H7874 power supply

[Robert Armstrong]

>Looks like for this enclosure an ATX supply could well work.
> For my VAX my notes say it didn't.

  A VAX would certainly be harder.  You'd have to kludge up the ACOK and DCOK signals for one thing, which I don't think the R400x uses.  It'd be really handy to find even a schematic for the backplane so we could see which signals are actually connected, but I did some searching and came up with nothing.

  Oh, and I did try to power up the H7874 on the bench for testing, but it wouldn't turn on there either.  It probably has a minimum load requirement, or it needs some signal from the backplane to turn on.  FWIW, the R400x does not have a power switch - the only way to turn it on or off is thru the H7874, either by the circuit breaker or the power control bus.  The R400x also has several large power resistors on the M7493 SCSI connector module; those may be there to provide a minimum load for the power supply.  

Bob