 I'm using is connected via 5 split-out BNCs
on its end; on the OTHER end is the standard VGA connector - that
connector plugs into not the video card, but a little "black box"
which performs the plug-n-play identification. That little widget
plugs into the PnP-compatible video card (Matrox Millenium).

Thus, even though BNCs are used at the monitor end and the monitor
itself can't communicate anything useful, the information is
none-the-less communicated.

A hack that works.

  13.49) Building a 5 BNC cable

This is straightforward, if time consuming and tedious.

The five coaxial cables (75 ohm, RG59 typical) are wired as shown in the
table.  The corresponding VGA connector pin numbers are in ().

     Coax Center         Coax Shield
    Red Video  (1)      Red Return (6)
    Green Video  (2)    Green Return (7)
    Blue Video  (3)     Blue Return (8)
    H Sync (13)         Ground (5,10)
    V Sync (14)         Ground (5,10)

Tie pin 11 (ID0) to Ground to indicate a color monitor.
Leave pin 12 (ID1) open.

Make sure that the lengths of the cables are fairly well matched - to within
a couple of inches - to assure that the 3 color channels line up precisely.
(One foot of cable is about 1.5 to 2 ns of delay which is significant for
a 10 ns dot clock!).

Also note (see the other sections on BNC cables) that you will lose your
Plug and Play capabilities without the direct control connections to the
monitor (or for monitors without these featuers).

That's it!

You will wish that your fingers were about 10 times smaller than they are,
however. :-)

  13.50) Using a workstation monitor on a PC

These are nearly always fixed frequency monitors with a scan rate that
is not compatible with typical SVGA cards.

They may have a special connector like a 13W3 or 3, 4, or 5 BNC
connectors.  Some have a non-standard connector.

While these normally use standard analog video signal levels,
you have a couple of problems out of the starting gate:

1. The fixed scanning frequencies of most of these monitors are not
   directly compatible with typical SVGA standards.  Many high end boards
   like the ATI ProTurbo can scan at 1280x1024 probably at an appropriate
   refresh (horizontal is going to be the critical one) rate.  Also, boards
   that allow software adjustment of size (like the ATI) are in effect 
   changing scan rates as well so that gives another degree or two of freedom.

   However, many typical video cards do not provide this degree of flexibility.

2. The monitor needs sync-on-green (3 BNC connectors), composite H and V
   sync (4 BNC connectors and 13W3) or at least a VGA to BNC adapter cable
   (5 BNC connectors).  Your VGA card normally puts out separate syncs.

   Many video cards have a software mode (probably accessible in the
   setup program) to enable composite sync output so for these at least
   there is no problem with a 4 BNC monitor.

   You can build a circuit to generate the required video for a 3 BNC
   monitor if you are so inclined.  See the "Sync on Green FAQ" for detailed
   information and schematics.

3. What you do for booting since the default will be VGA (at least for
   DOS/Windows.  If you only use your PC at one fixed high resolution,
   than this may not be that much of a problem..  

There are specialized boards that will emulate standard VGA/SVGA modes
using a fixed frequency monitor.  For more information, see the document:
"Notes on Approaches to using Fixed Frequency Monitors on PCs".

  13.51) Tweaking the deflection rate of a fixed frequency or non-standard monitor

Pulling a fixed frequency monitor by more than a few percent will likely
be a problem.  I know this is not the answer you were looking for but
getting a new inexpensive video card may be a better solution.

Other types of monitors - XGA for example - may be variable or multiple
frequency but incompatible with VGA/SVGA.  Some adjustment may be possible but
how far you can go will depend on many factors.

If not, you are looking for an adjustment called horizontal oscillator,
horizontal frequency, or horizontal hold.  If you do tweak, mark everything
beforehand just in case you need to get back to the original settings.

There is a slight risk of damage, particularly when lowering the horizontal
rate as this increases peak current to the horizontal output transistor.  This
may result in immediate failure or more stress on components resulting in
failure down the road.  I have no idea with your monitor.

An alternative that may be possible is to use the setup or install program that
came with your video card to decrease horizontal size and then adjust vertical
size if needed.  This would best be done while monitoring with a scope or
multiscan monitor.  A byproduct of software adjustments to size will often be
a change in the scan rate of a few percent which may completely cover what you
need.  The reason this may work is that these adjustments vary the length of
the H and V video back-porch which affect the total scan time.

I know I can do this with my ATI cards.

Also see the document: "Approaches to Using Fixed Frequency or Non-Standard
Monitors on PCs" which includes a specific modification to permit an IBM9517
XGA monitor to be used at VGA/SVGA scan rates.

  13.52) Displaying TV on a computer monitor

My general recommendation is that if you have the space, buy an inexpensive
TV - the quality in the end may in fact be better.  And, it will be usable
without tying up your expensive monitor and (maybe) PC.

Some older monitors like the Mitsubishi AUM1381 and Emerson CGA (which also
has a speaker) include a composite NTSC input jack requiring only a baseband
video source like a VCR.  These do produce a very nice picture.  However,
most newer auto-scan VGA/SVGA monitors do not go to low enough horizontal
scan rates.  To display NTSC or PAL on these requires a scan convertor
(likely to be very expensive) or at least a scan doubler (less expensive
but not as good).

