Vintage Kenwood Receiver Volume Issues

[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.

[floobydust]

I think the FM 38kHz MPX carrier signal strength comes in on 13,14 to Qc6 which amplifies it and DC9 detects it and if strong enough MPX signal, turns on Qc7 and starts the party.
You can adjust the Stereo/Mono switchover threshold with trimpot VRc2, if the circuit is weak and can do anything. Lc9 might be out of tune but leave that for later.
Measuring emitter voltages on Qc6, Qc5 will tell what they are doing. Is the S meter doing OK, shows reasonable signal strenght?

I have no idea what "BEACON OUT" is going to GATE IN on terminal 6. Qc4 and Qc5 are a switch to slightly change Qc6 trigger level, I think.

[wasedadoc]

Q4 and Q5 form a Schmitt trigger circuit. Explanation and that same circuit at https://en.m.wikipedia.org/wiki/Schmitt_trigger

Purpose is to prevent repeated switching between stereo and mono mode when signal strength is wavering around the decision level. Once the level goes down enough to make it switch to mono, the level needs to increase well past that to go to stereo. And vice-versa.

[David_AVD]

Note that in a lot of cases, the rated voltage and current draw of the stereo lamp is very important to it's correct operation.

[Arty30]

Thanks for all the information. The stereo lamp is the closest I could get to 6V 33mA at 28mA. The signal strength meter works, but never gets above half even from a 100KW station transmitted from a mile away, not sure if this is an issue or not, but the audio is still very strong barely needing any volume to be loud. The other thing that confuses me is the threshold equations for the Schmitt trigger end up being the same due to R28 and 32 both 10K,or am I missing something? I also simulated it in LTSpice, but didn't get anywhere.

[Arty30]

Q5 emitter voltage is 0.7V, so it's passing ~1.5mA. Q6 emitter voltage is 0V, so it is off.

[Arty30]

Additionally I found the 38kHz carrier and audio at point 14 by VR2. It looked good to me, I could see the waveform change with the music like I would expect with a carrier and audio wave. The amplitude was 50-60mV, but I have no idea what it should be.

[Arty30]

Since this Thread has gone dead in the last couple days probably due to the now misleading title, I am going to start a new one specifically on unravelling the MPX section of this receiver.