My CTC-16XL Project.

[old_tv_nut]

It's the triode section of V24, labeled "Chroma Reference Oscillator Control." Sorry I did not use the schematic name. It's also called a reactance tube, because it amplifies the reactance of C111, 4 pf, by the gain of the tube, due to connecting C111 from plate to grid. Any stray capacitance between the grid and the plate is also amplified in a triode, and is referred to as Miller capacitance. Anyway, the purpose of this circuit is to vary the capacitance attached to the oscillator circuit according to the control voltage developed by the phase detector. It varies the capacitance when the control voltage from the phase detector varies the gain of the triode.

The phase-locked loop works this way: the phase detector develops a voltage depending on the relative phase (and frequency, within limits) of the oscillator and burst. If the oscillator is tending to be too high in frequency, the control voltage will increase the gain of the control tube, thus increasing the capacitance loading the crystal, and reducing its frequency, and vice versa. L31 is tuned to get as close to the right frequency as possible when the detector output is zero; then it can correct either way as things drift with temperature or other factors. That is why you short point J to ground when adjusting A16 for zero beat - you are making the control voltage zero.