29 Aug
2014
29 Aug
'14
1:27 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Robert Jarratt
Sent: 29 August 2014 12:04
To: 'General Discussion: On-Topic and Off-Topic Posts'
Subject: RE: Mysterious rtVAX 1000 Failure
I have done some reverse engineering on the board that houses the rectifier and which has
the outputs to the fans. I had been assuming that the fans were powered by this board
alone, possibly through that linear regulator, and not through the SMPS, which led me to
think that if the fans don't work it must be this board rather than the SMPS (I am not
an electronics person, so pardon me if that was a silly thought).
However, I now see that it is impossible for this to be the case. The rectifier portion of
the board is isolated from the portion which powers the fans by an optocoupler, so the
power is not coming from the rectifier. In which case it must be coming from the SMPS on
the other half of the clam shell, through a connector which I had assumed was an output
from the rectifier board, but appears to be an input.
As power is getting sent to the SMPS half of the clam shell from the rectifier, that means
the fault has to lie in the SMPS side of the clam shell. This fills me with dread, as the
board is much more complex on this side, I don't have schematics, I am no PSU expert,
and I still can't see any physical damage, although there *must* be some.
Regards
Rob
-----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
up.
-----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
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?
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