It's been 5 years since I was able to get my old Soviet amplifier (https://www.dropbox.com/s/g6cxnqiooz2vrus/amp_speakers.jpg?dl=0) working with your help (https://hydrogenaud.io/index.php/topic,100351.0.html). It hasn't popped a fuse since, but recently I noticed a low humming noise on the left channel. This hum is always present on all input channels to a certain degree except one, where the noise is much louder and is present on both channels.
I have tried disconnecting everything but the speakers, but the hum is still present. If anything, disconnecting the input cables actually makes the hum louder. What could be causing this?
I have the full schematic here (https://www.dropbox.com/s/nf320uu0h1xgnu3/amp_scheme_full.jpg?dl=0). I've marked the input selector part of the schematic, do tell if you need more info.
On possible reason and the most probable is dry electrolytic caps. You ned to test them.