Notes on the Troubleshooting and Repair of Computer and Video Monitors


  12.17) Setup menus will not go away or hieroglyphics on screen

Both these problems could be caused by a faulty microcontroller or
its associated circuitry.  However, bad connections in the vicinity
of the controller logic could also be at fault.

Unless you see something obvious, you will need schematics.

  12.18) Setup Adjustments Lost

Many modern monitors have RAM, somewhat like the CMOS SETUP memory in your PC,
that store all factory adjustments.  When power is lost, there is power
surge, lightning strike nearby, nuclear detonation or EMP, it may
have put bad information into the ram and thrown it out of adjustment.  There
is a way to get into the service mode (depress and hold a secret button
down and turn set on, special combination of buttons on the remote, etc.)
and then use the remote to reinitialize and adjust the problems out.


The service manual will be essential to have any chance of successfully
reinitializing everything without causing damage due to incorrect settings.

If it's not an adjustment problem you probably have a bad part - somewhere.

If you do manage to get into the setup menu and are willing to take the
risk without service information, try not to make any unnecessary changes
and document every change you make!!!  That way you can go back if you do
anything wrong (hopefully).

  12.19) Monitor doesn't work after being in storage

So the monitor you carefully stuffed in a corner of the garage is now totally
dead.  You swear it was working perfectly a year ago and just have to get
that state-of-the-art Commodore 64 up and running!

Assuming there was absolutely no action when you turned it on, this has
all the classic symptoms of a bad connection.  These could be cold/cracked
solder joints at large components like transformers, power resistors, or
connectors and connectors that need to be cleaned or reseated.  By 'no action'
I mean not even a tweet, bleep, or crackle from anything.

To narrow it down further, if careful prodding with a well insulated stick
does not induce the set to come on, check the following:

1. Locate the horizontal output transistor.  It will be in a TO3 metal
   (most likely on an older set) or TOP3 plastic package on a heat sink.
   With power off, measure collector to emitter with an ohmmeter - in at least
   one direction it should be fairly high - 1K or more.  Then clip a voltmeter
   on the 250 V DC or higher scale across C-E and plug in and turn on the set.
   Make sure it is well insulated.

   * If the problem is in the low voltage (line) power supply, there will
     be no substantial voltage across C-E.

     You should be able to trace from the power line forward to find the bad
     part though a schematic will help greatly.

   * If the problem is in the startup circuit or horizontal oscillator/driver,
     then there will be something on the order of 100-160 V across C-E.

     In this case, a schematic may be essential.

Note: don't assume that the metal parts of the chassis are ground - they
may be floating at some line potential.

There is also a slight chance that there is a low voltage regulator
in addition to the horizontal output, so don't get them confused.  The
horizontal output transistor will be near the flyback transformer and
yoke connector.

  12.20) Cheap monitors with multiple intermittent problems

If the monitor is a non-name or the company has since gone belly up (no
surprise, right?) you may have a monitor with one of those circuit boards
best described as bad solder joints held together with a little copper.
In this case, prodding with an insulated stick and the use of a few select
4 letter words may get it going.  The circuit boards may be double sided with
what were called 'rivlets' for vias.  The rivlets were relatively massive -
literally little copper rivets - and they were not adequately heated or
tinned during assembly so there were bucketloads of cold solder joints
that show up during middle age.  I repaired one of these by literally
resoldering top and bottom of every one of the darn things with a high
wattage iron.  Or, the soldering just may be plain, well, horrible.  Carefully
going over every connection is the only solution.  Sometimes, removing the
solder from suspect joints, cleaning both the component lead and trace, and
then resoldering will be needed if corrosion has set in.

  12.21) Monitor has burning smell

Assuming there are no other symptoms:

If this appears after extended operation - an hour or more - it may
just be a build up of dust, dirt, and grime over the years.  After
understanding the safety info, some careful vacuuming inside may help.
Just don't be tempted to turn any screws or adjustments!

Dust is attracted to the high voltage section in particular - even the
front faceplate of the CRT collects a lot and should be wiped with a damp
cloth from time to time.

