Re: A sick computer monitor

[David Bowman <David_Bowman@GEORGETOWNCOLLEGE.EDU>]

Regarding the latest message from Ludwik, concerning the ongoing saga of
his sick computer monitor:

> ...
> But today old symptoms started to appear more often and
> finally the monitor became black. I was turning it on and
> off (sometimes successfully activating the screen). Finally
> even this stopped working. How to shot down a Mac when
> the monitor is black?

Ludwik, apparently your sick monitor has a problem in its horizontal
deflection circuitry that has gotten worse with time.  At first the
problem only seemed to affect the amplitude of the horizontal deflection
pulses and this seemed to mess up the pincushion correction modulation on
those pulses.  But as time has worn on the problem has gotten so bad that
the horizontal deflection pulses have stopped altogether (i.e. their
amplitude went to zero).  Presumably, the problem is in the horizontal
deflection oscillator or the horizontal deflection output amplifier or
the flyback transformer or the horizontal deflection yoke (the 2nd, 3rd
or 4th possibilities being most likely).  Since the horizontal deflection
pulses power the primary of the flyback transformer, and the CRT gets its
high anode voltage from a step-up secondary winding on that transformer,
this means if the horizontal deflection signal is lost the screen will go
black.  Another secondary winding on this transformer powers the
horizontal deflection yoke.  My guess is that maybe a short developed in
the flyback transformer or the horizontal deflection yoke which then
overloaded the horizontal deflection output circuitry and reduced its
amplitude.  This then messed up the needed amount of pincushion
compensation.  This was temporarily alleviated by your manually resetting
of the amount of that pincushion compensation and maybe resetting the
width control.  But the short got worse and the overloading of the
horizontal output stage final burned out the horizontal output
transistor, maybe a resistor or two, etc. in that stage. at which point
the flyback transformer no longer was energized and the CRT no longer got
any high voltage to accelerate the electron beams to the screen, and the
screen went black.

> At that point I borrowed an old Mac LC monitor and all
> symptoms disappeared. Will it last? I hope so; the computer
> was on for many hours and all has been normal so far.

At least this has made your computer usable.

> I think the AppleVision monitor is really sick and needs a
> professional hand. I wish I could buy a used replacement
> monitor for peanuts, as Bernard did yesterday.

I don't know the policies of various Apple repair service centers are,
(possibly they are required to always send broken monitors back to the
manufacturer for rebuilding) but maybe, if you are lucky, you could
persuade someone at such a service center to let you have ("for peanuts")
someone else's broken AppleVision monitor.  Just make sure that the
problem with that monitor was not anything to do with the horizontal
deflection circuitry.  You could then use the other monitor to
cannibalize parts off of to fix your monitor.  In particular, you could
swap the module containing the horizontal output stage, the flyback
transformer, and/or the deflection yoke.  I expect that if you could get
a hold of such a used broken monitor that such a strategy would probably
fix your problem.  If you found your local Apple dealer uncooperative
maybe you could get a deal "for peanuts" on such a broken monitor from
your university's service technician in their IT department.

When swapping parts related to the high voltage part of the monitor be
very careful about the static charge that may remain on the anode after
the monitor is disconnected from the power line.  If you do not properly
discharge this filtering capacitance you could get a nasty multi-kilovolt
shock.  Also be sure to clean off any dust from the high voltage leads
and from deflection yoke since such dust can be a path for shorts, arcs,
etc.

David Bowman
David_Bowman@georgetowncollege.edu