I've racked up a PM1000 using the PSU-4448. Everything worked great when I first powered it up. Used it with a condenser mic - no problem. Later that day, nothing. The output cap (C37) had blown, so I replaced it. Everything works great, then that blows in under an hour. All voltages appear correct. I've used a 50V and 35V cap. Maybe it's the voltage leading up to the output cap?
So is it your PM1000 board having this problem? C37 is a 33uf/25V according to the Yamaha schematic.
It's a DC blocking cap for the output stage of the PM1000. My guess is you may have a short somewhere on your PM1000 board that's impressing a voltage across the capacitor more than it's rated 25Volts.
Check your PM1000 board.
Have you recapped this PM1000 board? Maybe the capacitors on this board are leaking or shorted internally... being 1974 era, it's probably a good idea for a recap.