If the symptoms develop quickly - in a few minutes or less, then there
could still be a dust problem - a power resistor may be heating a wad of
it but other possibilities need to be considered.

If not dust, then probably in the power supply but realize that TVs don't
have a nice metal case labeled 'power supply'.  It is just a bunch of stuff
scattered around the main board.  Without identifying the part that is
heating, a diagnosis is tough especially if the set really does
work fine otherwise.  However, if a series regulator were faulty and putting
out too much voltage, the set could appear to work properly but in fact
have excessive power dissipation in certain components.  If cleaning the dust
does not solve the problem, you will probably need a schematic to identify
the correct voltages.

  12.22) Static discharge noise and picture tube quality

This question came up with respect to a large screen TV but may apply to large
screen monitors as well.

"I bought a 29" TV a couple of weeks ago and I have noticed that after being
 switched on for > about 15/20 minutes, whenever the picture changes from a
 "light" scene to a darker scene, the set makes a crackling noise. It sounds
 as though there has been a build-up of static and it is being discharged. I
 have never noticed this in a TV before and I was wondering if this is normal
 and acceptable behaviour for a large-screen TV?"

It probably is normal.  Whether it is acceptable is a personal matter.  In
some geographic areas no countermeasures are taken at all... 

When the scene changes from bright to dark, the beam current is reduced to
practically zero. As a result, the high voltage rises. (The high voltage
supply has a relatively high internal impedance.) The high voltage is
connected to the inside layer of the picture tube. A voltage change on the
inside will also cause a voltage change on uncovered parts of the outside,
especially on the part of the picture tube that is hidden under the deflection
coils. This causes little sparks between the picture tube surface and the
inside of the deflection coils and this is accompanied by a crackling sound.

On the better picture tubes, a dark "anti-crackle coating" is painted on the
picture tube near the deflection coil.  This is a very high impedance coating,
dark black, much darker than the usual aquadag coating over the rest of the
picture tube. You should be able to see the difference.

If, on the other hand, the outside of the picture tube near the deflection
coil is not coated then you have a problem. Then you will hear strong
crackling also at switch-on and switch-off. Normally you shouldn't see such a
'cheap' picture tube on the European market...

The area of the picture tube around the anode connector is also not coated,
for obvious reasons. Normally that should not cause any significant
sound. Same goes for the front of the screen and neither should the anode
cable crackle.

In a dark room you should be able to see from the tiny blue flashes where the
sound comes from. This is perhaps best observed at switch-on and switch-off
(with a black picture on the screen). Try and keep the back cover mounted !

  12.23) Loudspeakers and monitors

Loudspeakers incorporate powerful magnets - the larger the speaker, the
larger the magnet.  However, anyone who goes ballistic when the mention
is made of a loudspeaker near a TV or monitor, should take their Valium.

The fringe fields outside the speaker box will not be that great.  They
may affect the picture perhaps to the point of requiring degauss.  The
normal degauss activated at power-on will usually clear up any color purity
problems (assuming the loudspeakers have been moved away).  At worst, manual
degauss will be needed.  The CRT will not be damaged.  The maximum
field - inaccessible at the voice coil -  is quite strong.  However, even
for non-shielded loudspeakers, the magnetic field decays rapidly with
distance especially since the core structure is designed to concentrate
as much of the field as possible in the gap where the voice coil travels.

Speakers specifically designed for use with multimedia computers have
(or should have) specially shielded magnet structures which will minimize
these effects.  However, if you see any indication of discoloration, move
them to a greater distance.

However, keeping unshielded (e.g., megawatt stereo) speakers away from
CRTs is a good idea.

Now, you really should keep your superconducting magnetic resonance imager
magnet at least in the next room.....

  12.24) Should I replace all the electrolytic capacitors if I find a bad one?

When a bad capacitor is found in a monitor, the question of course arises
as to the likelihood of other capacitors going bad in short order.
It might be worth checking (other) caps in the power supply or hot
(temperature) areas but you could spend you whole life replacing **all**
the electrolytics in your older equipment!

