VAX 4000-500 PSU Overload?

Robert Jarratt robert.jarratt at
Tue Dec 15 14:38:19 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Robert
> Jarratt
> Sent: 13 December 2015 15:47
> To: 'General Discussion: On-Topic and Off-Topic Posts'
> Subject: RE: VAX 4000-500 PSU Overload?
> I have been able to remove the leaking ones from the +12V and -12V board,
> just have to wait for the new parts to arrive. The two on the 5V and 3.3V
> board have not leaked, are not bulging, and are harder to remove without
> removing the bottom heatsink or pulling from above as you suggest. I have
> not been able to reach them with my ESR meter either, unfortunately. I am
> tempted to leave them, although pulling from above is an option. Does
> anyone else agree with the pull from above solution? In case you have not
> seen it, the design makes it very hard to get at the underside, you have
> desolder a load of transistors to do so, and they are not in easy
> either.

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.



More information about the cctech mailing list