A1200 post electrolyte damage: Alice overheating?

[Boing-ball]

Today's experiment: supplying Alice's Vbb from an external source. I wanted to test the behaviour of this 1200 without the +/-12 V supplies (only +5 V), so I wired up a connector and hooked it to my bench power supply. Since I have a second output on this supply, I set it for 3 V below the ground for the 5 V supply, and connected it to one end of C2X (which I'd also replaced with a 10 uF electrolytic cap with a 100 nF ceramic in parallel).

Results:Running without +/-12 V makes no difference to the glitch, the health of Vbb or the CCKQ clock signals. Running with the extra capacitance also did not help noticeably. But supplying a negative voltage to Vbb seems to keep the CCKQ signal good and allows the system to keep running! If Vbb rises above about -0.8 V the CCKQ signal goes bad, but if Vbb is lowered again below -1.5 V, it comes right again. Alice doesn't draw any appreciable current on Vbb.

Overall power consumption was about 1.57 A @ 5 V (7.85 W), and interestingly, improving the Vbb strength (lowering the voltage from -1 V to -3 V) makes Alice run cooler and with lower current draw from the 5 V supply.

I'll need to test a bit more and figure out how to make this a permanent fix, but it's looking hopeful that I now have a working 1200. I've tested it today with internal and external floppy drives, a mouse, but no keyboard, and so far no demos or games I've tried have shown any unexpected issues.

Some great problem solving there. Keep us updated…

[Boing-ball]

Just a small update: I've heard another report of similar issues with Alice's Vbb voltage collapsing, but there the root cause was the C2X capacitor going bad (low capacitance), so that should also be considered if the same symptoms arise: graphics and sound goes crazy after a short time running OK, Alice runs hot, XU9 GAL runs hot, Vbb voltage and CCKQ clock voltage collapse, but cooling Alice avoids the failure.

My fixed A1200 is still going strong, so supplying Vbb externally seems an acceptable solution (provided C2X is OK).

Do we know if this is also the same issue on the A4000?
Great diagnosis work. It’s always been documented that Alice can get hot. I always placed Heatsinks on mine and the GALs.