Buzzing right channel on a Sony TA-3200F

[partyface86]

Merry late Christmas everyone

I am summoning the DIYaudio forum experts again for help.

A few days ago I bought a beautiful Sony TA-3200F power amp for about 30$. The amp works great, however there is a light buzzing noise present on the right channel. The left channel is perfect.

DC offset is almost 0V and the bias is at 25mV as it should be. However one of the emitter resistors is burnt a bit, which might be a sign for some kind of overload.

Can anyone tell me what might the problem be. I am only an amateur audio enthusiast with a little electronics knowledge so any help would be greatly appreciated

 

[partyface86]

I also found that the the right channel (the buzzing one) runs a bit warmer then the left after a while...

 

[rayma]

A few days ago I bought a beautiful Sony TA-3200F power amp for about 30$. The amp works great,
however there is a light buzzing noise present on the right channel. The left channel is perfect.
DC offset is almost 0V and the bias is at 25mV as it should be. However one of the emitter resistors is burnt a bit,
which might be a sign for some kind of overload.

Could be a bad electrolytic that's just in one of the channels, perhaps as a coupling cap (there are are least 4 of those per channel).
The emitter resistor is probably ok, (some coloration after time is normal) since the offset and bias are good.

 

[partyface86]

Could be a bad electrolytic that's just in one of the channels, perhaps as a coupling cap (there are are least 4 of those per channel).
The emitter resistor is probably ok, (some coloration after time is normal) since the offset and bias are good.

The amplifier is direct coupled, at least that is what the service manual states. But why coloration on only one of the channels, and why one channel runs warmer?

 

[rayma]

The amplifier is direct coupled, at least that is what the service manual states. But why coloration on only one of the channels, and why one channel runs warmer?

Like I said, there are at least 4 signal electrolytics in each channel. One is an input coupling capacitor. Probably the problem is one of those.
This will require a technician-level skill to track down and repair.
Here's the schematic. Sony TA-3200F Manual - Stereo Power Amplifier - HiFi Engine

 

[partyface86]

Thanks, but I already have the schematic, mine is the 51,501 model. The only input cap that I can find is a 0.47uF/50v mylar capacitor, and one 0.068uF/50v mylar capacitor for the subsonic filter switch

 

[partyface86]

Like I said, there are at least 4 signal electrolytics in each channel. One is an input coupling capacitor. Probably the problem is one of those.
This will require a technician-level skill to track down and repair.
Here's the schematic. Sony TA-3200F Manual - Stereo Power Amplifier - HiFi Engine

I also found that the power switch has a secondary switch that acts as a short circuit between the inputs and ground when the power is off so the amp doesn`t continue playing until the main filter caps discharge.

When i activate that secondary switch the buzzing stops, and I am sure that the amp is still on as a very faint hiss is heard trough both speakers

Edit: The secondary switch does exactly is I described. The amp still plays a few seconds if I unplug it from the mains with the power on. When unplugged the buzzing is not heard for those few seconds until the main caps are discharged, and the amp stops playing completely. The signal wires are very close to the mains power wires. Can the signal wires pick up the buzzing from the mains wires?