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Macintosh IIsi possible video glitch
Posted by: MrArthegor on 2026-03-20 13:22:29
Hello everyone,

I’ve recovered a Macintosh IISi in good condition except for the PSU which had a significant capacitor leakage. The electrolyte was practically in the motherboard PSU connector.

Naturally, I removed all the caps, thoroughly cleaned the remaining electrolyte and re-capped the PSU. I’ve also re-capped the Logic Board.

Now the computer starts but the video output signal is strange with a wobble in the background. I’m using an OSSC with a standard DA-15 to VGA adapter but I’m struggling to determine if this issue stems from the OSSC or the Macintosh. (Of course, I only have the OSSC to test and my other old monitor doesn’t support RGB.)

Here’s a link to the video of the effect. If anyone has an idea I’d be very grateful.

Thanks for reading and sorry if I make any English mistakes; it’s not my native language.

Posted by: Byrd on 2026-03-20 14:27:40
The wobble/shimmering looks like a result of the OSSC or your LCD sync - can you try another monitor LCD or ideally CRT.
Posted by: MrArthegor on 2026-03-20 23:48:29
Thank you for your comment, indeed, It’s an OSSC issue I was able to find a monitor with sync on green (pretty hard to find in LCD, one 4 monitor I have check only 1 was ok)

I’m going to try to fix the OSSC settings, but before that I need to make the IIsi more reliable because it’s seem to have sporadic issue with shutdown, and sad Mac boot
Posted by: bibilit on 2026-03-20 23:49:26
Yes, nothing wrong with it, your LCD is not playing nice with the IIsi.

As Byrd says CRT is the best option.
Posted by: MrArthegor on 2026-04-12 04:16:11
A follow-up on this issue for anyone who encountered the same problem.

Ultimately, the OSSC wasn’t the culprit. After my last post, the Mac state became increasingly erratic with long death chimes occurring on eight out of ten boots. Initially, I suspected the PSU was the problem due to severe electrolyte leakage (the electrolytes were even present on the motherboard connector with the PSU wire’s capillary action).

Yesterday, I attempted to use BlueSCSI and clean the board but forgot to put the RAM module back. To my surprise, the Mac booted perfectly but crashed when I reinstalled the RAM. Since the RAM is known to be good, I reflowed the SMD cap I had replaced and the Mac now seems to behave normally.

In short, it appears the issue was a bad solder on one of the SMD caps, which caused the problem. If anyone experiences a similar issue, please check your solder and don’t hesitate to reflow it.
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