VAXstation/MicroVAX 2000 CPU/FPU overheating?

[Tony Duell]

> 
> >>> Do you know that the fan is the original one? Is it possible that 
> >>> somebody has replaced the fan with a lower airflow one in a misguided 
> >>> attempt to make the machine quieter?
> 
> A good idea, but I don't think this is the case. According to the technical

OK... Not having seen the machine, all the rest of us can do is make 
guesses, most of which are likely to be wrong :-)

[..]

> I've not done measurements, however the fan seems to be operating correctly,
> good airflow, doesn't sound like it's dragging etc. The sound and airflow
> are not noticibly different from the other two boxes.

It sounds like it's OK. What I normally do to detect a dragging fan is to 
watch it when I turn the power off. If it stops very quickly, it's time 
to dismantle it and clean the bearings...


> 
> According to the technical manual, the fan is supposed to be "variable speed".
> It's a 12V DC fan, however it is powered via a 7905 (-5V) regulator which
> has it's reference through a resistor divider which includes a thermistor.
> So presumably it speeds up as the cabinet gets hot - since the thermistor is
> in the power supply, which has somewhat of a separate airflow path (metal
> plate between PS and boards) it may not detect the "hot spot" forming at
> in the other airflow path.
> 
> I'll have to do some measurements. I could try hard-wiring the fan to full
> power, however this thing *should* work as it is designed, and the fan supply

True. On the other hand, if making the fan run at maximum speed all the 
time keeps the machine running, it's worth doing that mod (IMHO) to save 
the problem of replacing the CPU or FPU chips.

-tony