Date: Tue, 29 Jun 2004 14:37:45 -0700 From: Ken <[EMAIL PROTECTED]>
Acquired a 7100/80 a couple of days ago with hopes of replacing a 7100/66 as a Sonnet G3 upgraded NuBus Mac.
Swapped the logic boards as the /80's case was in rough shape. Reassembled and booted to test the assemble. With the case top off, it booted fine and seemed "happy."
Slid the top back on and hit the power key; Got the boot chime and then it stopped at the grey screen with a cursor arrow. PRAM reset 5 times; nothing but the grey screen.
Started parts swapping to isolate the problem. No luck.
Tested all parts except the logic board in the original 7100/66. Boots and runs great.
Did I get the final few minutes of life out the old thing when I first tested it with the top off?
Most likely the heat sink grease between the PPC601 chip and the heat sink has dried out and is no longer conducting heat properly. When you boot up the chip runs for a minute or a few seconds before overheating.
Remove the motherboard again, remove the heat sink (push the four clips out from the back of the motherboard), clean off the old dried heat sink grease from the CPU and the heat sink. The old stuff will look like dried white toothpaste.
A foam swab and isopropyl alcohol works for cleaning the stuff off, but cotton balls will do a pinch, just be careful about leaving fibers behind.
Place a small dab of new grease on the little square metal die in the center of the PPC601 and replace the heat sink.
Warnings: While doing this do not bear down on the PPC601. They are prone to cracking if one presses down on them too hard.
Really just use a little dab of heat sink grease when replacing the stuff. The grease just needs to fill the imperfections between the heat sink and the CPU die, which are caused by neither surface being perfectly flat. As you can imagine, this requires very little grease. The grease is conductinve, and if it squishes out and runs all the way to the edge of the CPU it may get on the pins that surround the CPU and short some of them out.
You can get serviceable heat sink grease at Radio Shack for about $3. It comes in a little blue and white tube which is likely to be packaged on a card and hanging on a peg board somewhere in the store near the soldering supplies.
I've seen a number of 7100s with your symptoms and this cause. It could be something else, but this is a cheap easy fix well worth trying.
Jeff Walther
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