  12.25) Black powder being generated inside monitor?

You have just noticed a black powder spontaneously appearing from inside
your computer monitor.  What is it?  The monitor seems happy as a clam.

Well, it is probably just air-born dust that is collecting there due to
the air flow in your area and high voltage static fields.  The monitor is
acting like an electrostatic dust precipitator.  If there were really black
powder being generated inside, I would expect you would smell something
really really bad and the monitor would not continue to be happy.

  12.26) Sweet little old ladies and TVs from attic

The following story is specifically for a TV but the same applies to any
electronic servicing.  Always confirm the customer's complaints first!!

Then verify that everything else works or you will never know if your
efforts have affected something unrelated.

(Original request from rogerj@apex.com):

>A sweet little old lady has duped me into repairing her old G.E. 13" color
>TV. Wanted me fix bad volume pot..... "oh it has such a good picture"...
>she says.

>Stupidly w/o even turning it on, (big mistake) I begin to open the set.
>After 15-20 min. of travail, I discover that a previous "repairman" has glued
>the case shut!

>Now w/ set open, I turn it on and this picture is LOUSY. Bad color, and very
>poor convergence. But I don't know if I'm to blame for banging it around 
>trying to open it up. Also, no hor. or vert. hold. (fixed that w/a few caps)
>This things probably been sitting around for a few years.

Well, you certainly did not kill the caps.  Anything that sits for a few
years - probably in a damp unheated attic - is suspect.

Did you find the adjustments on the yoke assembly tight?  If so, you probably
did not move anything very much either.  She may remember the good picture
it produced before being stuffed away in the attic.

> Anyway after going through all the adjustments, the convergence at the sides
> is still bad and the horizontal size is a tad insufficient (w/no adjustment
> available)

Could be that the convergence (including pincushion) circuits are still
faulty - not just misadjusted.

Other things that can effect horizontal size while still giving you a complete

1. Voltage to horizontal output transistor low.  Is there a voltage regulator
   in your set?  The one I have has none.  I assume your line voltage is ok.

2. Increased resistance or inductance of the yoke windings.  For all
   you know, the yoke may have been replaced with the wrong part.

3. Yoke improperly positioned on tube neck.

4. Excessive high Voltage.  This is usually not adjustable.

I bet the thing hasn't worked properly in 10 years.

  12.27) Disposing of dead monitors (CRTs and charged HV capacitors)

I don't know what the law says, but for safety, here is my recommendation:

Treat the CRT with respect - the implosion hazard should not be minimized.
A large CRT will have over 10 tons of air pressure attempting to crush it.
Wear eye protection whenever dealing with the CRT.  Handle the CRT by the
front - not the neck or thin funnel shaped envelope.  Don't just toss it
in the garbage - it is a significant hazard.  The vacuum can be safely
released (Let out?  Sucked in?  What does one do with an unwanted vacuum?)
without spectacular effects by breaking the glass seal in the center of the
CRT socket (may be hidden by the indexing plastic of the socket).  Cover the
entire CRT with a heavy blanket when doing this for additional protection.
Once the vacuum is gone, it is just a big glass bottle though there may be
some moderately hazardous materials in the phosphor coatings and of course,
the glass and shadow mask will have many sharp edges if it is broken.

In addition, there could be a nice surprise awaiting anyone disconnecting the
high voltage wire - that CRT capacitance can hold a charge for quite a while.
Since it is being scrapped, a screwdriver under the suction cap HV connector
should suffice.

The main power supply filter caps should have discharged on their own
after any reasonable length of time (measured in terms of minutes, not
days or years).

Of course around here, TVs and monitors (well, wishful thinking as I
have yet to see a decent monitor on the curb) are just tossed intact
which is fortunate for scavengers like me who would not be happy at
all with pre-safed equipment of this type!

  12.28) Apple/Sony monitor dies after variable length of time

The following discussion relates to failures of the X-ray protection tap
on a Sony part affectionately known as the 'big red cap' or the HSTAT
block in some Sony manufactured monitors.

