VT101 8085 CPU Fault

[Robert Jarratt]

A little while back I posted about a possible 8085 CPU fault on my VT101
terminal controller. After a session with the logic analyser, courtesy of
another member of this list, we determined that the 8085 was indeed bad. We
substituted it with another 8085 and the terminal controller started working
normally.

However. This has revealed a fault on the monitor control board. There is
the occasional vertical wobble. Sometimes the image is stable, then it will
wobble up and down a bit, sometimes just a bit, sometimes more frequently,
then it will stabilise for a few seconds, then it will wobble again, etc
etc. I know it is the monitor control board because we sent the Video Out
output to a separate monitor and it was perfect.

I have checked the ESR on all the electrolytics, they are all fine. Prior to
all this I had replaced C439 with a higher voltage rated component, as
recommended here: https://trmm.net/VT100. I used a 100uF part, rated at
100V. I had also replaced C437 as that had a high ESR prior to seeing this
problem.

Is there anything else I should check?

Regards

Rob