Mystic Color Classic Screen Distortion

[christcc2]

I have a Mystic Color Classic with the VGA mod done to the analog board. Display geometry has been fine until last week, when the bottom is now compressed and squished vertically - and a portion of the top third of the screen appears stretched vertically. The picture is also shifting up the display on it's own after a reboot.

Thoughts on next steps to address? I'm comfortable adjusting the pots for the CRT and have the non-conductive tools for making those adjustments. I have the service manual as well. I did adjust geometry on the display 6 months ago.

From what I understand the previous owner did do a recap of some portion of the analog board when he did the VGA mod. I've owned this Color Classic for about a year and a half.

 

[jonschultz]

The fact that it’s shifting around vertically makes me think it’s related to the TDA8172 (vertical deflection IC) or components around it. You could check electrolytic cap CF10. This stuff is inside the RF cage at the center rear of the analog board, so it’s unfortunately not that convenient to access, but doable if you’re determined. There are lots of places that I could recommend scoping if you have a scope and are comfortable taking measurements with the CC running.

 

[christcc2]

Does someone have a schematic they can share?

 

[jonschultz]

Actually, CF10 might be just outside of the RF cage (I don’t have my board in front of me at the moment to check), so it might be easier to access than I initially thought.

 

[jonschultz]

Does someone have a schematic they can share?

I have a partial schematic here, but others have more complete ones. @ArjenCNX recreated the whole board and has the schematic posted in one of the recent CC threads.

 

[christcc2]

Well I just rebooted the machine and now it looks like this:

 

[jonschultz]

Another place to check is on pin 7 of the TDA8172. You should get around 5V here. What you’re seeing could be related if it’s way off. In any case, modulation of the DC component of the vertical deflection (ie the vertical centering) is affected by components in that area.