We have experienced similar problems with our X-channel preamp on our AMX 400. This is also an "old style preamp". We concluded that there was
too much stress on the solder joints especially when the side-srews were
tightened. No more problems occurred after we left the srews untightened.
Cheers,
Marco L.H. Gruwel
marco@herzkreis.uni-duesseldorf.de