Update: as I'm always suspicous about double faults, I checked the A4U1 mixer as far as I could and also the 300MHz amp. It turned out the 300MHz Filter was misaligned. After realigning, I got about the same level for the 20MHz and the 580MHz Signal at the output of the mixer.
Maybe someone tried to "repair" E18 by randomly playing with the most accessible alignments.
cheers
Martin
Martin