Greetings, distinguished repair specialists,

I have been using a 21" Hitachi CM813ET CRT monitor for some time now. It
exhibits the strangest phenomenon every now and then. Prompting me to write
this message is the fact that it today appears stronger than ever. I have
gone through the excellent monitor repair faq at
http://repairfaq.ece.drexel.edu/sam/monfaq.htm, but I'm still a bit
uncertain of where I should start looking. I'll describe the problem in
detail and hopefully you guys can give me some pointers.

First of all, the phenomenon either happens or it does not happen. If it
does not happen, everything is perfectly normal. The picture is crisp and
centered, nothing is out of the ordinary. When it does happen, however, the
picture shifts about 1 cm to the left and becomes jittery. Mainly,
consecutive horizontal lines are not aligned perfectly, but individual lines
can shift left or right on each redraw, making the screen look like it
shimmers very nervously.

The amount of jitter varies greatly. At times, the jitter is barely
noticeable, with movement range of only 1 pixel or so. Sometimes it can
extend to several centimeters and occasionally the monitor completely loses
horizontal sync, drawing nothing but messed up lines on the screen,
accompanied by a high-pitched whine. The whine is not there during lesser
jitter or normal operation.

As I said, the condition looks like someone is using a pot with an integral
switch to control it. It's easy to detect when the condition is "on" because
the image is shifted left. Then the amount of jitter ranges back and forth,
never jumping quickly from total chaos to relative order or back, but
gradually shifting between these states.

Thus far the condition has been fixed by simply turning off the monitor for
a second or thirty and then turning it back on. It might exhibit some jitter
for a couple of seconds after being turned on, but would then settle down
and work properly. Please note that I'm not confusing this with the normal
startup-degauss-wobbling of the screen. This time, restarting the monitor
doesn't seem to help and as I'm writing this, the monitor is turned off,
disconnected and resting so that I can see if extended cooling time does
anything for it.

I should mention that I have ruled out all external noise sources as the
cause of this by operating the monitor in different rooms and with different
computers, without having a cellphone with me or anything such. Bad scan
rates are also out of the question because once this condition exhibits
itself, the same jitter is visible even in the monitor's own "power saving"
logo that displays for a couple of seconds after disconnecting all input
signal, or after turning the monitor on without input signal.

None of the monitor's adjustments have any effect on the phenomenon, nor
does alternating between the two inputs or degaussing the monitor. Whacking
or physically abusing the monitor has no effect either. Nothing
distinguishable from the normal drifting anyway.

My understanding, based on the monitor repair faq, is that there is clearly
something wrong with the horizontal deflection circuitry. Since percussive
maintenance has no effect, I would almost be willing to rule out a cold
joint as the primary reason, focusing more on a possibly faulty component
somewhere either in the power supply of the horizontal deflection or in the
late parts of the signal path.

Without leading you guys any further astray with my possibly mistaken
conclusions, I would like to tap to your collective intellect in order to
gain insight as to what precisely look for when I finally open the beast up.
If any of you have experience with similar problems, or can supply the
schematics of this (or similar) Hitachi monitor, please don't refrain from
replying. Your help is deeply appreciated.

--

Arto Rantala