Electronics > Repair

Vintage Kenwood Receiver Volume Issues

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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.

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