Workgroup Server 9150 Freezes Without Additional Cooling - Can anyone check their WGS VR temps?

Mac84

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Sep 4, 2021
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Hello all,

I recently did a live stream (link) where I attempted to repair a client's WGS 9150. (120MHz). The symptoms were that the system would freeze after being on for a period of time.

The voltage regulator behind the ROM slot had some visible singing of the board, so I decided to replace it. They are part Lt1085, I believe this is a data sheet for them.

In addition, while one of the pins did have 12 V being read, the other two pins read negative MV values. After replacing both voltage regulators (the other being right next to the CPU) the voltage regulators appeared to be happier. However, they seemingly run very hot. I've attached a digram below showing the voltages I got on my voltage regulators (after replacing them) in the event that helps anyone in the future. :)

With the system on (running a screensaver) it would freeze after under 5 minutes of use. I decided to try leaving the system on (after letting it cool down) with a small USB powered fan over the system. This seemed to work and the system ran without an issue for over 10 minutes (running an After Dark's flying toasters).
Before I shut off the machine I recorded the temperatures of both voltage regulators and the CPU heatsink. With the small fan pointed at these, the voltage regulator behind the ROM slot (away from the CPU) measured 185 F°, the voltage regulator by the CPU measured 160° and the CPU measured about °120. 🥵🔥

If anyone happens to have a WGS 9150 and a thermal camera, I'd be very, very curious to know what your voltage regulators measure!

I may re-cap the board, maybe the old aging caps are making the voltage regulators work extra hard. If anyone has had any similar experiences with this type of heat or components, I'd love to know more.
 

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Reactions: Drake

trag

Tinkerer
Oct 25, 2021
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Kind of late replying...

The voltage regulators are resistance based rather than Buck or Switching type. That means that they basically convert all the excess voltage to heat to get the desired voltage. So it's not surprising they get hot.

I've seen similar issues on the PowerMacintosh 7100. In many cases the heat sink grease between the heat sink and CPU has turned to chalk.

Have you removed the CPU heat sink, cleaned it and the CPU and applied new heat sink compound?

Do not overapply. Usually the stuff is conductive and you do not want it running onto the pins of the PPC601. I killed one that way, once....
 

Bolle

Tinkerer
Nov 1, 2021
48
82
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+1 on applying new thermal paste. Also check if the Peltier is actually good.
Also there's a fan attached to the back of the power supply assembly so it sits right on top of the CPU heatsink once the board is installed in the machine. Make sure that fan is running.