Hello everyone, first of all sorry to the OP for hijacking this thread and not adding anything to solve his issue, but as he said it ranks high in search and with this newly found cap plague on the SC-88 I believe it will continue to be relevant in the coming years as well.
After seeing this thread I inspected my SC-88 and found all 5 10µf surface-mount caps to have leaked, so I decided to give a recap a try even though it was working without issues until that point. I removed all of the caps with hot air (I used 300°C, which did cause the surrounding ICs to get hot for a moment, even with the smallest nozzle available) and when testing they had near zero capacitance left, so it is quite surprising how the machine could still function in that state. I couldn't get the 100µf cap off with hot air and tried it with pliers, and unfortunately lifted the positive pad and adjacent traces on C38; I reconnected this with a wire to pin 5 on CN1, and also connected the neighboring C39 to the new cap.
Having replaced all the caps I put it back together and it came up with a blank screen on first powerup (not like the OPs one with the black boxes though, it was completely blank), but after that worked fine with a quick test. I found my initial soldering work to be too shoddy to leave it for good, so I decided to redo it. Putting it back together I made the mistake of not connecting the CN3 ribbon cable, so it powered up with the LCD off; when i corrected this and first turned it on it would somehow jump to the test tone pitch setting, but not react to any input. Since then, it seemingly powers up normal, showing the SC-88 writing and listing these few settings, and stays on part 1 with stock settings, but reacts to neither of the front buttons nor MIDI input 🙁
I have really no idea what happened as it did work fine after the first try, could that loose ribbon cable have shorted something on the board possibly? I don't have any other explanation right now but I could get a cheap logic probe if it helps with troubleshooting. Or maybe it could be some kind of damage on the internal connectors from all the unplugging? I already checked continuity on all caps and that's not the issue, also checked the ICs for possible bridges. All of the ICs stay cold when powered up, only the tone generator IC gets very mildly warm to the touch. Quite a long shot but any ideas would be appreciated...