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.