jensenmann
Well-known member
I have an early Manley VariMu on my bench which is humming way too much. In the beginning the hum was acoustically audible and is visible on the FFT. It has three mains transformers and my impression was that all three were humming/vibrating to a degree that it wasn´t usable in a mastering environment from it´s acoustic output alone.
I did the obvious things like recapping and replacing the rectifiers which have suffered from heat. A zener diode in the HV regulator circuit was defective and created a lot of overtones >250 Hz across the entire spectrum. After that step there were still overtones visible on the FFT but hum @ 50, 150 and 250Hz was dominating. The mechanical vibration of those trannies made me suspect and I replaced all carbon comp power resistors which completely erased all overtones >250Hz. What´s left now is the hum at 50, 150 and 250Hz. Since 100Hz isn´t a problem the PSU should be fine now. At this point the mechanical vibration of the mains trannies went down to an usable level.
Tube heaters are regulated. That means the 50Hz hum has to be magnetic interference. By the lack of better ideas I made an mumetal shield from selfadhiesive foil for the output transformers. That helped 6dB @50Hz and 3dB @150 + 250Hz. But there´s still hum in the signal at a too high level.
The last thing I found out is that the power consumption is 90W while Manley´s specs say 80W. But I have no idea what I should check now. I´m out of ideas. If you guys have a hint that´d be highly appreciated.
I did the obvious things like recapping and replacing the rectifiers which have suffered from heat. A zener diode in the HV regulator circuit was defective and created a lot of overtones >250 Hz across the entire spectrum. After that step there were still overtones visible on the FFT but hum @ 50, 150 and 250Hz was dominating. The mechanical vibration of those trannies made me suspect and I replaced all carbon comp power resistors which completely erased all overtones >250Hz. What´s left now is the hum at 50, 150 and 250Hz. Since 100Hz isn´t a problem the PSU should be fine now. At this point the mechanical vibration of the mains trannies went down to an usable level.
Tube heaters are regulated. That means the 50Hz hum has to be magnetic interference. By the lack of better ideas I made an mumetal shield from selfadhiesive foil for the output transformers. That helped 6dB @50Hz and 3dB @150 + 250Hz. But there´s still hum in the signal at a too high level.
The last thing I found out is that the power consumption is 90W while Manley´s specs say 80W. But I have no idea what I should check now. I´m out of ideas. If you guys have a hint that´d be highly appreciated.