VAX 4000-500 PSU Overload?

[Robert Jarratt]

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] 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,
I
> 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
to
> desolder a load of transistors to do so, and they are not in easy
locations
> 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.

Thanks

Rob