VAX 4000-500 PSU Overload?

Holm Tiffe holm at
Thu Dec 17 09:52:41 CST 2015

Robert Jarratt wrote:

> > -----Original Message-----
> > From: cctalk [mailto:cctalk-bounces at] On Behalf Of Holm
> Tiffe
> > Sent: 16 December 2015 17:27
> > To: General Discussion: On-Topic and Off-Topic Posts
> > Subject: Re: VAX 4000-500 PSU Overload?
> > 
> > Robert Jarratt wrote:
> > 
> > ...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 intend to do some more analysis and testing before putting the PSU back in
> the machine. I just tried using a current-limited bench PSU on the outputs
> of the PSU to see if one of them behaves oddly. I suspect the -12V output is
> not right, it sucked up more than 1A with just 0.3V across it. The other
> outputs sucked up much less current and increased slowly.

..sounds at least interresting. Maybe you have a bad Tantal Capacitor
somewhere on the boards..
Try to pull boards until the current decreases...

> Of course it could have been a totally invalid test, but I know in some
> circumstances it can help to see if the output stage and the crowbar is
> working. 

Whatever you want..  I'm curious what you will find out.
> > 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..
> > 
> Some people suggest removing those batteries altogether, to avoid damaging
> leaks.
> Regards
> Rob

The machine is build to contain those NC Packs, and yes I know that gases
from the Cells will corrode the electronics around it.
But the machine had some years running time on it and the corrosion effects
are pretty small, so I think with the fitted new battery it will last some
additional and I could handle this.



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