Marantz SR4000, One Channel Noisy When Using AUX

[bzenic]

Here is my issue:
Both channels working well on FM with earphones and/or speakers.
When I use AUX input things are different: one channel shows the typical "bad transistor" symptoms: crazy change in volume with distortion and noise.
I was thinking that the most likely culprit would be a transistor in AUX path, would it be tone control amp?
Any thoughts?

 

[dankik]

When I use AUX input things are different: one channel shows the typical "bad transistor" symptoms: crazy change in volume with distortion and noise.

I don't think it is a bad transistor. Take a look at the schematic below.

All signal paths go through the same selector switch (AM, FM, Phono, Tape3/Aux) and the Aux input does not have the phono preamp circuit or the AM/FM front-end circuits, but goes directly into the selector switch. If the other inputs sound OK, then I would suspect dirty switch contacts. I would try to apply Deoxit to the switch to clean the contacts first.

 

[bzenic]

Thanks a lot, I will try that and post the outcome.

 

[hofferwood]

The selector switch is mounted on the main board, and cable operated. They are known to get dirty and tough to clean.

 

[bzenic]

Yes, it is not obvious what part to clean.

 

[bzenic]

Well, I sprayed CRC contact cleaning spray into the switch and it seems it's fixed the sound on previously noisy channel at least for now.
I'm very thankful to @dankik who saved me from torture of searching for and replacing the transistors.

 

[dankik]

You're welcome. I'm glad it worked for you.