VAX 4000-500 PSU Overload?

Holm Tiffe holm at
Fri Dec 11 01:42:01 CST 2015

Robert Jarratt wrote:

> My VAX4000-500 will no longer power up, with the PSU starting up and then
> immediately shutting down. I suspect a possible short somewhere. I have
> measured the resistance of the load presented to the PSU by connecting
> probes to the backplane sockets used to power the machine. The odd one is
> the 5V load. With all the boards in and drives inserted I measure a
> resistance of about 4R. As I pulled out boards, drives and fans, it
> gradually crept up to 6R. So with nothing connected to the backplane I get a
> 6R load across the 5V supply.
> To my inexperienced mind, that seems a bit low. Should I expect such a
> value, or should I be dismantling the box to investigate possible shorts or
> failed components on the backplane?
> Additionally, the 12V side seems to be charging a capacitor as the
> resistance slowly climbs to about 130K. Is that reasonable? Again, nothing
> but the backplane.
> The 3.3 and -12V show very high resistance at all times.
> Thanks
> Rob 

I have an 4000/300 which had PSU Problems too (don't know for now if this
uses the same PSU).
When I got the machine the PSU powered up only sometimes after switching on
and when it was running it shut off itself after some time.

I'm strongly suggest to change all the Electrolytics in
the PSU you can reach, all the brown Nichicons where dry an dead.
One bigger cap (100i0µ)on a PCB lost electrolyte and pissed it all over the
There is a big rectangular Cap in that PSU that I've left there since
I don't have a usable Replacement and the primary caps where still good so
far. The PSU itself is a really bad design from sight of repair, you can't
properly replace the caps on the switcher PCB since you could'nt dismount
the heat spreader so I suggest that you pull the Caps from above and solder
new one from above as I finally did.

Now after some use of the machine the PSU has stabilized somehow, the
machine runs flawlessly.

Another thing is that small PCB on the DSSI storage Backplane, the Caps
there where dead too. I thing it is the termination Power regulator and in
my case the bad caps "simulated" a bad disk, I could'nt initialize this
disk properly before I've changed the Caps there too..


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