[Mondial]

I think I an beginning to understand the connection from the stereo beacon lamp to the preamp equalizer. When you are in "FM Stereo" mode, you should only hear stereo stations, mono station will be muted. This is accomplished by biasing the preamp transistors off with the voltage from the lamp circuit. The voltage at 4F should be about 24 V receiving mono, which when applied to the emitters of the preamp input transistors, biases them off and mutes the audio. When receiving stereo, the voltage at 4F drops to near zero and allows the preamp transistors to function normally.

The problem is that your beacon circuit is not working correctly. When receiving stereo the voltage at 4F should drop to less than a volt. You seem to have about 20 V at that point whether receiving stereo or not. So the muting circuit thinks you are receiving mono and it mutes the preamp.

Once you figure out why the voltage at 4F is not switching properly, the "FM Stereo" function should work correctly.