Macintosh IIci keeps blowing its fuse

[DistantStar001]

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?

 

[franklyn]

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.

 

[DistantStar001]

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.

Okay. First of all, thanks! this was helpful. So far I've got over this thing several times and only found 3 shorted components. The big one was the transistor at Q1 (K1341) for which I managed to get a replacement. the others were a zener diode (ZD1) and another (I guess regular) diode at D2.

Apparently, ZD1 is a 18v zener diode according to post 9 on this thread:

Thread 'Schematics for Delta PSU'

Nov 27, 2021

Looking for a Delta SMP-120EB PSU schematics

pretty sure a Zener diode at least is bad.

Found on Quadra and Centris units.

• bibilit
• Replies: 27
• Forum: Peripherals

However, I was unable to identify the diode from D2. It's simply marked with a 2 on one side and an 18 on the other. My local electronics store was unable to find a match and I can't seem to find a parts list or schematic for the PSU. Can anyone tell me what this diode is?

Side note: I tried taking a picture of it with my iPad, but sadly, the focus wouldn't cooperate.