VAX 4000-500 PSU Overload?

Peter Coghlan cctalk at
Thu Dec 10 16:37:22 CST 2015

> 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?

Your meter used on a resistance range will not give you a good idea of the
load presented to the power supply.  Most of the load is semiconductors which
will not behave normally as they will not get enough voltage from the meter.
The one case where it may be helpful is if you measure a dead short or very
close to it, indicating something like a shorted decoupling capacitor or maybe
a sliver of solder or other conductive object bridging some contacts somewhere.
6R is not low enough to indicate this.  It will cause a bit under 1A to flow
from the power supply. 1A is likely much less than the machine normally draws
on the +5V supply.

I would suggest disconnecting the power supply completely from the machine and
putting a dummy load such as a 6V headlamp bulb or bulbs on it instead.  If the
PSU works normally on the dummy load, reconnect the machine and look for some
component getting hot.

If the PSU continues to trip when on a dummy load, try looking for shorted
rectifiers and/or smoothing capacitors on the output side of the PSU.  If the
output voltage is trying to climb too high, maybe a crowbar trip is operating?
(As Tony always says, don't disable the crowbar to try to find the problem!)
> 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.

It seems reasonable but again, your meter on a resistance range is not going
to tell you much here due to the semiconductor load.

> The 3.3 and -12V show very high resistance at all times.


Peter Coghlan.

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