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Mac IIci PSU recap advice

bigmessowires

Well-known member
Thanks for the suggestion! The desolder and resolder is something you're doing pretty routinely on your boards to fix power-up problems? I probably won't try that on this IIsi board, because I'm running out of patience with it and I'd be afraid of accidentally making things worse rather than better, but it's a good idea to keep in reserve if I need it.

I was able to test my idea of replacing R115 without actually modifying the board, by connecting a diode and 1K resistor between +5V and PFW on the PDS slot. That doesn't actually replace R115, but it puts another resistor in parallel with it, lowering the overall resistance. Though it's not the "right" solution, it works fine with my FET-based soft power circuit and ATX PSU. We'll see what happens with the original PSU once it's recapped.

 

bigmessowires

Well-known member
Here's the conclusion of my Mac IIsi story.

I replaced all the electrolytic capacitors in the IIsi's power supply, except for the two big ones (they looked fine and I didn't want to mess with them). The good news is that the PSU no longer smells like stinky fish. The bad news is that I still have the same problems with with PFW that I saw with the ATX power supply. So the PSU recap project was ultimately no help, other than to eliminate the fish smell. :)

Even with the recapped PSU, the computer would either turn off immediately after turning on, or turn off at some random time 30+ seconds after turning on, or turn on and stay on but refuse to turn off. Bolle is probably right that the problem lies with the 68HC05, but I'm tired of screwing around with this machine. So for the PFW signal I added a 5.1K ohm external pulldown, as well as a diode in series with a 1K ohm pullup connected to +5V. With this it seems to reliably turn on, stay on, and turn off from soft power. Obviously it's not a great long-term solution, since something is wrong with the logic board and may get worse in time. The extra parts were stuck into pins in the PDS slot, so I've also lost the ability to use the PDS slot for anything else. And the sound no longer works - it's very quiet and sounds scratchy. Hard to believe this logic board was recapped just a few years ago and was working fine as recently as last year.

But then... I thought everything was OK with this work-around, but as I was typing the above paragraphs, I noticed that the hard drive sometimes randomly spins down for a brief moment and the computer freezes, even though the mouse still moves. I'm not sure, but I think the system fan also slows down when this happens. It happened several times in a row while booting from another disk, right at the moment when it tried to mount the main SCSI disk. I'm not sure if it's a problem with the disk, or the logic board and PFW, or the power supply I just recapped, but I'm out of patience and not going to work on this any more.

Let us all take a moment of silence for this poor sad IIsi.  xx(

 

Elfen

Well-known member
There are two tiny caps on the IIsi PSU Daughter Board that causes 90% of all PSU problems on the IIsi. Replace them and the IIsi comes back to life... most of the time.  As an added insurance, one can recap the rest of the PSU.

I remember this from a former member who was here and sold caps and repair services. Here is his old website about the IIsi information (and the two caps involved). Hope it dos not get me into trouble...

http://maccaps.com/MacCaps/Capacitor_Reference/Entries/1990/10/5_Macintosh_IIsi_Sony_Power_Supply.html

 
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