Vintage Kenwood receiver FM MPX Issues

[Arty30]

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.

[wasedadoc]

The 0 to 53kHz stereo MPX signal goes into the base of Q1.  The tuned circuit in its collector is for the 19kHz pilot tone.  More 19kHz selectivity by the tuned circuit in Q2 collector.  Frequency doubling to 38 kHz by the two diodes alternately feeding Q3.  38kHz filtering in collector of Q3 and feeding the 38kHz into the demodulators D5-D8.

Meanwhile the MPX signal from Q1 emitter goes via the rejection filters into the centre tap of Lc3.

As I wrote in your other thread Q4 and Q5 are a Schmitt trigger, probably acting on IF signal strength.  Q6 has a feed of the 19kHz from the collector side of Q1.  Amplified by Q6 and filtered by tuned circuit in its collector.  Rectified by D9 to make Q7 start to conduct when 19kHz level is sufficiently high.  Q7 collector into Q8 and Q9 which are another Schmitt trigger driving the stereo lamp.  Also if the lamp is off the high voltage at Q7 collector goes back to the centre tap of L2 and reverse biases D2 and D3 so that their pulses do not reach Q3.  With no 38kHz into L3, AM demodulation of the L-R signal on the 38 kHz carrier does not take place and the mono L+R is present at both outputs.  (As was also said in your other thread the resistance of the lamp is part of the Schmitt trigger design so needs the proper lamp.)

The Schmitt triggers are not about "switching rapidly".  They are to prevent "dithering" between mono and stereo which would otherwise occur when the reception is hovering around some single threshold level.  A Schmitt trigger has hysteresis, giving two thresholds.

[Arty30]

Thank you very much for the high level explanation of this MPX design. I will see if I can trace signals sometime tomorrow or Thursday, I did some today with not much idea what I'm looking at other than finding a 19Khz signal around Q1 that bounces/changes with the audio. As for the important lamp resistance, how precise should it be? I obviously can't measure the faulty bulb but the schematic specifies 33mA at 6V. the closest I got was 28mA, about 15% higher resistance I suppose. The other thing I was confused about is the link you gave on the other thread to the information about the triggers specified upper and lower threshold equations based on a few key resistances, but these resistances were the same resulting in the same upper/lower threshold?