[guenter1910]

Intermittendly the signal gets distorted and very quiet. Sometimes it disappears and comes back after a while and sounds ok. This happens on and on. It seems to be contact problem (?) - I was thinking of the tube circuit and soldered the connections of the tube pcb again but this does not help. It happens on both inputs line and mic.
Any hints?
Thank you in advance!
Guenter

[JPK]

Try tapping on components and connectors with a pen to try to locate the defective connection.
Resolder all solder joints that look suspicious.
Monitor a constant sine signal with your AC voltmeter at points TP1, TP2, TP3, TP4, TP5, TP6 to try locate the stage where the problem occurs.

[guenter1910]

I can measure an AC Voltage on all TP's except TP2 and 3. I applied a 1 kHz sin wave and I can hear it ok. I can hear the changes in gain and distortion and I see it on the frequency analyser. But there is no measurable AC voltage on TP 2 and 3. Although I can see it on the oscilloscope feature of the multi meter. The signal is there but quite low compared to TP 4 for example, but it seems to have some kind of a DC offset - it is not on the 0 line of the oscilloscope. All other signals on the other TP's are at the 0 line. I don't know if this is normal, as my testing right now with mic, line and HiZ inputs was all ok....