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Power Mac 7100 - No Chime, No Boot

saybur

Well-known member
I've got a problem child Power Mac 7100/66AV that I've been trying to fix for a while, and was hoping for some thoughts from knowledgeable people here about the next steps to take.

The issue: from either an ADB keyboard or the rear button, the system powers on (front LED lights, there's a slight "click" from the speaker, PSU fan spins up), but then nothing happens - no chime, no video, zilch.

When I first got this system, it worked fine; it had been in climate controlled storage and was apparently treated well beforehand, so fairly clean, no obvious cap issues, etc.  I replaced the PRAM battery, re-applied some good thermal paste to the CPU per reports here & elsewhere about the stock stuff going all dusty, blew out the dirt that was inside, and ran it for an hour or so before putting it up on the shelf for an eventual re-cap.

After a few months of intermittent use, mainly swiping parts from it while I was working on other systems, the 7100 was no longer booting correctly and started exhibiting the behavior in the title.  After checking the battery in-circuit (which read 3.6V) I figured it was just the usual Mac cap issues.  So far, I've tried the following:

  • Replaced the electrolytic SMD caps on the logic board
  • Tried to run the system with just the ROM SIMM, onboard video, and the soldered 8MB RAM
  • Re-seated the ROM SIMM a few times
  • Swapped the PSU with a known-good unit from a IIci (the 7100 PSU runs fine in the IIci, as I recall from work a few months ago)
  • Verified that the HDD/CDROM power rail has good +5V & +12V
  • Did an inspection of the board for obvious scorching, traces rotting, corrosion, etc.
This logic board is immaculate and the caps did not appear to have a chance to leak before I replaced them.  The CPU heatsink is getting nice and warm while the system is sitting there doing nothing, so I think that it's correctly dissipating heat away and the die isn't cracked.

I'm not entirely sure where to go from here.  Does anyone have any ideas?

 

saybur

Well-known member
Oh, and mods, sorry, this should obviously be in the NuBus category.  I'll try to actually read the category before submitting next time xx(

 

Macdrone

Well-known member
not sure.  Power supply is usually the culprit,  but you say youve done that.  I have a pile of them and have yet to kill one so not real sure.

 

trag

Well-known member
You didn't get heat sink grease on the CPU pins around the edge of the CPU did you?  The stuff is mildly electrically conductive and will cause shorts.  I killed a PowerComputing Power120 that way, back when that was a new and shiny machine.  

Now days, I would clean the stuff off with a solvent spray, but back then I didn't know a good way to clean it off of the tiny delicate pins.

 
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saybur

Well-known member
You didn't get heat sink grease on the CPU pins around the edge of the CPU did you?  The stuff is mildly electrically conductive and will cause shorts.  I killed a PowerComputing Power120 that way, back when that was a new and shiny machine.  

Now days, I would clean the stuff off with a solvent spray, but back then I didn't know a good way to clean it off of the tiny delicate pins.
I don't think so, but that's a good idea, I'll double check.  The stuff I used is (theoretically) supposed to be non-conductive, but I'd rather not rely on that saving my bacon.

Also, thanks Macdrone, I'll also double check the power supply in circuit to see if there's anything wonky.  I don't completely understand the PS On-Off / PFW line from the PSU, but here's a shot:

1) it's connected to the +5V trickle to start the system

2) is pulled low by the Mac to signal the PSU to shutdown (?)

Does anyone know if it is monitored in any way as a power-good signal?  Or is that generated somewhere else on the board?

 

saybur

Well-known member
Pulled the heatsink off and after cleaning off the residual grease, saw a discoloration between two halves of the die, one half hued purple, the other blue:

https://imgur.com/a/kpHiU

It sadly matches the description given for a cracked 6100's 601 here:

https://68kmla.org/forums/index.php?/topic/28904-dead-6100/

So, that's probably it.  I'm guessing I fouled up, either with too little grease (it looked a tad dry on one side, IMHO), pushed too hard when re-applying the cooler, etc.  Sigh.

Anyone have a second opinion?  I'd love to be wrong!

 

Unknown_K

Well-known member
Since a G3 PDS slot upgrade disabled the on board CPU could this be a way around saving the machine (other then either finding somebody to replace the soldered on CPU or getting a new MB)?

 

Bolle

Well-known member
You sure the PDS upgrade disables onboard on those machines? I think for Sonnet at least you need the extension to load or otherwise it will run on the 601 instead of the G3.

 

alk

Well-known member
I don't believe the G3 PDS cards do anything to the CPU. I'm reasonably confident that they require an extension to offload processing from the soldered 601.

 
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