AKG C-12VR Power Supply Troubleshooting

bmack · Dec 10, 2018

Hello All, I'm having issues with an AKG C-12VR power supply and hope you could help. It was brought to me by a client who said the 250mA fuse has blown ( seen at the bottom of attachment coming off the 3volt winding of the PT).

I replaced the fuse, checked voltages and everything tested good. Used it for a couple weeks with no issues, returned to owner and the fuse blew again within a day. The fuse is associated with the heaters for the Mic's tube. The tube tests fine and when replacing the fuse, I adjusted the heater voltage to 12.6v as per the schematic.
This happened 3 more times so I finally went to the clients house to check for electrical issues in his studio/attic of his house. With a multi meter I measured the the hot and cold legs of his house power. Checking the cold legs I got 8 voltsAC and 15 voltsAC in different rooms.
The house is about 130 years old and likely has some remnants of an old North American wiring system called "knob and tube" wiring (no vacuum tubes are present , it's just a primitive ungrounded wiring system that passed the bare wires through ceramic insulators- knobs with a tube through the middle). I've tried telling the client that I can't find any issues with the mic and that he should have his house wiring checked. He's not happy with my answer and I don't know enough about house wiring to give him specifics.
Maybe this belongs in a house wiring thread and not in a mic repair thread, but wanted to see if anybody else has any opinions/theories about the fuse blowing.

Thanks

granger.frederic · Dec 10, 2018

Hi,
I fixed the same issue last year by replacing all the electrolytic capacitors.
stop using it , or you'll cut the transformer...
regards

bmack · Dec 12, 2018

Thanks for your reply. Although I didn't mention it already, I did remove the capacitors from the circuit and check them with a capacitor/ESR tester. They were all good, so I left them in place. I also checked the regulators and they were good.

If it were the capacitors, wouldn't this be happening in my studio as well?

granger.frederic · Dec 12, 2018

beware that most ESR testers often test at high frequency , not between 50 to 100hz...
i'd check that, and if the tester works in a higher frequency, then i'd change all the electrolytic caps, including in the mic.
regards
fred

bmack · Dec 12, 2018

Yeah, I've seen mixed reviews on those cheap ESR testers. There are a few enough electrolytic caps in there that I could change them all faster than testing them. Will give that a shot and see if it helps.

Best