"This is a (Apple) Sony 13" monitor, 4 years old.  After being turned on
 for 30 minutes, the display goes completely blank and the front LED goes
 off.  If the power is shut off for 10 minutes or so, it will come back on
 for another 15 minutes or so, then go blank again, etc.  The +120v and
 +65v from the power module is still present when it blanks out, but no
 other voltages (+12, +960, etc) are present on the main circuit board. 
 I've been told it might be the HV capacitor is bad; would like to hear a
 2nd or 3rd opinion before buying a new capacitor."

That is the same diagnosis a friend of mine got for her monitor with that
identical problem.  Replacing the capacitor did fix the problem.

That 'big red capacitor' is a Sony part which includes some kind of low
voltage sense connection as well.  It is used to shut the monitor or TV
down should the HV increase resulting in increased risk of X-ray generation.
Unfortunately, the resistors inside often go bad causing the unit to shut
off erroneously.  The guy at the place where she got it repaired said that
the capacitor is one of the most common problems with those monitors.  $70
for the part + $50 for labor, ouch!

These used to be only available from Sony.  Why can't Sony design monitors
like everyone else?  Sure, I know, theirs are better (well, except for the
unsightly stabilizing wires on Trinitrons!).  Now, however,  less expensive
replacements can be had at places like Computer Component Source.

For testing, it may be possible to disconnect the sense output.  With shutdown
disabled, the monitor should continue to run BUT WITH NO X-RAY PROTECTION.
Therefore, this should only be used for testing - a replacement will be

Note: On some models, the sense wires need to be connected during startup
or else it will never come on.

CAUTION: On some models (like the Sony CPD1302), the sense signal may be used
for actual HV regulation.  Thus, if the sense wire is disconnected, (or the
divider inside the Hstat block fails open) there is no feedback and it is
possible for the high voltage (and probably B+) to increase until the HOT
(and possible other components) blow.

(From: Duke Beattie (beattie@wsu.edu)).

The low voltage connection of the 'big red cap' is part of the "X-ray
protection" circuit.  If the high voltage to the crt goes to high it is
supposed to shut down the whole thing.  Unfortunately the sensor inside
goes bad and puts out the wrong voltage and that shuts down the world.
The part is available at "Computer Component Source" for about $30, it is
a "M041" (Sony/Apple part number"  These things go out with great regularity.
So if your Apple monitor shuts down this is probably the culprit.

(From: A.R. Duell (ard12@eng.cam.ac.uk)).

On some of the older Trinitrons (certainly on the 13" Trinitron monitor
I have), the HSTAT pot is connected as a potential divider on the EHT 
supply. The slider of the pot is connected to the static convergence 
electrode, but a tap on the lower end of the pot goes to the protection 
circuit.  Something like this: 

            Static Conv Electrode
                                     |             |
                                     o            _|_
                                 Protection      ///

If the EHT rises too high, then the voltage at the protection point also 
rises, and a shutdown signal is sent to the scan processor.

All those resistors are encapsulated in the HSTAT block which has an EHT 
input from the flyback, a Coaxial EHT output (EHT and Hstat electrode) to the
CRT, an earth wire, and a 2 core cable (earth and Protection) that goes 
to the scan board.

Unfortunately, if those resistors change in value, then the protection 
circuit may operate even at the normal EHT voltage. And as they're all 
potted in one block, you have to change the complete unit.

(From: Neil brown (nbrown@whispa.co.nz)).

When your monitor works do you see faint diagonal white line on it?

If so the cutoff need adjusting and it will cause the symptoms you
describe exactly, If it doesn't come on after a "rest" then yes it may be a
bad cap but I have realigned a lot more than I have replaced HV caps!

Also on the adjustment board there is a resister that goes and pushes the
cutoff up high, from memory it is a 1 M resister and it drifts up high.

