VT101 8085 CPU Fault

Peter Coghlan cctalk at beyondthepale.ie
Sun Feb 15 17:34:48 CST 2015

Robert Jarratt wrote:
> 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?

I've no experience of the VT101 so these are just general suggestions:

- Examine the symptom closely and try to figure out exactly what is happening.
  Maybe it is a change in height, a change in linearity, a shift of the whole
  raster, a loss of synchronisation or something else I haven't thought of.
  Different answers will suggest different components to examine more closely.

- Tap the board with an insulated tool to check for intermittent contacts.

- Check for dodgy preset potentiometers or other controls in the vertical area.
  If the whole raster is moving up and down, look particularly for a vertical
  shift control and see if adjusting it slightly produces the same effect.

- Put a scope on the power supply to relevant stages in the vertical section
  and watch for a change when the wobble happens.  Monitoring waveforms in
  the vertical section might also reveal something.

Peter Coghlan.

More information about the cctalk mailing list