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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).

Questions:

  • 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?

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    \$\begingroup\$ So it only happens when you use the headphone output from your TV??? Headphone outs are much higher power (voltage) than a typical RCA 'line level' signal. Like many times more. You're not doing the amp any favors by driving it with that signal. Make sure the TV and amp are powered by the same AC socket, that might be all you need to do. Does the TV not have RCA outputs for audio? Use those instead of the headphone jack if possible. \$\endgroup\$
    – Kyle B
    Feb 8, 2023 at 0:05
  • \$\begingroup\$ Thank you, you are absolutely right. Forgot to mention that I have connected the cable to another output on the set top box (dtv receiver) instead. Much healthier levels and no DC bias. \$\endgroup\$ Feb 8, 2023 at 6:55

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To further confirm the problem, add an external DC blocking capacitor. The issue should disappear then as well.

And yes, indeed, the DC bias can be a source of all those problems you describe. Some multiplexers and other analog chips may not be well protected and providing external hard DC bias may latch them up. In that case they may pull down the internal power supplies, or control lines, or output lines, etc.

Verify the power supply voltages to the chips that take the audio inputs, in both normal and “sound stuck” conditions. There’s a fair chance that one of the supplies shared with the CPU is dragged down, potentially heating up the chip(s) that latched up due to DC bias. The DC bias may also be propagating via ESD diodes in the chips to the rails, and partially powering up the circuits, causing e.g. the reset lines to be “wonky” and not properly reset the CPU, preventing the startup, etc.

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  • \$\begingroup\$ Thank you so much for this explanation. sounds like it could have even been worse then: "chips heating up". \$\endgroup\$ Feb 7, 2023 at 23:46

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