Yes leaky cap, because the one doing the muting has around 50V across the capacitor, with the power switch shorting it out via a resistor to bias the FET hard off. Slight leakage, with a high impedance supply to the positive side, means it will drop the voltage biasing the FET down to pinch off the gate, and reduced supply voltage means the voltage with the leakage is just not enough to fully turn them off. Not surprising the amplifier side works at 15% of the mains power, after all it just has to have enough to provide bias voltages, though the tuner side definitely would not run, as there it needs more than that to have the oscillators and mixers operate.
Good troubleshooting. I miss the days when schematics and parts were readily available, and easier to follow.
Thanks Dave!
i am amazed that such low voltage would power that unit on and produce sound...great job as always
the moment u said Stereo Receiver from 1979, i knew the culprit must be the capacitors
Diagnosis is always the secret sauce.
👍👍
Yes leaky cap, because the one doing the muting has around 50V across the capacitor, with the power switch shorting it out via a resistor to bias the FET hard off. Slight leakage, with a high impedance supply to the positive side, means it will drop the voltage biasing the FET down to pinch off the gate, and reduced supply voltage means the voltage with the leakage is just not enough to fully turn them off.
Not surprising the amplifier side works at 15% of the mains power, after all it just has to have enough to provide bias voltages, though the tuner side definitely would not run, as there it needs more than that to have the oscillators and mixers operate.
Those purplish colored electrolytics were always a problem especially in Sony tvs and a lot of Japanese electronics.
Be well Steve.
:o)❤
NAD is crap