Help with 1176LN H Funky FET? (Q1)

[jwhmca]

Trying to calibrate this guy.... suspecting Q1 as a probelm...

Symptoms include:

1. Huge change over warm up cycle.
2. Jumpy output and GR levels.
3. 20 ratio = nothing, then switch off and back on... then it works.
4. Intermittent operational all over the place.

Basically, the thing is wompess, and I can't really get it to settle down and be repeatable. I have already done a recap.

Do the FETs fail?
When they do is it always hard short or open? Or do they go crazy?

 

[PRR]

Sounds to me like bad contacts. Connectors, solder joints, switches, pots.

JFET seems pretty unlikely.

DC voltages readings could give clues. If the +13V (say) rail goes 6V or 17V when you smack or switch things, hmmm. Measure at the power supply but also out in the circuit.

 

[gyraf]

..clean/check the rarely-used bypass switch - located all the way ccw on attack potentiometer..

Jakob E.