28 Aug
2014
28 Aug
'14
3:32 p.m.
I have started playing with the rtVAX 1000 I collected last year. It was
running OK when then was a sudden bang and the machine stopped. Not even the
fans will run. There was no smell and no smoke, so I knew immediately that
it can't be the mains filter capacitors, which always fail on these power
supplies.
I have opened it up and cannot see any damage in the PSU, the CPU, the
memory, or any of the other boards. I am sure it must be the power supply,
but I really can't see any damage. The power supply is one of the ASTEC ones
common in the MicroVAX II.
Is it one of those 115V/230V PSUs that autosenses the applied voltage rather
than has a voltage selector switch? I've had an autosensing PSU that got
itself stuck in 115V mode and the supply here is 230V. The result was one of
a pair of VDRs in black rubber sleeves failed with a loud bang at switchon.
The debris was contained within the sleeve and it took quite a while to figure
out what had gone bang then a lot of headscratching to figure out why...
Another possibility might be an exploding tantalum electrolytic decoupler.
They can leave little evidence that they were ever there but it is hard to see
how one could permanently take out all outputs from the PSU in one go. It
is a good match for a failure some time after switchon though.
Some of the DEC PSU's I've come across have to be left switched off for
a few minutes before they reset after an overload but I doubt you've had it
switched on all the time since the problem happened either.
Regards,
Peter Coghlan.