IIci boot issues - makes crackle then turns off

[Durosity]

I have a IIci which is causing me bother. It was working several years ago before I removed its capacitors and stored it ready for a re-capping, which I've only just got around to doing. When I turn it on it does power up for a moment, the LED light flickers, the speaker makes a bit of a crackling sound, then it turns off. It's only on for 1-2 seconds.

So far I've tried the following:

Known good PSU (tested with another IIci)
Confirmed all caps (tantalums) have been installed correctly and in correct orientation
Swapped UC13/UD13/UE13 with chips from my other IIci, same issue on faulty system, other system working fine as it was,
Removed and cleaned up the sound chips which did have corrosion under them but no sign of any damaged traces or vias (same for the above UC13 etc chips)
Tried a different ROM in the rom slot.
Replaced ram modules

Anyone have any insights that could be causing this behaviour? I've had problems with the soft power circuitry on a Iicx before, but that just resulted in nothing powering on at all.

 

[ClassicHasClass]

This really does sound like a power or power circuit problem. Check the components on page 1 of https://github.com/nickpunt/nubus-s.../bomarc/Macintosh IIci Schematic (bomarc).pdf - do they all look operational?

 

[Durosity]

This really does sound like a power or power circuit problem. Check the components on page 1 of https://github.com/nickpunt/nubus-se30/blob/master/Schematics/bomarc/Macintosh IIci Schematic (bomarc).pdf - do they all look operational?

Yeah, the power circuit seems like the most likely issue. I thought I was on to something earlier when I realised there was continuity between ground and the 5v rail, but then I tested the same pins on my working IIci and found the exact same.. still trying to figure out how that works in my head.

I’ll have a look though the schematic and see if it helps pinpointing the cause, although I have to admit I’m rather crap at reading these things.