Hello, This is a continuation of my '69 Kenwood TK-140X receiver repair. The lingering fault is that the FM appears to not be switching over to stereo mode and only outputs mono sound. I am not very well informed about how FM MPX decoding works and that is partially my goal for this thread along with repairing this board. I caught on to this fault when the stereo lamp wasn't illuminating and then I realized it was more than that. I will post a schematic of the entire MPX board so this makes sense, but I have tested the part that illuminates the lamp (Q7-9) by pulling high stereo defeat on pin 7. I was also informed that Q4-5 make up a Schmitt trigger so it will engage stereo without switching rapidly. this section appears to me to never change state at all. pin 6 labelled Gate In is at roughly 3V when tuned to a station and 0.3V when not tuned, this to me seems sufficient to switch the state and enable Stereo mode and the lamp through Q2 and Q6. Any help explaining the operation of this circuit and possible paths of troubleshooting would be greatly appreciated, I am out of my depth on radio type circuits but am hoping to learn. Thanks.
Arty.