68kMLA Classic Interface

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LC 1: no chime, no display, fan spins (w/ photo)
Posted by: 9166188 on 2022-01-15 02:29:14
Got a LC w/o a working PSU, but the board looks decent. So, an ATX PSU is used instead to provide 5V and12V. Sadly the machine turns on without any sign of life except the spinning fan.

I have soaked it in hot bath and have it fully recapped already but still no change. I had tried with and without battery, with and without RAMs. Still nothing happens. No sound, no video.

Any ideas? I probed the legs of each capacitor and found some have low voltages (-0.7V, 0.9V, 1.5V, etc) and some registered no voltage at all. Would these help to pinpoint where the problem may lie?

Screenshot 2022-01-15 at 6.15.40 PM.png
Posted by: joshc on 2022-01-15 08:17:11
I see gunk/cap goo around the legs of various ICs. How did you clean the board?

The Egret (UD8) and Apple Sound Chip/DFAC (UB10) need to be clean for a successful start.

Even the legs around the CPU don't look that clean, maybe its just the photo but it looks like it could be cleaner.

1642263377089.png

1642263396864.png
Posted by: 9166188 on 2022-01-16 17:54:55
Thanks a lot for the detective work! I soaked it in hot water with detergent for some hours. And brushed the board especially around the legs. I will give it another go. Is vinegar a better solvent in this case?
Posted by: mdeverhart on 2022-01-16 17:58:18
I usually use a cotton swab with some rubbing alcohol to remove cap goo. A toothbrush with some IPA would also work well to get between the legs. Of course, it’s always possible (even likely) that goo has gotten under the IC, in which case your best bet is to remove the IC, clean with IPA, and then reinstall.
Posted by: joshc on 2022-01-17 03:17:27
Isopropyl alcohol should be used to clean the board. Water/soap is fine as long as the board is soaked/rinsed with isopropyl alcohol. A brush may be needed to get rid of dirt in-between IC legs.
Posted by: cheesestraws on 2022-01-17 03:37:54
Thanks a lot for the detective work! I soaked it in hot water with detergent for some hours. And brushed the board especially around the legs. I will give it another go. Is vinegar a better solvent in this case?

Water and isopropanol dissolve different things easily (being polar vs. non-polar solvents), so using both has a certain amount to recommend it to get rid of general grot. Using vinegar on top is usually unnecessary unless you're trying to remove fairly specific things, mostly battery residues.

If you use both, use the IPA second and make it a thorough washing. IPA both breaks up the surface tension of the water (making it less likely that droplets will get wedged) and helps it evaporate (I think because it forms a lower-boiling azeotrope with water, but I may be wrong—chemistry is a bit of a mystery to me).
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