Studer 269 Phantom Power Question

[echo zulu]

I have a potential issue with the phantom power on this mixer and I'm hoping that someone who's familiar with this particular board will respond.

The (unloaded) PSU outputs 48v when cold, but drops about 5v after it's been on for about an hour. Is this typical behavior for this supply? It was recapped with high quality electrolytics by the previous owner and I don't see anything obvious wrong with the circuitry.

 

[Bo Deadly]

Sounds like bad power silicon. When silicon parts (transistors, diodes, triacs, etc) get hot, they can get burned up inside and start to loose their switching capability. Meaning they don't turn on or off all the way and / or become slow. They might behave ok when cold, but when they warm up, the bad behavior begins.

You might identify which part is responsible by blowing cold gas directly on an individual part and see if it fixes the problem. Specifically, get an "Air Duster" canister with a straw and slowly blow cold gas directly (put the straw tip as close as you can without touching it) on a particular part like transistor or diode or rectifier and then watch the voltage. If the voltage starts to go back up, you know that part is at least partly responsible. But if one part like that is bad, you might want to replace anything that is handling a lot of power. Particularly parts where the PCB or part itself looks discolored.