H7140 PDP11 PSU

You beat me to it ;-). That was what my last message (sent a few minutes ago suggested). It could be the transformer, but it's unlikely. The trap for the unwary (and it caught me the first time) is to assume that T1 is a normal mains transofmrer. It isn't. It's the transformer of a switch-mode power supply. The clue is that one end of the primary widnign fo this transfotmer goes to the rail that DEC call '300V', that is the positive side of the mains smoothing capacitors. Most of the circuitry for this SMPSU is on the bias and interface PCB. I think I mentioned the chopper transistor in one of my earlier messages. I would start by checking that. If it's shorted, then soem other components, or mnaybe even PCB tracks, will be open. I will try to think of some sensible tests to do on this circuit. The main problem is that it's not isolated from the mains, so you cna;t just clip a 'scope onto it. If you try that at the very least you'll blow fuses, more likely you'll do more damage to the unit (and to the 'scope probe). But as I said, start by finding nad checking the chopper transistor. -tony