68kMLA Classic Interface

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Macintosh IIci keeps blowing its fuse
Posted by: DistantStar001 on 2026-05-23 23:07:41 Just as the title says. I have a Macintosh IIci that blows its fuse as soon as it gets power. This is definitely a PSU problem, as the logic board boots fine with the working PSU from my IIcx. Unfortunately, from what I understand, this PSU is the less reliable non-ASTEC (614-0009) variety. My IIcx and other IIci both have ASTEC supplies, so it's not like I can swap parts or compare readings between them. Replacements are over $100 on eBay, so I'd like to fix the one I've got. The problem is, I don't have a lot to go on when it comes to symptoms. The thing was working fine for months (likely years before I got it) and then it just blew its fuse (and every other fuse I trie to put into it). I tried a partial recap, but haven't sourced all that I need yet. However, I don't think that's the issue. Only one of the capacitors showed any sign of leaking (1000mf 16v). and replacing it and the others I had didn't make a difference either. Besides, as I understand it, the fuse is on the high-voltage side, where the one questionable cap I found was on the low side. Side notes: Yes I've already recapped the logic boards on all three machines. And as mentioned, all boards boot. As for my problem machine… Any ideas?
Posted by: franklyn on 2026-05-24 09:41:00 Switch-mode power supplies consist of a primary circuit (mains voltage) and a secondary circuit (5V and other low voltages). If the mains fuse blows, first check whether the rectifier (diodes) has a short circuit; next, check whether the high-voltage main electrolytic capacitor has a short circuit. If no problem is found there, inspect the main switching transistor and its surrounding components—e.g., diodes. All of these checks are performed within the primary circuit. Caution when working with the power supply unit: 300 to 400 volts are present at the main electrolytic capacitor—this poses a life-threatening risk.
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