Electronics > Repair
Vintage Kenwood Receiver Volume Issues
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floobydust:
Slow down, first make sure the tuner and MPX section has proper power.
It's weirdly powered by the pilot lamp power (D3, C313, R309) for around +9V for +B2 in the MPX. How are the other pilot lamps, they affect that voltage if burnt out or LED it will go high, and there is a 7.5VAC/6.3VAC transformer tap to see what you have.
The other important rail is B+ 14V regulated from QC9. Many 16V caps on that 14V rail too, they might be a bit cranky.

Tuned to a stereo FM station, see if QC7 gets any base voltage at all. The stereo-mono (STEREO DEFEAT) switch seems to short that out to force mono mode.
The actual current drain of the stereo lamp (30mA) sets the hysteresis-amount of the stereo/mono switch Qc8/Qc9 make. It can be hard to get a proper lamp.
With a burnt out stereo lamp, Rc9 27k (by discriminator) front end might get stuck in stereo but not sure.
Arty30:
Thank you, sorry for jumping around. I have checked that +B2 is 9V, although two of the four dial lamps were replaced with LED since that's all I have, and I found a small stereo lamp that seemed to do fine on 6V, but I'm not sure it's current rating.
Arty30:
OK, here's the measurements you were interested in, Q7 had 20mV Base-Ground and 17mV Base-Emitter, and for the sake of thoroughness, collector-gnd: 1V, emitter-gnd: 0V, not much going on there. Additionally I measured the current of the stereo lamp at 6V and it was 80mA, so I replaced it with one that was 28mA at 6V, this made no difference. Both voltage rails were also good at 9.4V and 13.7V.
floobydust:
Qc7 having no collector voltage (power) there is a problem. It might be shorted, or a bad connection there. I would dig a bit.
Arty30:
OK ,so here are some data points. I decided with the help of my brother Alex, to apply approx. 1V to pin 7 the stereo defeat input to artificially drive Q7 on, and voila, the lamp illuminated. I don't believe the issue is in that section so I began looking at Q4-6. Q4 was on, but so was Q5 which seems odd so I created a sketch to highlight my findings. It appears Q4 is supposed to be on, but is not. I even replaced Q4 with no change. Hopefully this makes sense and someone can explain what is going on (or wrong). Thanks.
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