(I posted over on 68kmla too, but thought i'd try here in case anyone has ideas)
I have a Q610 whose lower video is corrupt when running in thousands of colors. Interestingly, dragging a window from the bottom to the top brings some of the corruption with it.
Here is what I've tried in the last few weeks:
1. Different VRAM - no change. I even bought new "purple" VRAM.
2. Continuity checks for VRAM SIMMs and chips - nothing broken. I checked between VRAM and CLUT, VRAM and djMEMC and the bus transceivers U20, U23, U24, U25.
3. I got a cheap USB 24mHz 8 channel logic analyzer, so I hooked it up to all pins on the VRAM SIMMs. All +5V and grounds are working, and the rest have changing signals. I also hooked it up to the CLUT and the PD pins all have signals on them. I suspect my logic analyzer may not be fast and big enough to a useful, decodable signal but I'm new to logic analyzers so I'm not completely sure. I saw enough to see changes in signals everywhere however.
4. MacTest shows onboard VRAM is fine - when testing with no SIMMs. The VRAM test fails with SIMMs.
With SIMMS:
Video RAM Failure at address $F9080200!
Failed data pattern = $FF0000!
The only thing left I can think of is that U25 - which I had to replace because it was taken out with a cap - is somehow out of spec. But since the machine otherwise boots and operates normally I'm thinking this probably isn't the case. If I'm not mistaken it handles D0-D7.
Does anyone have pointers on what else I could try to fix this issue?
I have a Q610 whose lower video is corrupt when running in thousands of colors. Interestingly, dragging a window from the bottom to the top brings some of the corruption with it.
Here is what I've tried in the last few weeks:
1. Different VRAM - no change. I even bought new "purple" VRAM.
2. Continuity checks for VRAM SIMMs and chips - nothing broken. I checked between VRAM and CLUT, VRAM and djMEMC and the bus transceivers U20, U23, U24, U25.
3. I got a cheap USB 24mHz 8 channel logic analyzer, so I hooked it up to all pins on the VRAM SIMMs. All +5V and grounds are working, and the rest have changing signals. I also hooked it up to the CLUT and the PD pins all have signals on them. I suspect my logic analyzer may not be fast and big enough to a useful, decodable signal but I'm new to logic analyzers so I'm not completely sure. I saw enough to see changes in signals everywhere however.
4. MacTest shows onboard VRAM is fine - when testing with no SIMMs. The VRAM test fails with SIMMs.
With SIMMS:
Video RAM Failure at address $F9080200!
Failed data pattern = $FF0000!
The only thing left I can think of is that U25 - which I had to replace because it was taken out with a cap - is somehow out of spec. But since the machine otherwise boots and operates normally I'm thinking this probably isn't the case. If I'm not mistaken it handles D0-D7.
Does anyone have pointers on what else I could try to fix this issue?