  12.29) More on the Apple/Sony 'big red capacitor thing'

(From Terry L. Wright (terryl@wolfenet.com)).
The big red thing has been called a capacitor, a voltage tripler and a
diode assembly not to mention other less polite names.  It is in fact at
the root of the failure in this monitor but does not necessarily need to be
replaced.  You will find a low voltage shielded wire comes from the red
block.  It goes to a four lead jack and plug which connects to the main
board.  The two pins that the shielded cable goes to are marked ground and
Href, short for high voltage reference.  If these two pins are shorted
together the unit will no longer shut off by itself.

Why does this work?  Because the red block contains a voltage divider, the
output of which tells the main board if the 25 Kilovolt supply to the crt
goes too high.  When the red block ages the relative values of the internal
resistors changes and the block output increases.  The main board
interprets this as excessive high voltage and shuts the horizontal output
down to protect the circuit and ostensibly to protect from Xrays.  By
shorting the output you can force the main board to assume that the voltage
is not too high.  Note that you have also disabled any protection that the
circuit may have provided from Xrays or high voltages.  Personally I do not
care about this as I have never seen this monitor fail in any way to cause
excessive second anode voltage.

Editor's note: failure (open) of a snubber capacitor across the HOT is one
failure that can result in excess high voltage.  Thus, I would consider this
a temporary 'for testing' solution unless you add some other mechanism for
detecting excess high voltage.  First confirm with a high voltage probe that
the monitor isn't shutting down properly - due to excess high voltage!  In
addition, the original problem may get worse and eventually affect the
convergence and other functions of the Hstat unit.  --- sam

(From: David J. Pittella (ddc_pitt@ix.netcom.com)).

I spent 8 years working for a very large Apple authorized service provider.

The original 13" Model MO-401 (not the MO401LL/B) actually had a bad run of
these high voltage capacitors. Apple did have a warranty extension on specific
date ranges of these parts,  I would doubt this is still in effect ... but you
could check.

The 'big red' high voltage capacitor is Apple P/N 910-0058, it is mounted
to the bottom of the chassis on this display. This part connects between
the flyback and the anode connector on the CRT, there is also small grey
cable from this device to the "D" (main) board.

The "C" board (on the neck of the crt) is notorious for cold solder joints
on the CRT connector.  I would always resolder these whenever I worked on
this display.

  12.30) CTX monitor intermittent or blows fuse

Initial symptoms are erratic startup or shutdown sensitive to temperature
or vibration.  Eventually, the monitor will go totally dead if the original
problems are not dealt with.

Look for a vertically mounted daughterboard. This board contains an IC
UT3842 which is the pulse width modulator IC for the switcher supply. ECG
makes a replacement although I don't have the number handy. Make sure
you check associated parts on this card for damage, as this circuit
usually fries pretty well.

The entire cause of these problems is generally bad solder joints on the
back side of that daughter board.  Unsolder it from the main board, and fix
those first.  Where a connector is used (P104) resolder this as well.  Then
replace Q101, the 18 V zener next to it (ZD101), and the .39 ohm resistor if
necessary.  Note: The zener is for protection only.  Therefore its exact
voltage rating is not critical - anything over about 6 V will work.

(From: Keith Scott (kscott@news.HiWAAY.net)).

Exactly! Every 14 or 15" CTX I've worked on had the MOSFET, zener and the
low ohm resistor toasted. BTW, they use the low ohm resistor as a fuse to
keep them catching on fire when the other stuff shorts out. 

(From the editor).

Once the fuse blows, several parts have gone belly-up and will need to be
replaced in addition to the soldering of the daughter board.

(From: Bill Rothanburg (william.rothanburg@worldnet.att.net)).

Replacing the fuse will not fix the monitor.  The odds are rather overwhelming
that you have been bit by the infamous CTX 'daughter board with bad solder
joint' flaw.  If you have the ability to handle a soldering iron, order the
repair kit from  CCS (1-800-356-1227).  This will contain all of the parts
and instructions on fixing this problem.  IMPORTANT!!!  Remove the daughter
board, resolder all of the joints on the connector, and reinstall the daughter

CCS sells a kit for $13.99, includes 2SK955, 1N5248 18V zener, .39 R, and
fuse.  #07-1512  800 356-1227   They also warn of solder breaks on plug
of daughter board.  The service manual is available from CTX for $15,
800 888-2120 (compared to $50 from CCS!!).

