I checked the board.

M - position uses 100n Film + 2u2u electrolytic

L - position uses 100n Film

H - position uses 100n Film + 100u electrolytic

As the envelope attack stayed long in every position it couldn't be the electrolytic caps I guess.

I tried to measure the diodes but the boards are too close to each other to measure every one of them. Disassembling is not easy. 

Anyway I put the module back in the case.

Surprised I was to notice that it works again! Am I happy Yes. Am I satisfied No, because I don't know what was wrong.

Karel.