Thanks for that explanation..... at first, it seems a complex way of doing it ..but it doesn't use that many components...so quite cheap to make. Just a question of idiots like me getting my head round the current 'routes' ..@rogs
Q11 is the key. PNP, emitter grounded, must have negative supply to start conduction.
The negative supply is coming from R35 (47K). If the PTC branch does not dump that current to ground, it will turn on Q11.
But there is still a connection through R34 (6.8K). Yes, and 2 volts plus 200 ohms at 1.16 ma is 2.32 volts negative. 341 ua is still to be accounted for.
R33 (150K) is bringing 328 ua and R40 (100K) is bringing 138 ua. Thus, Q11 is off if the LED is on.
If the PTC string goes to high resistance, the 1.16 ma through R35 (47K) will pull the base of Q11 negative and turn on Q12.
But -- if R33 or Z5 are open circuit, and /or C17 is short circuit, them surely you have got the situation where the LED will still be on, but Q11 will also be turned on, the amp will never come out of mute?...
(would also explain the lack of +/- 15V at the op amp....)