  12.31) Gateway Crystalscan CS1572 jiggling

"I have a Gateway CS1572 FS monitor.  Recently, a high pitched whine
 accompanied by faint dark lines scrolling from top to bottom appeared.
 Initially the problem disappeared after a warm-up period, but now it is
 constant.  Can anyone give me info on: solving similar problem, or a
 source for schematics on this type of monitor.  Gateway wants me to send
 it to MAG, but that sounds like big $$$."

Other related symptoms: Wiggling raster, possibly only at higher scan rates.

R331 is a common failure in the power supplies of Gateway CS1572 monitors.

It is supposed to be 91K. 1 W but gradually increases in value until regulation
is compromised.  While it is marked 1%, hand selecting a 5% metal film resistor
that is within tolerance will work fine and even this may not be needed as
the voltage adjustment pot is in series with R331.  Therefore, if you have
the adjustment procedure, a 1% resistor is unnecessary in any case.  Then,
adjust the B+ to the value marked.

Note: It is probably a good idea to replace R331 for these symptoms even if
it tests good.  In some cases, it would appear that these resistors fail at
full voltage but not when tested with a multimeter.

If symptoms persist, check ZD302 (12.2 V?).

While you are in there, check for bad solder connections or damage to R302
and Q105 (swivel base hits these).

Chapter 13) Items of Interest

  13.1) How do multiscan monitors determine and store the scan parameters?

With modern SVGA multiscan monitors, once a particular resolution and scan
rate is set up, there is rarely a need to readjust size, position, and other
parameters.  How is this accomplished?

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

It's different for different designs, of course, but in general today's
'digitally controlled' monitors recognize various timing modes by counting
the horizontal and vertical sync pulses to determine the line scan and
vertical refresh rates.  Any input within a certain tolerance of a recognized
pair of frequencies here is assumed to be that timing, and a set of stored
numbers corresponding to that timing are then read from a memory and used
to set up the adjustments.  In most of these monitors, the various adjustable
parameters - size, centering, etc., - are controlled by voltages coming from
a set of D/A converters, so the stored information is basically just a table
of numbers that get sent to the D/As when that timing is recognized.

The number of both factory and user presets available varies from product
to product, of course, but there's usually somewhere between 8-15 of each.
The exact number is going to depend on how much memory is available, and how
many different parameters need to be controlled for each recognized timing.

Unless the output of the graphics controller is an exact match for the
timing used at the factory when the preset information was generated, there
may still be slight errors, for obvious reasons.  Fortunately, the widespread
acceptance of timing standards (such as those produced by VESA) are
reducing the severity of this problem.

  13.2) Monitor reliability with SVGA

There are parts in the monitor which may get hotter with SVGA but if it is
designed for SVGA resolution, there should be no problem (assuming you are
not running in an excessively hot room or with the ventilation holes covered).

A good quality auto-scan monitor should not mind switching screen resolutions
frequently (though doing it every few seconds continuously may stretch this
a bit).

Newer auto-scan monitors should also be smart enough not to blow up if
you feed then a scan rate which exceeds their capabilities.  However,
there are a lot of poorly designed monitors out there.

If it is supposed to run SVGA, use it at SVGA.  If it blows up,
switch to a different brand.  There are a lot of crappy monitors being
sold on their own and bundled with PCs.

  13.3) How high a refresh rate should I use?

It is the vertical refresh rate that impacts display appearance.  The visual
effect of too low a vertical scan rate is excessive flicker.

Up to a point, higher is better.  Everyone agrees that appearance improves
up to at least 70-75 Hz (vertical) non-interlaced but beyond this point is a
hotly debated issue (and a topic for a never ending discussion on your
favorite Internet newsgroup).  The use of interlaced scanning can reduce
apparent flicker for a given scan rate for typical gray scale or color images
but may result in annoying flickering or jumping of fine horizontal lines in
graphics and text displays.