For the case of older monitors with digital (TTL) inputs, see the section:
"Modifying a CGA (or EGA) monitor for NTSC or PAL input".

You can also buy video input cards complete with tuners ('PCTV') which
will put TV into a window and allow you to idle away the time you are
supposed to be working while watching 'Mork and Mindy'.

While various convertors are advertized to use a computer monitor
with video from a VCR or other source, keep in mind that if it sounds
too good to be true, it probably is like the claim of a $200 box for this:

OK, let me get this straight - this card/box will enable a 31.4 KHz horizontal
scan rate monitor (VGA) be used as a TV - yes or no?  It thus includes a video
A/D, full screen frame buffer, D/A, and all the other tuner stuff for under
$200?  I don't think so.  A scan doubler - which is a subset of the above -
will not result in a high quality picture since it will display pairs of
lines interleaved or leave alternate lines blanked reducing brightness.  Or
does the impressive advertisement leave out the key requirement that the
monitor sync at the NTSC horizontal scan rate of 15.734 KHz (most newer
monitor do not)?  Or is it a board that plugs into a PC and indeed does use
the resources of the PC including the VGA card and bus?

In any case, get a written money back satisfaction guarantee.

  13.53) Modifying a CGA (or EGA) monitor for NTSC or PAL input

These are often high quality monitors and would make nice TV displays -
especially as there are many no doubt gathering dust on their way to
the dumpster!

However, these are digital (TTL) monitors with respect to the video inputs
and proper linear video amplifiers may not even be present.  Therefore, you
may need to implement both the NTSC or PAL decoding as well as boosting the
signal levels to the hundred volts or so needed to drive the CRT.

The scan rate of CGA is the same as NTSC so deflection is not a problem.

For PAL (625/50) instead of NTSC, the vertical rate will need to be reduced
to 50 Hz but this should not be a problem.  The horizontal scan rate is close
enough (15.625 KHz).

Similar comments apply to EGA monitors that have a compatible scan rate.
EGA represents a range of scan rates between 15.75 KHz and 21.85 KHz so
this should not be a problem.

  13.54) Driving multiple non-daisy-chained monitors from one video source

It is not possible to just connect monitors in parallel.  The terminating
resistors (75 ohms) of each monitor will also be in parallel reducing signal
strength and resulting in various problems with cable termination including
ghosting, ringing, etc.

A simple circuit to implement a video splitter is shown at:


This is just a set of emitter following buffer amplifiers and should suffice
for many applications.  Various companies including Elantec, Analog Devices,
Maxim, and others have video amplifier chips as well but the basic approach
may be adequate for your needs.

  13.55) Displaying computer video on a TV

Assuming this means NTSC:

1. You need to convert RGB to NTSC - there are single chips for this.  Try
   Sony, Philips, Motorola, and others.  These will combine the R, G, B,
   H sync, and V sync into a single composite video signal using a minimum
   of additional components.

2. You need to match the scan rate to NTSC - 15.734 KHz horizontal.  Even
   basic VGA is twice this - 31.4 KHz.  If your video card can be programmed
   to put out interlaced NTSC rate video then this is easy.  If not, it is
   more difficult.  If you want to use anything higher res than VGA, it is
   a very non-trivial problem requiring the construction of a scan convertor
   which includes a video A/D, full frame store, interpolator/readout timing,
   video D/A.  Unless you are an experienced digital/analog designer, you
   really do not want to tackle any of this.

For the special case of VGA->NTSC, you may be able to get away with just
storing a single scan line since the horizontal frequency is (almost)
exactly twice the NTSC horizontal of 15.734 KHz.  A double buffer where
one buffer is storing while the other is reading out at approximately half
the VGA pixel rate should work. With appropriate timing, even lines become the
even field for NTSC and odd lines become the odd field (I may have this
backwards).  It is still not a trivial undertaking.  Also, keep in mind
that the quality you will get on NTSC will be poorer than the VGA due to
fundamental NTSC bandwidth limitations.  Also, flicker for line graphics will
be significant due to the interlacing at 30 Hz.  Even this is a non-trivial

The requirements for PAL are very similar.  For 625 lines systems, the
800x600 is the format that most closely matches the TV resolution.

You can also buy little boxes to do this.  Quality is general not great
as you are seriously limited by NTSC/PAL and the VCR.  Except for
presentations on existing TV rate equipment, it is probably not worth
the effort.  This is totally useless for any serious computer applications.

For professional presentations, modern video projectors are available that
use high resolution LCD panels and real-time scan conversion.  However,
they are quite expensive (up to $10,000!!!).

  13.56) What is Kell factor with respect to interlaced displays?

(From Bob Myers (myers@fc.hp.com)).

