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SX-780 Troubleshooting. Relay clicks on, very weak sound

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CaptainNemo

New Member
Hi everyone,

First of all, I love how helpful the AK community is here. It's amazing!

I got this SX-780 that had fading sound/ channel dropping sound. I recapped it using the list I found on this forum. All is good, i test it and it sounds good. I put everything back together and now it powers up normally, I hear the relay click after a few seconds but I have to turn the volume know 80% to barely hear an FM radio station. Same thing for Phono. Unfortunately, I haven't found any thread discussing this particular problem and since i'm an amateur electronics guy I couldn't figure it out myself.

Few things I've checked are :
Fuses: took them out and read 0 ohms - which i believe should be OK.
Darlington Powerpacks: Voltages seem to be good according to another thread.
Soldering and caps look good, no detectable problems

Any help would be greatly appreciated! I do have a DMM but I'll probably require a quick explanation how to check some parts, if need be.

Thank you so much.
 
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There are a bunch of REGULATED power supply voltages to check for.
+/- 39.5, +/- 24, +21, -19, +23.7, -23.1,

Also the unregulated +/- 39.5 at the power amps,

Have the guys help you check out the muting circuit Q28, q29, q30 - it's primary purpose is anti-switching pop, but it can do this if continuously muted. C405 is the time constant, if in backwards it could (no time to analyze that now) keep mute on all the time.
 
Check whether the same symptoms occur with the AUX or TAPE inputs. If so, the FETs which perform the muting function are not in those paths and therefore off the list of suspects (for now).

I've had problems with those muting FETs and have jumped them from S/D for diagnosis - Safely. As Mark stated, if that e-cap is in backward is may not allow the gates of Q28/29 to drive the FETs into conduction. Of course, if Q30 is shorted E-C the same would occur.

Again, it may all be eliminated depending upon whether the signal passes in AUX or TAPE.
 
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