VAX 4000-500 PSU Overload?

Robert Jarratt robert.jarratt at
Tue Dec 22 09:25:08 CST 2015

> -----Original Message-----
> From: Robert Jarratt [mailto:robert.jarratt at]
> Sent: 20 December 2015 07:04
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> Subject: RE: VAX 4000-500 PSU Overload?
> The problem is just how to physically do it. But in any case I don't think
it is
> necessary to do this now, as I have been able to reproduce the problem on
> the bench now. It looks like the 12V side starts up and then shuts down,
> sometimes after a few seconds, sometimes after a couple of minutes,
> sometimes it does not even start to output the right voltage. I am pretty
> that it is falsely detecting overcurrent. The problem is going to be
finding and
> fixing the part that is not working, there are a lot of tiny surface mount
> components that seem to process the signal from the current sense
> resistor....

I was just reading the BA440 enclosure manual, and realised that the PSU
will shut down the system if it thinks the fan has failed. When I have the
PSU on the bench I don't have the fan connected of course. This would
explain the shut downs on the bench even when under low loads, it could be
just that it is detecting fan failure. At this point I realise I could be
barking up completely the wrong tree in suspecting faulty overcurrent

This PSU is really hard to diagnose, because it is so complex, especially
because it has a pretty complex interface to the backplane, not just power,
but also, seemingly, a fair number of control signals. I can't really tell
why it is shutting down, which makes it difficult to know where to begin
once the basics have been checked.



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