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Hello,
This is my first post and I want to apologize beforehand if I have missed a thread that pertains to my particular issue. I have scoured the internet for several hours and still have not found a solution to the – what I believe to be – 120 Hz buzz coming from my Bassman AB165. This noise happens without an instrument plugged in and stops when I remove v4 or v2 but remains when I remove v3 or v1. The hum is almost completely gone with both volumes at zero but will appear at about 1-2 when either are turned up and get louder as volume knobs are turned up.
Recent repairs include: new power transformer due to shorted primary in original, all new cathode bypass caps, all new filter caps, new bias supply cap, all new rectifier and bias diodes, 100 ohm resistor in series with the middle grounded on filament taps, combination of new carbon comp and metal film plate resistors, three prong cord, removal of ground switch cap, new screen grid resistors and new grid stoppers.
I have double checked ground connections with a probe grounded at the power cord, double checked all the new capacitors, tested preamp coupling caps for DC leakage, measured DC resistance of the choke, substituted a 100 ohm resistor for the choke and carried out wooden skewer test with no changes or insights.
Plate Voltage measurements:
V1a = 240
V1b = 248
V2 = 300
V3a = 245
V3b = 297
V4a = 245
V4b = 241
I’m at a loss and any help would be greatly appreciated!
This is my first post and I want to apologize beforehand if I have missed a thread that pertains to my particular issue. I have scoured the internet for several hours and still have not found a solution to the – what I believe to be – 120 Hz buzz coming from my Bassman AB165. This noise happens without an instrument plugged in and stops when I remove v4 or v2 but remains when I remove v3 or v1. The hum is almost completely gone with both volumes at zero but will appear at about 1-2 when either are turned up and get louder as volume knobs are turned up.
Recent repairs include: new power transformer due to shorted primary in original, all new cathode bypass caps, all new filter caps, new bias supply cap, all new rectifier and bias diodes, 100 ohm resistor in series with the middle grounded on filament taps, combination of new carbon comp and metal film plate resistors, three prong cord, removal of ground switch cap, new screen grid resistors and new grid stoppers.
I have double checked ground connections with a probe grounded at the power cord, double checked all the new capacitors, tested preamp coupling caps for DC leakage, measured DC resistance of the choke, substituted a 100 ohm resistor for the choke and carried out wooden skewer test with no changes or insights.
Plate Voltage measurements:
V1a = 240
V1b = 248
V2 = 300
V3a = 245
V3b = 297
V4a = 245
V4b = 241
I’m at a loss and any help would be greatly appreciated!
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