The EMX ST860 got a lot of electronics in it with long signal paths and there are clock rate noise and artifacts, some very high frequency. Not all of it is audible but can cause strange effects with the digital parts of the amp, especally if the effects part is used on said channel. That is even more possible if the input is open. I'd suggest to switch the FX off on that channel and connect something without it giving a signal or better: shorting the input (that can easily be done with a patch cable in the amp channel and connecting the 2 poles of the jack at the free end). The test of the gain influence is mainly to locate at which step of the path it happens. All that is assumed the channel works fine otherwise and no other channels are showing the same behaviour.
Since it happens at all 3 potentiometers and all only into the +, I assume these can be excluded. Most likely other error sources are the OPs x01 (101, 201 etc, depending on which ch it is), x02, x03 and the electrolyte capacitors around them. These are the OPs in the same order for decoupling/buffering the input from the EQ-influence, preamp and buffer for effect. One OP IC consists of two amps, that means each of these ICs is used for two channels but they can break 'separately'.
I got the feeling I forgot something but I can't concentrate anymore for error analytics, sorry.
The schematics are free download somewhere, google it.
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