IIci Motherboard Shorting

[superjer2000]

Thanks to many of the forum posts here, I was able to run through a marathon recapping session yesterday, finishing a Classic, an SE/30 and a IIci (still 15 machines to go).

The Classic worked out great.

The SE/30 works great as long as I keep the Ethernet card that was installed in the PDS slot out.  If it's installed, I don't get any video.  One boot the screen went black-ish and then recovered but it has been fine since.  I haven't looked into that one any further yet.

The IIci was in pretty bad shape from electrolytic leakage.  Many of the pads were nearly half gone and very hard to really get clean.  That being said, I was able to check continuity for most of the pads to a point elsewhere on the motherboard for those pads where I could follow a trace and I completed the recap.

When I powered it up, I had a really brief startup chime and then the power supply clicked off and tried again.  It did this a couple of times. 

I pulled the board and I think I have a short somewhere.  There was continuity on the +5V and GND pins on the Power header, and on both sides of C14 Axial.  I cut C14 out and still have continuity across +5V and GND on the power header and on both sides of all of the small square capacitors on the board.

I have checked both sides of all of the capacitors I installed and I don't have continuity across any of them (other than C14 as noted before) (my multimeter beeps then the resistance increases as I think it's supposed to).  I'm not sure why this would be for those replaced capacitors, but not for C14 and not for all of the small square capacitors (MLCCs?)  I don't see any errant solder anywhere that might be causing this.

I thought I'd throw this out there to see if anybody else has encountered something similar or had any troubleshooting tips - I've searched but haven't found a similar fact pattern.  I'm debating pulling all of the MLCC capacitors one by one to see if one of them failed shorted. 

Thanks!

 

[Compgeke]

Recap the power supply. It behaves like my IIcx did when its PSU caps went out.

 

[superjer2000]

Thanks Compgeke.  The struggle I'm having is that when I check continuity on the motherboard power connector (with the power supply disconnected), I'm getting a short between +5v and Ground.  I don't think that could be right??

 

[superjer2000]

The system (other than the power management aspects) and power supply are fine.  I was surprised that there would be so little resistance between Power and Ground and assumed more was wrong with the board than is actually the case.  I swapped in a supply from my 7100 and same issue so I replaced the original Astec and shorted pins 9 and 10 and presto, the system booted up to System 7.5 once plugged in. 

I was able to find a few other instances of this exact same issue (press power from keyboard or back switch and unit half chimes and shuts down with a click from the Power Supply.)  I didn't find once instance where somebody actually fixed it.  There was a fair bit of capacitor leakage under the power supply and a few of the ultra tiny chips under the power supply look like they may be irreparably damaged.  The chips I'm talking about are noted by the arrows in this old Uniserver thread below.  They appear to be little tiny diodes based on the power circuit schematic.  Has anybody replaced these and knows what they are and where to get them?

If I can't figure it out, I may need to go the ATX route with an external power button.

Thanks!

 

[mattsoft]

Necro-resurrection but I just finished recapping a dead IIci and it has this exact same problem. +5v on the power header is shorted to ground. C14 is shorted on the + side. I haven't tried forcing the PSU on by shorting pins 9 + 10 when plugged in, maybe I'll give that a try. I've been tracing the power schematic and trying to find where the short may be, but cannot find it. Yes, my PSUs are all good. Hrmm...

UPDATE: Yup, shorting pins 9 + 10 while plugged in boots the IIci just fine. Something must have failed in the soft power circuit. I've tested continuity according to the schematic and cannot find anything broken. Must be a component. Any super common failures?

 

[mattsoft]

I removed UD13 and UE13 and it was a disaster under there. Do people usually just replace those ICs or is it worth cleaning and putting them back on?

 

[superjer2000]

I replaced those chips plus the small transistor diodes in the pic above.  Soft power issues went away after that.  However per the above thread the low resistance between +5v and GND is by design.  Might be all the caps on the board charging up.

 

[mattsoft]

Cool. Guess I'll order replacements and give it a whirl. I was shocked by how much crap was under those. Looks like a few bad traces too. Will tone it out first and patch the traces. Weeeee!

 

[superjer2000]

I agree - it's pretty crazy how much electrolyte collects under there.  In my reply I meant transistors AND diodes.  I did have to jumper some connections as well now that I think back to it.  There is a soft power schematic on here somewhere that you can use to diagnose trace issues.

 

[mattsoft]

Yup, found it in a related post. Thanks for all the tips!

 

[Quantum]

Yup, found it in a related post. Thanks for all the tips!

Sorry to reply to an old thread — I'm just wondering if you've since managed to resolve the issues with your IIci. I ask because my IIci, likewise, will no longer power up. It actually worked flawlessly, until I gave it a bath. Prior, it had been shoddily re-capped and was functioning but dirty; and, because I like to fix things that are not broken, I decided to bathe it...

 

[mattsoft]

It's been a long time, but IIRC, it was the replacement of 1 of those ICs that did fix my problem.