VAX 4000-500 PSU Overload?

[Robert Jarratt]

> 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