10 Dec
2015
10 Dec
'15
7:15 p.m.
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!
Jon