Recapped IIci / IIcx Rapidly switches off and on once warmed up

[MrFahrenheit]

Hey, new to the forum here.

I have accumulated quite the vintage Mac collection, and as everyone has experienced, the leaky capacitor issue is a real pain.

Luckily I learned how to remove and solder on new caps.  I've been using tantalum caps, and I've got at least 2-3 of the IIci/IIcx motherboards where I recapped them, and they turn on just fine, but once they've warmed up, the just shut off and then power back up.  However, they don't power up completely, they immediately turn off again, and on and so on.  The cycle starts with the first shutdown/power up I hear the full chime, then it goes quicker and quicker and the chime gets less and less until it's just clicking.

The boards appear to be visually fine.  I did not note any extra chip legs corroded or traces damaged more than any other board, and I cleaned with IPA99 the entire board.  The power supplies are known good (not recapped), but they work fine with other IIci/cx motherboards.  

On the IIci I only have connected monitor, keyboard/mouse, and I'm using an external SCSI (Jaz drive) which works fine on all other machines.  On the IIcx I add the required video card.  Video card works fine in other machines.

On the IIci I am not using the cache card.  RAM has been switched around to rule it out.

When I say warmed up, it's a different amount of time for each machine.  The quickest is after 5 seconds or so.  The longest runs fine for 10-20 minutes at least until I run a Norton System Test on it, where it shuts off once the hard work on the CPU / video begins.

Before you ask, yes, the power switch is in the horizontal 'I choose when to shut it off' mode, not the vertical 'always stay on' mode.  I have rotated it to see if it makes a difference.  It was suggested to me to remove and clean the power switch.  I don't know if that's the solution here.  My guess is there could be a chip that is causing the issue.  Thoughts ?

 

[jessenator]

Welcome!

My first thoughts are the PSUs themselves. If they haven't been recapped, as you've said, I would look into recapping them as well. I did my Quadra 700's PSU (same basic type PSU) and there were a few bulgy caps, even though it would boot. And after ~30 years, the caps are likely bad or going so, just like the logic boards.

Take a look at this thread for the different PSU models as well as capacitor lists for a couple of the compact-desktop PSUs:





I do know the IIsi PSU's daughter-board can cause this same symptom to manifest itself, but it could very well to the same in these. A whole re-cap isn't a bad idea, all the same.

 

[MrFahrenheit]

Welcome!

My first thoughts are the PSUs themselves. If they haven't been recapped, as you've said, I would look into recapping them as well. I did my Quadra 700's PSU (same basic type PSU) and there were a few bulgy caps, even though it would boot. And after ~30 years, the caps are likely bad or going so, just like the logic boards.

Take a look at this thread for the different PSU models as well as capacitor lists for a couple of the compact-desktop PSUs:





I do know the IIsi PSU's daughter-board can cause this same symptom to manifest itself, but it could very well to the same in these. A whole re-cap isn't a bad idea, all the same.

I haven’t recapped my power supplies however known good power supplies were doing this on the trouble motherboards but worked perfectly fine on other recapped boards. I also tried 3 different known good power supplies. If it were the PSU, would it not exhibit symptoms on another motherboard, and wouldn’t it go away if it tried a different PSU?

 

[jessenator]

Gotcha. Most of the issues I'm finding from old FAQs and threads are around the instant-off symptoms, but I haven't heard of this where it's post-warm-up. That's interesting. Thinking of the other models, not sure how much the board management differs from the II's to the quadras, to the 7100, but I'm not an engineer but typically, if it works, it works.

However, they don't power up completely, they immediately turn off again, and on and so on.  The cycle starts with the first shutdown/power up I hear the full chime, then it goes quicker and quicker and the chime gets less and less until it's just clicking.

^^^This is what makes me think it's bad caps in the PSU itself, but if it works on other machines… very strange.  I've read that the PSU requires a 5V sense to be able to boot, but that shouldn't be an issue in it cycling power, could it?

My next suspect would be some unseen electrolyte goo somewhere. From my own experience, my SE/30's board was exceptionally clean from the naked eye, but I didn't have any sound at all until the board was super deep cleaned. Not the same as power issues, but just shows to go you that there might be some hiding cap goo causing issues.

A bit of a long-shot, but maybe a faulty power switch? Again, not an engineer, so I don't know specifically how it would fail, if it just turns on. And even if it was related to thermals, it doesn't get that hot… sorry for the textual thought process here.

Sorry if that isn't helpful, but maybe a more-technically savvy member could chime in here.

 

[bibilit]

https://68kmla.org/forums/index.php?app=forums&module=forums&controller=topic&id=54957

probably something like that.