Guru
Joined: 11/02/2018
Location: AustraliaPosts: 3313
| Posted: 05:12am 16 Jan 2023 |
Q5 and Q6 play no part in amplification. The only way they affect the signal is to limit the signal peaks so unless they are short circuit they won't be the problem.
With the power off simply measure the resistance between the emitters. Repeat with the probes reversed.
If you get a low reading either they or C15 might be a problem. Disconnect one emitter and check C15. If the resistance has risen C15 is probably ok.
Leave Q5 or Q6 disconnected power on, apply a signal and see if it works.
If not is the signal getting through R28 and C17 to the base of Q7?
Your PDF indicates both base-emitter voltages of Q7 should be 0.55V The collectors should be about 30V above ground.
The anode of CR9 should be -10V.
If all those are correct Q7 is probably ok.
Q3 and Q4 also play no part in amplification. They monitor the emitter currents of Q1 and Q2, applying extra negative feedback to the righthand base of Q7 if the currents get too high.