NAD C326bee pre amp does not pass signal

Hi everybody. I need some help in finding the issue. I got an used NAD C326BEE how was staying in protection mode. I changed all electrolytic caps and it was back. Unfortunately the sound was missing. Quick checks with other unit show that the power stage is working but the preamp doesn't. So I decided to trace the signal by my pocket oscilloscope and the signal is stopping exactly before the output.
So I'm pretty sure there is a problem with the gate circuit of JFET-s Q101 and Q301 because they behave exactly the same. But I stuck on the circuit which should control them.
Can someone help.
Attached the service manual.
 

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Welcome to the forum!

So it sounds like you've established that Q101 and Q301 are biased into cutoff state. Is MUTE asserted, i.e. is there logic high at J705B, pin 5?

If Mute is at 0v, problem must be along the path back to the Q101/301 gates. The gates should be about 0V if FET switches are conducting, vs. pulled to about -20V if being muted. If MUTE control is active, you can override MUTE by shorting base to emitter at Q162. Let us know if this doesn't lead you the problem.

Good luck!
 
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Thanks for your advice. I will try it. Yes the MUTE is controlling this path and I'm considering what can happening if Q101/Q301 are bypassed?

When I checked pin5 /MUTE it was 0.28V with MUTE activated and 0.24V when MUTE is not activated from the remote control.
 
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Here are the V measurements results
IMG_20230811_083051.jpg
 
I’m very embarrassed, but my description of muting state is backwards. /MUTE is active low, so low voltage on the on the connector pin 5 should assert muting. Your remote should cause that pin 5 to switch to the high state; about 0.6V should appear on Q142 base and Q142 collector should switch to about 0V. It appears this is not happening.

As a test, you can short Q142 collector to ground and you should get audio signal appearing at outputs.

Do other remote functions work? Can you mute manually from the front panel? Trace control line from R723 back towards the processor and try to discover why it’s not responding to mute requests.

Another thought: if your multimeter has a diode test function, use it to apply bias to Q142 base junction. That should also enable audio.
 
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Are you sure about q142?!? It is a power transistor on the power stage?!?
All remote functions are working. The device doesn't have physical button on the front panel so the only way is trough the remote control. When activated the main status led on the front panel is blinking which I thing means that the processor did his job.
 
Very sorry. Q162, the one you've annotated with measurements. Fuzzy print and fuzzy eyes. :(

The Mute control seems to originate at processor U503. Check there for activity in response to Mute from the remote. Is there continuity from the processor pin 2 to R723?
 
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I checked again the active / not active mute voltages on pin 5. So results are as previously posted 0.28 / 0.24 - activ/na.
Pin 2 (U503) is well wired till r723.
I checked also pins 3 and 4 which should be feeding U503 MUTE command. The VCC is +4.9 but the ground bother me with its 0.05V. I took the ground from the main bord and it shouldn't have any voltages, right? Could some smd grounding resistor make that mess?
 
Thanks, that helps.

There's a good chance the muting circuits will work when control signals are applied. As mentioned earlier, you can confirm enabled audio by grounding collector of Q162 or by applying base bias. Either should activate audio.

Let us know if you still have trouble. Good luck.