VAXstation/MicroVAX 2000 CPU/FPU overheating?
dave06a at dunfield.com
Tue Feb 26 20:00:53 CST 2008
>>> 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
manual, the power supply and fan should be the same in the main system
cabinet and in the matching expansion boxes (same chassis) - I have two
expansion boxes (tape and disk) and all three have exactly the same fan
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.
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
seems to be working, as noted above, it behaves virtually identically to the
other two boxes. I've done some googling and have not found any refernces to
other uVAX2000's having this problem.
Removing the color display buffer board should allow for better cooling of
the mainboard, as the two boards are large and sandwiched together in a
fairly thin air duct. But again, I think if this were a common cause of this
problem, there would be references.
> > Could be for all I know. Anyone got a fan CFM spec for the 2k? :)
> The Technical Manual seems silent on that point. I'll write and complain
> If Dave can say what the fan actually is, I'll take mine apart and
> see if it's the same. Might be interesting to see if my fan works at all
> or not too :-)
The fan in mine (as well as the two expansion boxes) is a:
made by Matsushita
I didn't have to take any of my boxes apart, the fan label can
be easily read through the rear air grill.
I would like to know if your machine gets as hot as mine - with the
main cover removed and nothing else, after a few minites, mine gets
a very noticable "hot spot" on the flexible shield covering the upper
(solder) of the mainboard above the CPU's (near the front of the
machine, near the center but more on the drive side). Unfortunately
you can't access the CPU/FPU heat sinks without removing the board
(unless you have a thin temperature probe) ... on mine, both the CPU
and FPU sinks get to the point where hanging on to them would probably
result in skin damage (blister etc.) - it seems to me that the FPU
(the one closest to the RAM array) get a bit hotter, but both are well
into the "uncomfortable hot" range...
Given that they both seem to run directly from the 5V rail, and that is
well with spec (something like 5.01 when I measured) I can't think of
any external problem that would result in that much current draw and
sill have the machine appear to operate normally (can't really think
of an internal problem either). Really has me stumped - there seems to
be reasonable airflow through the cabinet, nothing seems to be blocked
etc. It just gets so hot it dies... if the system were that "close to
the edge", I'm sure I would have seen references to more problems. I
did find a few references to thermal death when operated in very hot
environments, but I keep my office on the coolish side.
dave06a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
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