Bush DAC 90a loud hum

[John Earland]

Originally Posted by Sideband

UL 41's are rare and expensive. You might be able to fix the old one by burning off the residue inside the glass around the pins. You need a high voltage to do it (around 1,500V at several milliamps). Join the two heater pins together (1 & 8) and then apply the voltage across any two pins (except 1 & 8 of course). The idea is to 'arc' the internal deposits away that are causing the leakage. I have seen a device made up from an old camera flash unit which seemed to do the job quite well.

That sounds like an interesting experiment. I don’t think I have the equipment but I’ll hang on to it just in case I do!