I am looking at a Kenwood HiFi receiver (KR V5090) from the late 90s that sometimes loses sound on one channel.
Granted, it can just be replaced, but I always like to give devices a second chance by attempting to repair them.
With this Kenwood I've found that power cycling the unit a couple of times can quickly "invite" the problem.
Since both channels are routed over the same speaker relay contacts, I think that it can't be the DC protection/overload circuit that's tripping here, because then both channels would go silent.
When the fault occurs, power cycling the receiver usually brings back the sound on all channels again. If not, sound may still come back more or less randomly, sometimes right after you adjust the volume a bit, often with a loud/scary 'plop' sound coming from the speaker(s).
When playing around and giving it some more random power cycles, I found that the receiver even refuses to come out of standby in rare cases (the standby board mains power relay doesn't click then). Alternatively, it turns on only 'halfway' with an empty VF display and no sound, as if the microcontroller that is supposed to initialize various components has crashed.
Out of curiosity, I took the unit home for further inspection and measurements. To my surprise, the issue doesn't occur here - at all! The unit played okay for two days and power cycling always works as expected. Also, there's no DC offset on the outputs to speak of.
Next, I brought the unit back to its owners and did some 'in situ' measurements. I had a feeling some device around it negatively influences the receiver.
Now the situation gets interesting: the amplifier gets its audio signal from a LCD television's headphone jack. When I plug/unplug just one of these RCA connectors, there is always an obnoxiously loud "plop" sound from the speakers.
If I try this with another signal source, that doesn't happen at all. This made me very suspicious, because the remaining RCA plug still keeps the two devices connected. There should be no potential difference anymore between them causing a bang like that.
Measuring the tip of the RCA plug coming from the television to GND reveals a 5 VDC offset.
My theory is now: the receiver itself is fine, but the DC offset from the television on the input causes a variety of problems, such as:
- loud bangs/plops when connecting/disconnecting input cables;
- poor performance (lower overall volume/gain);
- instability (unit refusing to come out of standby sometimes).
- Does any of this make sense / could this be true?
- Can I safely blame the televison?
- Why aren't audio inputs first going through capacitors (they would block the DC part)?
Looking at the schematic, the RCA socket connects directly to an input selector chip (
NJU7313AL IC); I don't see any capacitor inbetween. Perhaps the input circuitry reacts in a bizarre way that upsets the amplifier?