ESX Audio In sound problem

[Kukuruzo]

Hi everyone,

I have an SD version (I also had SM version before, had no such issues as below) of ESX and it has problems with the Audio In input.

If I use it for send(out3/4)/return(audio in) with external pedals
or sometimes I plug the guitar to Audio In and play. The processed signal sounds fine for some time then it starts to fade out (usually left channel) and eventually I will bare hear the signal (could sound distorted/muffled sometimes) until I switch Audio In to Mic position and turn the Audio In knob to max. Then the sound comes back with the insane volume and blows my ears. After that I can switch the selector back to Line level and it will work for a short period of time. Then it will start to fade out again. I know that it's a hardware problem, but probably fixable. I think that it is not related to the tubes since I have tried another pair with the same result. I can solder simple things if needed. Could you (or anybody here) advise something on how to fix it? Thank you