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The schematic is available from Studer's archive service.
I forget the name of the service, but I found it on the Studer website.
I had to send an email and request for the specific schematic, but a nice woman sent me a link.
I downloaded it as a .pdf
It sounds like the bypass contacts on the pedal are not working... this may be right in the pedal switch itself. With unit in off position, use an ohmmeter to check continuity between in and out cable tips.
You must be on 50Hz power. You don't really need a schematic to repair the unit. Since the hum is twice power line frequency, the likely cause is the electrolytic filter cap for the high voltage has failed. The cap should be obvious in the unit. It will be near rectifier diodes and have a voltage rating above 50 volts. To test, bridge the one in there with another cap observing the correct polarity. If hum changes, replace the original cap.
I suspect the inut power circuits have a failure. Electrolytic filter cap and the votage regulator (probably a 7805 type) may have failed. IF the 7805 failed, then there is likely a short on the 5 volt supply to the electronics. In short, unless you are adept at electronics, you need to take it to an electronic tech for repair.