Okay, interesting discovery tonight. It's only when both the TDA2040 heat sink (AGND1) and the input jack ground (AGND) connect with the chassis that the signal drops out. So that would seem to short the two grounds together, which I assume are suppose to be isolated?
This would mean that the thermal paste between the TDA2040 and its heat sink was the only thing preventing this from happening previously until it wore away?
AGND1 has a circle around it in the circuit diagram AGND does not (and input jack ground actually says "CHAS", what does that indicate?
Update: Circled Ground No. 5018 Noiseless (clean) earth (ground): To identify a noiseless (clean) earth (ground) terminal, e.g. of a specially designed earthing (grounding) system to avoid causing malfunction of the equipment.
This would mean that the thermal paste between the TDA2040 and its heat sink was the only thing preventing this from happening previously until it wore away?
AGND1 has a circle around it in the circuit diagram AGND does not (and input jack ground actually says "CHAS", what does that indicate?
Update: Circled Ground No. 5018 Noiseless (clean) earth (ground): To identify a noiseless (clean) earth (ground) terminal, e.g. of a specially designed earthing (grounding) system to avoid causing malfunction of the equipment.