The SHG servo threshold setting is broken and outputs zero volts. This makes the SHG incapable of good green output, since it will just lock to zero volts output. Obviously no green means no squeezing so this needs to be fixed.

Turning the knob makes the signal jump a little with the polarity depending on the direction of adjustment. But then it goes back to zero.

Also the knob itself is broken and seems detached from the circuit assembly. 

I guess the potentiometer connection is broken, but turning the knob maybe applies a little pressure on the circuit that is then released when I stop adjusting. Initially I thought the circuit was preferring to go through some capacitor but looking at the schematic it seems not likely.