Macintosh IIci On-and-off

[twillkickers]

Picked up an old IIci from my local junkyard and cleaned it up, recapped the motherboard, and cleaned up a bunch of corrosion from the leaked capacitors with my ultrasonic cleaner. I was able to boot it after that, and it ran for a while. However, I then had issues booting the machine. Messed around with it for a while and it booted again, but the display started flashing on and off rapidly- I could see the desktop as the display flashed on and off, the computer was not rebooting but it seemed to be losing power and regaining it again. I know the issue is not with the monitor as the monitor still works fine. After that particular incident, I recapped the power supply. There was not any visible capacitor leakage inside the power supply. However, after this incident and the powers supply recap, computer will not boot at all now.

What might be going on? Is this a common issue? Any help would be appreciated. I already put a lot of effort into this old machine and now it seems dead for good. I hope I can resurrect it!

 

[jeremywork]

I picked up a cheap "as-is" IIci with similar symptoms. Initially it turned right on, soft power worked. It seemed fully functional, but turned off without my input after an hour of use. I turned it back on and it lasted another 35 minutes and did the same. The following day it would only power on if I held the motherboard power button in; pressing it instantaneously would only power the machine instantaneously. With the button held in long enough, it would appear to begin booting, but quickly displayed a sad mac screen. By the end of that day it did not respond to any form of power (as the seller had stated.) Swapping in known good PSUs did not change my symptoms, so I'm expecting motherboard capacitors should be replaced, and were just temporarily working due to extended time discharged and/or being hot or cold in shipping. None of my symptoms affected the video output though, it remained clean whenever the system was able to power on. Perhaps one of the capacitors on the motherboard is attached to a damaged trace. This can be hard to find but can cause all kinds of issues.

 

[twillkickers]

Interesting. That doesn't match my symptoms though, because all my motherboard capacitors have been replaced and are good. Any other ideas? Do I need to swap out those yellow rectangular capacitors on the power supply?

 

[jeremywork]

I haven't worked on those PSUs, but given the motherboard's been recapped, I might try to find a different PSU to borrow/buy and see if that takes care of it before doing too much more work...

 

[jeremywork]

I could see the desktop as the display flashed on and off, the computer was not rebooting but it seemed to be losing power and regaining it again

Was the mouse/system still interactive under this? I'm thinking this may be a result of VRAM(system RAM) retaining data if the PSU suddenly starts sending a pulse instead of steady power. I does take a bit of time for data to be lost from volatile memory.

 

[twillkickers]

I did not check at the time because I unplugged the system as soon as I could to avoid permanent damage.

 

[jeremywork]

I'd expect, especially if you could hear the fan/relays click on and off, it will be something in the power delivery either on the PSU or on the motherboard. On my 840av a corroded motherboard trace was reused with a new capacitor, and it often worked fine, but would occasionally switch on and off a few times quickly and lose power (there was no flashing in the video though.)

I'd say the power supply is still the best place to start looking, but don't rule out the motherboard entirely even though it's been recapped. If you have or can find a Maxa Snooper, it will also give LED readouts for the power supply voltages via a Nubus slot. I'm sure this could be done carefully with a multimeter as well.