VAX 4000-500 PSU Overload?

Robert Jarratt robert.jarratt at
Sun Dec 20 01:04:07 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Mouse
> Sent: 20 December 2015 01:46
> To: cctalk at
> Subject: Re: VAX 4000-500 PSU Overload?
> >>> But getting something in series to measure load is going to be
> >>> really awkward, [...]
> >> As in, you don't have any such thing to put in series, or you don't
> >> see a good way to get it in series?  [...]
> > I think the suggestion was to put an ammeter between the PSU and the
> > backplane.
> Yes...but ammeters (for nontrivial currents) are generally just voltmeters
> across current-sense resistors, and it's the current-sense resistor that
> to carry the current and thus requires attention.

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
sure 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....



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