I got an old Noresco 2470 II (Canadian Rotel rx-202 of some sort) from my uncle and have been trying to fix a bit of a strange problem.
The sound of the transformer gets in the signal path and makes its way to the speakers, its normally fairly quiet and is pretty much inaudible if I have anything being played at the same time. However it will often constantly do one of three things:
1. Stay quiet for hour long lengths of time
2. Occasionally crescendo up to the "amplified" level, and then either stay at the for a random amount of time, could be 1-3 seconds, 15, or move on to #3, or not stay at it at all and immediately go to the "unamplified" level. When it returns to the unamplified level from any amount of time it always happens instantly, not with a decrescendo, and makes a slight popping sound (at the same volume as the previous sound, you just can tell its a little different)
3. Stay at the amplified level
I should also mention there is no "default" level on startup, even if you wait a bit between to let the capacitors drain it can start up loud or quiet. Usually if you shut it off in the loud state it will start up back into it though, at least for that sitting.
I don't measure any ac voltage on the input or output grounds (or any of the PCBs), however there is a measured 7V dc relative to the ground form the wall (ill need to bring a scope home to check if that's a dc wave or a constant level). Connecting any of these ground directly to the wall with an alligator clip doesn't have any effect on the noise however.
The only other measurement oddity is that the positive outputs on the left channel have ~38 ohms of resistance to ground while the ones on the right have ~132.
As for the possible fixes I've tried, there was a passthrough for the wall power that was tied to the power switch, so it crossed over everything from the front to back twice, right next to the output wires. as mention before tying the reference ground to the wall ground. I previously made sure no components or wires were leaning against each other just incase any vibrations were being picked up. Removing the lights in case those were what was making the noise and not the transformer, then removing the transformer and putting it off to the side on carpet, so any vibrations wouldn't be carried over (I suspect the wires coming from it would still carry the vibration however.
I can't find any explanation of why the noise from the transformer doesn't constantly get amplified and only sometimes, if its in the signal path. (I assuming its cause by that since its the same as the output on the speakers, and the noise floor from just raising the volume is a much higher frequency and random, and gets louder or quieter changing the volume knob (which happens before it goes through the amps) while the other sound does not). I could try using some direct ac power in and fully remove the transformer just to confirm the "amplified" noise does only happen while the transformer is in there, and it is not just a different noise that masks over it because its the same frequency.
Also tracing the outputs to the tansistors, the only components inbetween are a fuse and an inductor (one on each channel), then it goes to the 2nd transistors, then the first. So I can pretty much narrow down that its a signal that is being amplified which comes from after the volume pot and before the transistors that do the amplifying. The only other thing it could be are the inductors, but there are seperate ones on each channel and the noise comes out of both channels loud/quiet at the same time.
