VAX 4000-500 PSU Overload?

Robert Jarratt robert.jarratt at
Sat Dec 19 01:30:48 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Maciej W.
> Rozycki
> Sent: 17 December 2015 16:18
> To: cctalk at
> Cc: cctech at
> Subject: RE: VAX 4000-500 PSU Overload?
>  NB the two brown capacitors seen in the middle top of your photo are
> among ones that leaked in my PSU; an SXF marking is barely recognisable on
> the edge of the lower one in your photo.  After considerable effort
> this year I succeeded with removing the intermediate heatsink blocking
> access to these caps, by desoldering the four rectifier dual-diodes
> the heatsink in place -- at the cost of losing a leg from one of the
smaller ones
> (a Motorola MBR3045PT) and some damage to the PCB.

These caps are on the 5V/3.3 board, and after close inspection, do not
appear to have leaked. I have been unable to reach them with the probes from
my ESR meter either. Given that the leaked ones were on the 12V board I had
just replaced those, and cleaned up the leaked electrolyte and then tested
the PSU again. It did work briefly, with no ripple on 5V, but I did not have
access to measure 3.3V ripple. I have had suggestions to just pull these out
from above and solder them in from above, rather than go through the
horrible procedure you describe. I am going to use a dummy load today to
test the PSU on the bench to see if it shuts down outside the machine. The
problem will be to know which of the outputs is causing the shutdown. I
wonder if it is possible to test the PSU with one output board removed, so
that I can identify which board is the bad one? I know I already asked this,
but if anyone happens to know that would be great!

>  Fortunately MBR3045PT parts are still available and I was able to get a
> replacement, and the damage to the PCB is I believe not critical.  What's
> important the larger rectifier dual-diodes (84CNQ045; no clear indication
> the manufacturer) have survived intact, as these seem to have become
> unobtainium now.  Apparently the last die foundry capable of making these
> parts has discontinued them earlier this year due to lack of customer
> interest, so the only source remaining might be part recovery from
> broken equipment.
>  Overall I think I'll need better tools to be able to desolder such stuff
in a
> more repeatable and less destructive way.  The thick legs of the
> 84CNQ045 parts combined with the large volume and consequently thermal
> capacity of the intermediate heatsink seem to be able to take heat away
> virtually instantaneously.  I'll appreciate your advice on choosing a good
> soldering/desoldering station, capable of handling such high-current (and
> consequently highly heat-conducting) parts.

Well, when I tried to do this on a previous H7874 I had, a friend of mine
(on this list) helped me out with this on the 12V board. We used one of
01384?MER=e-bb45-00001001 without causing damage. I now have one and it
works OK, though be careful if, like me, you use some pressure to bend pins
upright while trying to desolder, the barrel is not actually that strong!

>  For the curious and possibly to provide some information on the parts
> involved I've documented progress with the disassembly of this module at:
> <>
> (large photos!).  Removing the offending caps revealed C322 and C323
> designation underneath; these are 330µF/25V parts.  I decided not to move
> forward with installing replacements and reassembling the module, or
> proceeding with disassembling the other module without upgrading my
> soldering/desoldering tools first though.

Those pics show the full horror of this PSU and getting access to the parts!
Not for the faint of heart :-)

>  No idea.  I've only recently started finding my way with this system; I'm
> mostly a software person.  Admittedly quite a low-level one, but still on
> software side.  And it's been only earlier this year I discovered the
issue with
> the dreaded SXF caps, even though I've been using various DEC
> hardware/PSUs for over 15 years now.

Yes, I am similar, mainly a software person really, did a bit of electronics
at University, but not much at all. Rapidly having to learn more about PSUs!



More information about the cctech mailing list