"Computer Nerd Kev" schreef in bericht
. ..
Hello, this was going to go to sci.electronics.repair but it
seems that Aioe users are blocked from there, so I'm posting
it here instead.

I recently bought a broken "cocktail" style arcade game
machine made in 1980. Initial repairs were easy. I got the
machine to start by playing with the connections to the CPU
and logic boards and fixed the lack of green on the monitor's
display by soldering the green lead of the RGB signal back
onto the monitor board.

The problem I'm left with is that after about three minutes
(less if it has been recently used), the picture on the
monitor is reduced to some coloured vertical lines and a
whining noise is emitted. Before this, the display bends
inwards at the bottom (and a bit at the top) of the screen. My
assumption has been that this is a sync issue so the first
course of action was to look at the sync signal that goes from
the CPU board to the monitor with my Oscilloscope. I did this
expecting to see a slowly worsening waveform, but instead I
saw what seemed to my eye a quite nice saw tooth.

However what did catch the attention of my untrained eye was
that this waveform varied by less than a volt between 4 and
5V, except for a momentary drop to 0V at the end of the wave.
Period of the waveform was 90uS.

I only have theoretical knowledge of monitor sync signals, so
I didn't know what to expect, but it seems to me that this is
an oddly small voltage change for such a signal. I tried to
find something on the web that would tell me if this is the
case, but failed. Hence I ask here.

Thank you for reading.

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Apparently something wrong in your horizontal drive signal. Most likely the
line output transformer, maybe the horizontal coils but there are still some
other possibilities. Be aware that the coils are current driven.

petrus bitbyter