Proton AM455 Hum

[Carlosraj]

Dear Friends,

I am trying to fix a Proton AM455 amp. When I switch on the amp. at times it works normal but at times, it has sound but also a low hum frequency hum behind. At times, there is no sound at all but only the hum.There are four 4700UF,50V caps. When I measured the capacitance, 1 caps read 4200UF and the other 3 read 3900UF. The voltage across 2 caps is 44volts and the other 2 is 42volts. Are the capacitance and voltage reads normal?. The transformer has 2 pairs of secondary windings, one pair reads 31.9 volts AC and the other reads 35.9 volts AC. One more thing I noticed is, when the amp is working normal, as I turn down the mains voltage from 240 volts AC to 200 volts AC, the hum comes back. I think this is abnormal. I have tried turning down the voltage on other amps before. Even at 100volts AC, the amp was still working and no hum. Can someone please help and explain why the hum,

Thanks

Carlos

 

[rsavas]

Using a DMM, monitor the AC signal riding on the DC supplies to determine if the caps are failing under bias. A scope would work even better if you have one available.

Rick