VAX 4000-500 PSU Overload?

Robert Jarratt robert.jarratt at
Sun Dec 13 05:21:18 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Maciej W.
> Rozycki
> Sent: 11 December 2015 01:48
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
>  No leaking ChemiCon SXF-series capacitors inside?  They seem to suffer
> from some kind of design or manufacturing defect and consequently the
> electrolyte gets through the seal eventually even in parts never used.  A
> number of DEC PSUs have these capacitors, including the H7874 PSU which
> BA430/BA440 VAX 4000 cabinets use.

Just opened it up and there are indeed leaking caps, the brown ones. There
is also a strange deposit on a couple of screws (picture here:, but the screws are nowhere near the leaking
capacitors. Any thoughts on that?


>  Unfortunately replacing the parts is not easy as heatsinks block access
to the
> soldering pads and you need to desolder main rectifiers first to gain
> You need to clean any electrolyte spills too as they will cause corrosion
> shorts.

Yes, I have had one of these PSUs apart before and know how horrible it is
to get at these parts. I will however go ahead and replace these caps.

>  I observed similar symptoms with the failed PSU as you do: it started up
> briefly, enough for diagnostic output to start coming from the system
> through the console port, and then within a couple of seconds the PSU shut
> down.

I don't have an easy way to apply test loads to all the outputs at the same
time. Is it possible to remove the final output boards for each "half" of
the output and test one at a time without causing the PSU to shut down?



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