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Vintage threads from the first ten years

Reverb hum

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5/19/2000 2:43 AM

Just Blackfaced a 69 Bandmaster Reverb AA768. Replaced chocolate drops, ceramic caps, filter caps, etc. Works and sounds great except for nasty reverb hum. Sounds like a single coil hum. When the reverb knob is all the way down, no hum. As it is turned up, the hum grows louder. This hum happens whether or not the footswitch or reverb unit is plugged in! When the reverb unit is plugged in, I get reverb and hum together. When I touch, or come close to, the .002 cap on the reverb output jack, the hum gets a lot louder. Replaced tubes, checked voltages, all are ok. The gound connection checks out. I'm stumped.

Thanks for any suggestions.

5/19/2000 2:55 AM
DonJ	Jim, Check your cables to the reverb pan I had mySFPR do the same thing. Just a thought and worth a try. My cables were loose in the rca jacks.

5/19/2000 3:28 AM
M/J	What schematic did you use to black face it? M/J

5/19/2000 3:39 AM
Jim Martin	It was already very close to BF. Had to change the plate load resistors from 45k to 82k and 100k. Cut off the bypass caps on the power tubes and added a bias level to the bias balance pot. That's it.

5/19/2000 3:27 AM
Eric Eisenhower	I just restored a Gibson Minuteman with the same problem. Check the cables and the connectors. Pretty sure that's where the problem lies. -e

5/20/2000 4:18 PM
Tom	Had the same problem in a '71 BMR, The return RCA jack was bad. the reverb became intermittent, and hummed terribly when not working. The player would wiggle the cable and it would work again. Also the 220k resistor on the jack opened up during the jack replacement... Not sure if that was also a problem; so check both. Tom

5/20/2000 4:37 PM
JC	I'm new here, but once had a similar problem. I would check the ground connections of all the reverb circuit too. Pots, the tank itself... The reverb's tube socket gave me some problems too. JC

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