VAX 4000-500 PSU Overload?

On Thu, 10 Dec 2015, Robert Jarratt wrote: No leaking ChemiCon SXF-series capacitors inside? They seem to suffer 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 5 330?F/25V parts and 1 1000?F/25V part of this kind total in the H7874, scattered across its PCBs. All the 330?F 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 100?F/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