The Kell factor - which has to do with the fact that we're often undersampling
an image from the standpoint of the Gospel According to St. Nyquist - IS
a factor in the reduction of vertical resolution, but interlacing plays
a part as well.  This comes from at least two factors:

1. The monitor or receiver usually cannot precisely interleave the two fields.

2. More importantly, there are steps taken to reduce the interline flicker
   which reduce the effective vertical resolution.  This includes running the
   line width of the display somewhat larger than would otherwise be the case,
   and in interlaced cameras, discharging the entire screen (including the
   lines from the "other" field) after every field scanned.

Interlace is particularly troublesome on moving images, where you will often
perceive momentarily "missing" details.  There was a LOT of discussion
regarding the gory details of interlacing in the recent HDTV debates within
SMPTE and other groups.

  13.57) Weird phenomenon of the month

Talk about unusual.  This was posted to sci.electronics:

"Something VERY strange is happening, and I cant explain it.

There is a "ghost" on my TV screen of the text appearing on my
computer screen.  They are NOT hooked together in any manner.
They are about 4-5 feet apart.  Although, the antenna cable
runs within a foot of my computer.  I am wondering what causes
this to happen.  I have experienced interference, but this is more 
like a wireless second monitor.  I can turn off my monitor, and 
look over at the TV.  The text on the TV is scrolling up every 9
seconds. (like when the v-hold isn't adjusted.) Any Ideas?"

This is probably caused by RFI - radio frequency interference - from
a CGA or PC TV card being picked up on the TV's cable or antenna.  Only
CGA has a scan rate that is nearly the same as NTSC.  Any other PC
video scan rate would result in a torn up or rolling picture.

  13.58) Ultra cheap degaussing coil

Pack Rat Trick #457384

Next time you scrap a computer monitor (or tv), save the degaussing
coil (coil of wire, usually wrapped in black tap or plastic) mounted
around the front of the tube.  To adapt it for degaussing sets, wrap it
into a smaller coil, maybe 4"-6".  To limit the current to something
reasonable, put it in series with a light bulb (60-100W).  You need
AC current to degauss, so just put the bulb in series with the coil and
use the your local 120V outlet.  BE VERY CAREFUL that you actually
wired it in series, and that everything is properly insulated before
you plug it in  (A fuse would be a real good idea too!!)

A few circles over the affected area will usually do it.  Note that
it will also make your screen go crazy for a little bit, but this will
fade out within a minute or so.

Just a couple of points for emphasis:

1. The coil as removed from the TV is not designed for continuous operation
   across the line as indicated above.  In fact, it will go up in a mass
   of smoke without the light bulb to limit the current. The poor TV from
   which this organ was salvaged included additional circuitry to ramp
   the current to 0 in a few seconds after power is turned on.

2. Reducing the coil size by a factor of 2 or 3 will increase the
   intensity of the magnetic field which is important since we are limiting
   the current with the light bulb to a value lower than the TV used.
   You don't need to unwind all the magnet wire, just bend the entire
   assembly into a smaller coil.  Just make sure that the current is always
   flowing in the same direction (clockwise or counterclockwise) around the

3. Insulate everything very thoroughly with electrical tape.  A pushbutton
   momentary switch rated for 2 amps at 115 volts AC would be useful so that
   you do not need to depend on the wall plug to turn it on and off.

  13.59) Bob Myers notes on degaussing

A couple of comments: first of all, it makes no difference whatsoever
if the display is on while it's being degaussed.  (Oh, some people
DO like to watch the psychedelic light show, but it really doesn't
help anything for it to be on.)  Actually, there is a very minor case
to be made for degaussing while OFF, at least for the Trinitron and
similar tubes.  (The field of an external degauss coil CAN cause the
grille wires to move slightly, and they're a bit more flexible when
hot - so it is conceivable, although certainly unlikely, that you're
running a higher risk of causing the grille wires to touch or cross
and become damaged.)

Secondly, a good practice for degaussing is to slowly back away from
the monitor after giving the screen a good going over.  Once you're
about 5-6' away, turn the coil so it's a right angles to the CRT
faceplate (which minimizes the field the monitor is seeing), and THEN
turn to coil off.  This is to reduce the possibility of the field 
transient caused by switching the coil off from leaving you once again
with a magnetized monitor.

The last point is to make sure that you DON'T leave the coil on too long.
These things are basically just big coils of wire with a line cord
attached, and are not designed to be left on for extended periods of
time - they can overheat.  (I like the kind with the pushbutton "on"
switch, which turns off as soon as I release the button.  That way,
I can never go off and leave the coil energized.)

Oh, one more thing - make sure your wallet is in a safe place.  You
know all those credit cards and things with the nice magnetic stripe
on them? :-)

(Actually, I've got a good story about that last.  I was teaching a
group of field service engineers how to do this once, and being the
Big Deal Out of Town Expert, made VERY sure to place my wallet
on a shelf far away from the action.  Unfortunately, Mr. Big Deal Out
of Town Expert was staying in a hotel which used those neat little
magnetic-card gadgets instead of a "real" key.  Ever try to explain
to a desk clerk how it was that, not only would your keycard NOT let
you into your room, it was no longer anything that their system would
even recognize as a key? :-))

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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]