VAXstation/MicroVAX 2000 CPU/FPU overheating?

Sridhar Ayengar ploopster at
Mon Feb 25 12:33:20 CST 2008

Dave Dunfield wrote:
>> On the one hand, my wetware RAM QC department is glad I rememebred
>> correctly that the problem with that machine was overheating.  On the
>> other, I'm a bit disappointed it wasn't something simple and easily
>> fixed like a fan not turning properly.
> So far I've not been able to find a cause.
> I found schematics on bitsavers (thanks Al), but unfortunately they
> don't show power (or even most power/ground pins). As far as I can
> tell, the CPU & FPU are directly connected to bypass capacitors which
> are shown on the schematic as sitting on the 5V rail, so I believe
> they operate on straight 5V - The 5V rail is well within spec. Clocks
> look good as well.
> I have a feeling the FPU is knackered - it seems to be generating more
> heat than the CPU (hard to tell as they are mounted side by side and
> there is a fair bit of heat trasnfer through the board). Unlikely I
> can find a replacement --- Anyone got a VAXstation 2000 or MicroVAX 2000
> thats dead for some other reason?
> Odd thing is that everything seems to work - if I place a small fan
> to keep the CPU & FPU cooled to "just hot", it settles back down and
> all seems to work correctly. All self-tests pass (including FPU). It
> appears it would run indefinately in this state, but it's obviously
> not healthy. It seems unlikely that there would be an internal chip
> failure that would cause it to eat this much power and still perform
> it's intended function.

This may be a stupid question but bear with me.  Is the surface of the 
FPU chip getting hotter than the heatsink sitting on it?

Peace...  Sridhar

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