Hey Andy, I had a good communication with PRS support, and as I come from the EU I was contacted from PRS support in the EU, they were so kind and offered to send the wiring diagram to a service shop/guy of my choosing in my country. Have waited on a guy that is supposed to be really good, was recommended by a friend that works on many concerts here, but the guy did not return my calls and so I decided to become my own service guy ... AND ... have been repairing my MT15 up to an hour ago ... AND ... it looks like I have solved the noise/buzz issue. My MT15 is 2018 production year, what I found out was that the 1/4" jack sockets are obviously on the cheap side, they do not make a strong enough contact with the cable jack when plugged in. After a long and tiresome search for answers (you can look up my posts) I stumbled upon some good info on how to go about diagnosing amp. So, when I have cables connected to the FX loop it was ok, but when I tried playing with nothing plugged to the FX loop there was an ugly noise/buzz coming out of the speakers. But it all started gradually and very intermittent and was not able to point a finger at a specific issue.
So now I replaced the cheaper FX loop jack sockets with Neutrik sockets that are almost the exact same dimensions and it looks like I fixed the problem. The new sockets make a much better connection with the jacks, will probably replace the output sockets as well, but will wait and see if the front of the amp sockets also need replacement, they are a bit harder to get to as you have to pull everything out and would have to make spacers because the original sockets sit on L shaped pins that let them have some 2mm of space between the pcb and socket housing and the new ones do not have that ... the front pannel has pots that I do not want to resolder to a different hight
But you might have another issue, I was also looking for answers elsewhere, but found that, despite looking for bigger issues, mine was not too hard to repair. Well, will see tomorrow if I was right, it is a bit too late to play the amp at this hour.
--------------------------- the next morning -----------------------------
Yep, just played my little beast, silent as a sleeping kitten. With and without FX loop. Hope this is the end of my torment ... and did not need to take it to a service shop, well done Goran. Although I must say it would help to have some experience with desoldering pcb mounted parts beforehand, I would have done it in half the time, but now I have experience.
What I want to say is that maybe the source of the noise is not a bad grounding issue, but some other random thing like socket quality in my case.
