VAX 4000-500 PSU Overload?

[Maciej W. Rozycki]

On Thu, 10 Dec 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.

 No leaking ChemiCon SXF-series capacitors inside?  They seem to suffer 
from some kind of design or manufacturing defect and consequently the 
electrolyte gets through the seal eventually even in parts never used.  A 
number of DEC PSUs have these capacitors, including the H7874 PSU which 
BA430/BA440 VAX 4000 cabinets use.

 Gravity helps this failure to happen apparently as I haven't seen this 
kind of failure in a number of PSUs, like the H7821 or H7819, which were 
kept in their proper position throughout their life and where the affected 
capacitors are mounted leads up.  The design of the H7874 PSU puts the 
capacitors in a horizontal position though when the unit is in its proper 
position.

 You may want to inspect your PSU internally to see if there are any signs 
of failure around SXF capacitors; these have a brown sleeve.  Initial 
disassembly of the PSU is easy, you just need to unfasten a bunch of 
screws and undo three internal connectors.  There are 5 330µF/25V parts 
and 1 1000µF/25V part of this kind total in the H7874, scattered across 
its PCBs.

 All the 330µF parts were failed in a PSU I have recently obtained, 
however replacing all SXF capacitors is I'm told advisable even if there 
is no sign of failure as one is inevitable sooner or later.  While at it 
replacing a pair of ChemiCon SXE capacitors these PSUs have is also a good 
idea I'm told as they might be prone to the same failure, although I 
haven't seen one to happen myself.  These are 100µF/35V, in the same shade 
of brown the SXF parts are.

 Unfortunately replacing the parts is not easy as heatsinks block access 
to the soldering pads and you need to desolder main rectifiers first to 
gain access.  You need to clean any electrolyte spills too as they will 
cause corrosion and shorts.

 I observed similar symptoms with the failed PSU as you do: it started up 
briefly, enough for diagnostic output to start coming from the system 
through the console port, and then within a couple of seconds the PSU shut 
down.

  Maciej