VAX 4000-500 PSU Overload?

Robert Jarratt robert.jarratt at
Thu Dec 10 16:30:14 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?


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.





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