Electronics > Repair
Vintage Kenwood Receiver Volume Issues
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Arty30:
I don't have the schematic as I'm writing this, but does the FM tuner have a separate mono output or does the MPX section simply output the same mono signal on both L and R outputs? presumably the latter since in "Stereo" mode, I still get audio output. I did a quick test of the several diodes on board, they were good, but it may be good to note that this MPX board does exclusively use the often distrusted  2SC458LG transistors, although so far the one's I've checked tested ok.
David_AVD:
Audio is output from the FM tuner on the left and right lines in both modes.

Did you confirm that the audio output sounds like mono or stereo? Until you do that don't go replacing or adjusting anything.
Arty30:
I definitely won't go twisting things in any tuner circuits before I know how. The only thing I have to go off of is that in AUX  mode with my phone I can hear the audio change when I go from mono to stereo, this is not the case when listening to the radio. Is there a better way to be sure?
floobydust:
Many things can cause the receiver to seem stuck in FM mono.
Make sure the FM Stereo lamp is not burnt out, it also acts as a pullup resistor.
I would measure DCV on some of the transistors QC4-9 that will tell you what's going on there.
You'd have to check to 38kHz MPX carrier signal strength if you have a scope.

There are two trimpots VRc1 GATE and VRc2 STEREO DEFEAT if you can mark their position and turn them to see if it can switch modes to stereo at all.
Most of it is based on signal strength and if the FM signal appears weak, it will stay in mono.
Arty30:
Thanks, the stereo lamp was burnt out, but I have replaced it, could this be a sign of anything? I measured B-E voltage on Q7-9 closest to the lamp and it seems they are cascaded and currently they are permanently in the state where Q9 is off because Q8 is on because Q7 is off, so I don't suspect that section yet but I'll check the others and possibly play with those pots.
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