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Link below is to a section of the TH transmitter schematic taken from the patent PDF.
Let’s start at the output jack (96). Leading to the left is C326, a 10 nanofarad cap, leading to a switch SU100, which selects internal ATU and external ATU. In front of that switch is TX 301, a toroid transformer. It’s primary winding goes from ground to the junction of C343 and L303, and L303 connects to C325, which is another 10 nanofarad cap, then to collector junctions of TR 305 and TR306.
If C325 were shorted, the final’s DC voltage potential at the two transistors collector junctions would SHORT through L303 and TX 301 coils straight to ground…which would cause a drastic reduction in signal output, or kill it all together.
Since your seeing an incredible amount of signal output that you measured earlier…it is HIGHLY doubtful you have a shorted C325, as a shorted C325 would not only shunt the DC voltage present at those transistors collectors, but also shunt the RF signal there as well. This condition would not yield you the amount of RF signal you are measuring at the RF output jack.
Now look above the two final output transistors, which is the modulator circuit.
88 is the modulator current amplifier transistor which feeds B+ of 18VDC to the finals. It also supplies bias via R325. 18VDC is fed also to pin 8 of IC 301, the modulator input amplifier IC.
It is MORE likely that if there is a shift in supplied DC from that circuit..ie a DC imbalance and dancing with audio, which should be stable, audio or no audio, that will cause the two finals AND it’s bias to swing as well in relation to the audio applied. The supplied voltage and bias has to be stable. Modulation is done by embedding AC signal through that modulator circuit onto the DC voltage to modulate the RF signal passing through those finals.
IMO..taking into account your measuring a very high RF output, you have a DC imbalance taking place at the output of that modulator circuit, feeding too much DC voltage AND bias to those finals, and that DC voltage which should be stable, is bouncing up and down in relation to audio applied, further making the DC voltage unstable, and would cause your modulated RF envelope to look like it does on your scope..lop sided. Since the lobe increase is to the positive, that tells me the DC voltage imbalance is to the positive..meaning more positive voltage is going through those finals than there should be.
Take your scope in DC mode and probe at the top end of L326, the RF choke between the modulator transistor and the finals. Look for a steady DC voltage of about half the supply voltage of 18 VDC. It should NOT be any more than near HALF of that supply voltage. Then feed a low level tone into the audio input and note if that DC voltage swings UPWARD considerably.
I bet it does, and you will see it on the scope. At that same probe point, you can switch to AC mode on the scope and you should be seeing a nice sine wave from the tone. I bet you wont.
Conclusion…that IC 301’s DC biasing has been damaged internally to the IC, probably due to too much input audio applied and thrown off that steady DC biasing, which is shifting the modulator transistor’s biasing, in turn shifting that DC supplied voltage through it, feeding the finals too much DC voltage and bias, which it all swings and dances in relation to the audio applied..thus seeing way too much RF at the output.