In any case, you must not exceed the maximum scan rate specs of your monitor
and very high refresh rates may result in decreased graphics performance
particularly with DRAM based video cards due to bus contention between the
PC memory accesses and the video readout to the RAMDAC.

  13.4) Number of colors and monitor type

"I have a CTX CVP-5468 that will not do more than 16 colors in windows. It 
 is being driven by an Orchid Kelvin 64 VLB board, but had the same problem
 with an ATI card. When using it in linux under x-windows the same thing and
 more than vga and it goes blurry and very pixelated."

It is really not possible for this to be a monitor problem as the signals
are analog - continuous - the monitor displays whatever it is given and
does not even know the color depth except to the extent that cards are often
set up via software to use different scan rates for different color depths
(bits/pixel) often due to hardware memory/bandwidth limitations.

For the ATI in particular, I know that you can use ATI's DOS Install program
to set it up for each resolution and mode - try this.  I bet your monitor
is fine.

  13.5) Monitors, humans, and flicker

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

The flicker-fusion frequency for emissive displays such as
CRTs cannot be given as a single number applicable to all people, all 
displays, and all ambient conditions.  It is dependent on the
particular individual, the size and brightness of the display (and
the characteristics of the phosphor, if a CRT), the viewing distance,
and the ambient lighting conditions.

For a typical color CRT computer monitor, at typical brightness levels
and viewing distances, the image will appear "flicker free" to 90%
of the population by the time the refresh rate has reached the upper
70 Hz range; into the low 80 Hz range, and you cover 95% of the population.
Given the statistics, there are probably a few people who could still
see flicker by the time you got above 90 Hz, but there sure aren't
many of 'em.

The effects of the screen refresh rate on perceived motion have more
to do with the relationship between that rate and the ORIGINAL sampling
rate (i.e., ~60 Hz for standard video), and higher refresh rates are
definitely NOT always better in this regard.  Depends on the artifact in

  13.6) Is fluorescent lighting a significant source of flicker?

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

Actually, this is a myth.  Ambient light flicker is at best a second-order
effect in determining perceived flicker levels, and then only through
modulating the display's contrast ratio.  (Ambient light flicker isn't
even considered in the flicker calculations of the various ergonomic standards,
although the ambient light *level* is a concern.)

The notion that fluorescent lamps flicker and that this somehow produces
a "beat" with the screen refresh is simple to disprove.  First, if this
were so, 75 Hz screen refresh would appear WORSE than 60 Hz, since it's
farther removed from the line rate.  In reality, the reverse is true - and if
you REALLY want to maximize perceived flicker, turn OFF all the lights.  The
display will then appear to flicker MUCH worse, as one determining factor
in flicker is the APPARENT brightness of the screen (how bright the screen
is in relation to its surroundings).  Lastly, people don't realize that
fluorescents DON'T flicker at the line rate; being essentially plasma
displays wherein the plasma emissions exicte a phosphor, these tubes flicker
at TWICE the line rate - too high to be perceived.  Fluorescents show a
flickering appearance when they're failing, but that's a different
kettle of fish altogether.

(Also note that a large percentage of fluorescent lighting these days uses
electronic rather than magnetic ballasts.  Most of these do not suffer from
significant power line flicker (100/120 Hz) flicker as they are driven
at 10s of KHz by what are essentially switching power supplies.  Any variation
in intensity is at too high a frequency to matter.  This is true of most
compact fluorescent lamps, many cheap fixtures, as well as large (newer)
office installations or retrofits. --- sam) 

  13.7) Interlaced vs. non-interlaced monitors

The difference between interlaced and non-interlaced displays is in the
video timing.  Nearly all monitors can handle either.  Monitors are
specified as non-interlaced because for a given screen resolution and
vertical refresh rate, this is the tougher (higher) horizontal (H) scan
rate and it is desirable to minimize flicker in a graphical display (Fine
horizontal lines will tend to flicker on an interlaced display).  The H scan
rate is double the interlaced H scan rate since all scan lines rather than
just the even or odd lines are being displayed for every vertical scan.

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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]