VAX 4000-500 PSU Overload?

Holm Tiffe holm at
Wed Dec 16 11:26:36 CST 2015

Robert Jarratt wrote:

> > Tonight I replaced the leaking capacitors on the +12V/-12V board, and also
> > replaced the two similar ones that looked and measured fine. I put the PSU
> > back together, put in a couple of less important boards and drives, and
> the
> > machine now seems to power on OK. I measured the ripple using the power
> > connector for the front panel and that looks OK too. Unfortunately though,
> > that connector only sends out +12V, +5V and -12V. It does not have output
> > for +3.3V, so I have not, so far, been able to check the ripple for this.
> It is a bit
> > awkward to test on the bench with a dummy load.
> > 
> > As I did not replace the capacitors on the +5V/3.3V board, because the
> > underside is very hard to access, and I am reluctant to pull them off and
> > solder from above, I would like to be sure there is no ripple on the 3.3V
> > supply. So, I was wondering if anyone has any neat tricks for probing the
> > +3.3V supply with the PSU installed in the machine?
> > 
> > Next job is to replace the capacitors on the little DSSI terminator, they
> are
> > easy to change.
> > 
> I spoke too soon :-(
> The machine stayed on for a couple of minutes and then powered off. I
> suspect there is a problem with it detecting an overload that may not
> actually be there. Looks like I will need to get a dummy load and put it on
> the bench to see if it still happens there.
> I do recall that when it first failed it would stay on for shorter and
> shorter periods each time I tried it. So perhaps there is some component
> warming up and then causing a failure?
> Regards
> Rob

...sounds very similar to my experience with that PSU. The cure seems to be
simple run that machine then mine has stopped powering off now it seems.

I don't even know if the power off is because of an detected overload at
all, I had problems to even power on the machine the first time before
changing the caps. It would start only the approx 5 time if I switched it
on and I think that those problems are related.

I've installed VMS 7.3 and some additional packages and played for some
days with that machine and it is running stable now if I power it on.
The only thing that I still have to complain about is that the NiCad Pack 
is empty again a little to fast for my taste, the Battery was a new one..



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