Amplifier speaker relay problems.


Hi, I am working on an Niko Alpha II amplifier it is an older amp with the two large VU meters on the front. There is a problem when powering up that one chanel may not work. I have tracked this down to a relay that is in the speaker output path and is 24volt dc with two normally open contacts. It is on a printed circuite board and I wish to change it as cleaning did not help and I am afraid it may be degrading sound when working if not providing a good path. I uderstand this relay helps prevent thump on powerup and the sound from dithering out on shut down. Thank You for any advice.
guycom
the circuit is to charge the C308 to a reasonably high voltage so the Q306 turns ON and energize the relay coil.
Tried replacing cap C308 with several dif 100 uF caps but doesn't work. What would be the next thing to try? If I manually join the contacts in the relay, I get sound out of the speakers so it is something upstream from the relay. Maybe the relay needs replacing?
Dsrjohn,  AC voltage from one of the transformers taps produces positive voltage (D309,C309) for the status LED and the negative voltage (D308, C306, C307) to open transistor Q305.  When all for transistors Q302, Q305, Q303, Q304 are open then voltage on C308 builds up (to +21V) turning on transistor Q306 that activates output relay.
It appears that Q305 detects presence of AC voltage, Q303 and Q304 detect presence of DC voltage on the output (average of both outputs), Q303 reacting to positive and Q304 to negative output voltage, with time constant 1.76s.    Q302 appears to be controlled by Q301 that detects overcurrent in either channel.  When voltage on either of 0.22ohm resistors R301, R302 is greater than 1.3V  (two junctions) then Q301 activates Q302 turning off Q306 hence output relay.  It happens at 1.3V/0.22ohm=5.9A current draw from either output positive rail.

I'm afraid I did not simplify anything for you, but wanted to show 4 possible reasons: no AC voltage, DC on either output or overcurrent on either output.   It can be also faulty component - something is holding down voltage on C308 (something that activates one of four mentioned transistors).  Since you have schematics you can start measuring voltages (be careful).  Measure output DC voltage - it has to be more than +/-0.6V to trigger fault condition. Next voltage on C308 should be around 21V (voltages on schematic are in normal operating condition).  If it is, then problem is around Q306 or even relay itself (or shorted diode across it - D307).
If it is low, then something triggers it.  Check the voltage on the Base of Q302 and Q305 - should be less than 0.6V (most likely close to zero).  If it is then you most likely have bad transistor.  Measure what you can and post your findings.  Again, be careful.
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