cecilk
Active Member
I'm repairing an Airline (Montgomery Wards) GAA-2940 console and after noticing the sound quality was a little off tone-wise and distorted, I have tested and replaced all electrolytic capacitors in the circuit (many were way off tolerance and quite leaky).
The console now works fine - AM/FM both play and have decent reception. Tape inputs work fine. I'm waiting on a stylus so I can't test the turntable yet.
The problem? There is a noticeable 60 Hz hum when volume is at 0 on all channels (tape, phono, AM, FM). I've plugged the unit in to a few different outlets and turned off different lights and other components that I have in the room just in case there was any issue with interference.
From what I understand, a 60 cycle hum that is present on all channels usually indicates an issue with the power supply.
The console now works fine - AM/FM both play and have decent reception. Tape inputs work fine. I'm waiting on a stylus so I can't test the turntable yet.
The problem? There is a noticeable 60 Hz hum when volume is at 0 on all channels (tape, phono, AM, FM). I've plugged the unit in to a few different outlets and turned off different lights and other components that I have in the room just in case there was any issue with interference.
From what I understand, a 60 cycle hum that is present on all channels usually indicates an issue with the power supply.
- C18, C17A, C17B, and C17C are all new Nichicon caps. I over-spec'd them by accident so they are rated for 250v (I'm simultaneously working on a bunch of tube AC/DC radios and wasn't thinking).
- I've replaced R96, R97, and R98 with 1% metal film resistors.
- Power supply voltages are all pretty good - Not "spot on" but within range (-17v reads as -20.8v, +17 reads as +15v, +13v reads as +12v, +9.1v reads as +8.8v).
- I've tried varying the value of R96 to "balance" the positive and negative voltages (+17v and -17v) and got them spot on but this had no effect on the hum.