28 Aug
2014
28 Aug
'14
4:45 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Peter Coghlan
Sent: 28 August 2014 23:32
To: General Discussion: On-Topic Posts Only
Subject: Re: Mysterious rtVAX 1000 Failure
rather
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 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...
Mine has the selector switch. It did occur to me that the switch could have
been in the wrong position, it is actually hard to tell which position it is
in, although it looks to be on 220-240, which is right for me here. I would
expect the failure to be immediate if it was in the wrong position, or is
that not necessarily so? I will look inside the sleeved components to check
for anything there.
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.
I couldn't see any Tantalum caps when I thought of this. I have a working
PSU which I can dig out with a bit of difficulty, so I may be able to
compare and see if I can spot something.
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.
Indeed, some time passed between switch ons.
Thanks
Rob