VT101 8085 CPU Fault
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
> 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.
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