On Thursday, March 2, 2017 at 6:58:36 AM UTC-8, Bob Engelhardt wrote:
I have an old PC that recently lost its PS. An identical one was hard
to find,
... the PS is "400W", but the PC only uses 54W.

The biggest surprise was the PS's efficiency, or lack thereof. A
Kill-a-watt on the input showed 200w being used, and 54w output. 25%
more or less. I thought that a SMPS would be way more efficient than that.

Unless your machine is a loud-fans-and-multiple-XEONs type, 54W is a likely
power draw, and 200W is unlikely.

It is possible (on old Apple computers, it's common) to repair a dead power
supply. They're cramped and sometimes have unlabeled tiny chips, but
a check for burst capacitors and shorted rectifiers, and of course fuses, will often
bring 'em back to life.

If you find shorted power FETs, be aware that there's a gate drive which might
have failed, too; those repairs get complicated.