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Commodore 1902 Vertical Hold Problems


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Hey all, I have been slowly restoring a 1902 monitor and so far I have done a full recap including main, neck, and power supply board. I have performed the upgrade to allow it to accept analog RGB input as well, as this 1902 shares all the same internals as the 2002 (even the RGB mode switch) with just a few missing components on the board.

My problem: before working on the monitor, it had very unstable vertical stability when using the digital RGB input. After the recap, the problem remained. The issue also manifests when using the new analog RGB input mode.

When I say unstable, it's only in certain modes. In 80 column text mode, it's rock stable. In the desktop GUI on the 128 or Amiga, which is mostly grayscale, it's completely stable. But as soon as I go into any color graphics mode, it starts rolling and I have to fiddle with the V.Hold to get it to stabilize. Over the entire sweep of the knob, I have to get it to a fraction of a degree to get the picture to stop rolling. And when the scene changes, often the image starts rolling again till I adjust it to slightly different spot.

The image is rock solid stable using the composite/split y/c input across 2/3rd of the V.Hold sweep, so I figured the problem must be in the RGB circuitry. To that end, I have replaced all the parts highlighted below with brand new components, yet the problem persists. I have tried 4 different cables and 4 different computers and the problem exists across them all.


At this point, I don't know what to look at next. Any ideas?


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