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How to turn on a IIci PSU outside the computer?


Well-known member
Well, it's not the ADB filter. I swapped it out with one from a IIgs, and same issue. Auto-power on an no ADB ports. The same symptoms listed above.


Well-known member
@superjer2000 OK, some interesting developments. I replaced all three 3 ICs and ran a bodge wire from a missing pad (it had completely corroded away.)

Great news is that the computer now turns on. Yay! Bad news is that it always turns on no matter what. It's like I have the power switch on the rear pressed in. However, after it boots, neither of the ADB ports work. Some Lycosing indicates that non-working ADB ports is a bad ADB filter.

So back when I thought the filter could be bad, all the shorts disappeared when I removed it. So now I'm leaning back towards a bad ADB filter. It didn't occur to me that the filter is bunch of capacitors, and mine is completely shorted open. Fortunately my IIci has both DIP holes and IC pads for a filter. So I think I can steal a filter from another computer with ADB ports, it doesn't have to be the DIP version. I'll try swapping out the filter tomorrow and see how it goes.

I'm hoping I can just buy a generic filter, but I don't know what part to look for. I couldn't "find an alternative" on Digikey or Mouser.

In the meantime, I'm pretty happy with my rework. Especially my bodge wire. I got a nifty metallic blue teflon wire from the recycler. My handy work is all hidden under the power supply, though.

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That does look like a nice fix! As an aside, I had also replaced the small diodes and transistor on my soft power circuit, not sure it was necessary (and not saying it might be a factor for you) but it's crazy how much that cap goo spreads and what it eats. With regards to the continuous turn-on problem) I'm sure you removed the jumper wire from the PSU plug? (I have to ask as sometimes it's the most obvious thing).

I don't have any feedback on the ADB ports as I've never had an issue with ADB on any of my vintage Macs (50+ units). I'm assuming you tried another keyboard/mouse? I had a bad ADB mouse one time that caused a lot of weird issues.

Congrats on getting this far!

The Bomarc schematics for the IIci are available which might help to trace back the ADB problem as well.


Well-known member
So when I remove the filter and isolate the ground plane to the right (marked in blue), everything seems fine over there. When the filter is installed, the components marked in red are shorted. Likewise with the ICs that I marked already in the previous picture. 5v is always shorted to ground, which is why I think the computer is automatically turning on (and perhaps why my ADB ports are not working.)

So whatever is causing the short is on the left side of the board somewhere. I just don't know where. Problem is, that's 90% of the board.

I guess three's nothing for it but to just follow the 5v and check every capacitor and diode. Although, I think I'm going to start by systematically removing D1 through D7 and see if I can get the short to disappear. All those bitty diodes were in the line of fire of leaky caps. Even though they appear to test fine even in-circuit, maybe one of them is still bad.


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Well-known member
Sigh. Computer is back to not turning on after replacing the D3-D7 diodes and Q1, Q3, and Q4 transistors.

Also, as far as I can tell, it's still shorted.

I also can't get my hot air station to melt anything worth a damn. I've even set it to 500 C, and it might be be able to remove a tiny surface mount diode if I hold it there for a few minutes. Maybe the included tip is too big? Don't know. (practicing on a ewaste scrap board, not anything valuable.)
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New member
Perhaps the soldering iron tip is dirty? Post a photo here. Are you using anything to clean the tip?

Also sometimes it helps to use flux on the solder joint. Don’t use flux in a pen, it doesn’t work too well. Use gel flux that comes in a syringe. (Don’t breathe the fumes, use a fan with a carbon filter)

if you don’t have flux then melt a bit of new solder onto the old solder joint.


Well-known member
I spent a good 12 hours on this board today. I'm just out of ideas. Everything north of the lower ground plane near the ports has 5v shorted to ground. So all the capacitors north of it are shorted, and diode D10 reads both directions (if I pull the ADB filter and isolate the lower ground plane, everything reads normally, which is how I know the short is north of the lower ground plane ... which means the other 90% of the board.)

I also replaced today (with no change in the problem) the corroded ICs at UH1, Q2, and UA4. I also removed, cleaned, and resoldered (to no change) VD11, UA13, and UA7. They were also corroded.

Each time I removed any of those components, I tested for shorts, but it was still shorted.

I also tested every single resistor on the back of the board. They all tested fine except R66, R64, R42, and R36. Those all read 0.5 ohms regardless of their values. If I take them out of circuit, they read normally, and the short is still there.

Because 5v is shorted to ground, after I had initially replaced UB13, UD13, and UE13, the computer would start automatically no matter what. No ADB activity. So I had replaced diodes D3, D4, D5, D6, D7, D8, and transistors Q4, Q5, and Q1. The computer is back to not booting, and 5v is till shorted to ground.

I have completely exhausted my ability to diagnose this board, unfortunately. Unless anyone has any suggestions.