VAX 4000-500 PSU Overload?

Jon Elson elson at
Thu Dec 10 21:15:07 CST 2015

On 12/10/2015 04:30 PM, 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.
Most likely there is a failed capacitor somewhere.  Look 
over the boards for small Tantalum caps.  They may not show 
any outward signs of failure.  Finding the bad cap on a 
board with dozens of them in parallel is a nightmare!  You 
should be able to safely power the machine with only a 
couple boards at a time to find which one is bad.

And, of course, it could be the power supply itself!


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