29 Aug
2014
29 Aug
'14
4:03 a.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Robert Jarratt
Sent: 29 August 2014 08:40
To: 'General Discussion: On-Topic and Off-Topic Posts'; 'General Discussion:
On-
Topic Posts Only'
Subject: RE: Mysterious rtVAX 1000 Failure
Answering my own question, I decided it was safe to connect up the filter/rectifier half
of the PSU, and indeed it was. I have found the following:
The large capacitors, which I think are smoothing the output of the rectifier, are
charging up. The V+ and V- connector that goes to the other half of the PSU has about 320V
across it. However, there is another connector that output +15V and -15V, these are both
at zero.
The LM317T linear regulator appears to have no input and no output, it could be that I am
measuring it incorrectly though. I connected one probe on my DMM to the chassis and then
tried the three pins on the regulator, all at zero.
So I am guessing the output of the rectifier is not reaching the regulator, which I am
guessing runs the fans. It would seem that the rectifier itself, and the input filter are
all fine.
I still can't see any failed components, but the next step is going to be see if I can
work out the schematic by examining the board, so that I can narrow down where the failure
might be.
Regards
Rob
-----Original Message-----
From: cctalk-bounces at classiccmp.org
[mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Chuck McManis
Sent: 28 August 2014 23:09
To: General Discussion: On-Topic Posts Only
Subject: Re: Mysterious rtVAX 1000 Failure
If it actually made a 'bang' sound the most likely culprit is an
electrolytic capacitor. A visual inspection should find it, although
I've known them to fail pretty amazingly leaving the case intact
except for the circle on the top where the plates came flying out and landed
somewhere remotely.
The system is waiting for P_OK (power OK) before it will fire up, and
the fans are on the non-mains side of that PSU as I recall. Is it the
BA23 case? (most common case) there was an issue with the power
distribution harness on those where DEC used several wires in parallel
to carry the 5V current. Except they were not all the same length and the
short
one would some times vaporize.
The 'fixed' harness you can see all the
wires are the same length
which makes it harder to tuck in behind the back plane but it doesn't blow up.
Yes, it is a BA23 and I do have the "bad" wire harness. In fact I think all my
BA23 machines have the bad one, but I don't run them that often, and they are
not particularly loaded, so hopefully that will be OK.
The fans are powered from the half of the power supply that contains the input
filter and the rectifier board. The bulk of the power supply is in the other half
and does not appear to have any involvement with the fans. As the fans don't
work, this would suggest that the fault is either on the input filter board or the
rectifier board.
Is it safe to connect up the first half of the PSU to the mains, switch it on and try
to probe around to see if I can see where the fault lies?