The gain and amplitude of the oscillations are determined by the setting of P6. In addition to amplitude adjustment, this trimmer has a moderate effect on the oscillator frequency.
B7 is an input socket that allows the base voltage of transistor T4 can be controlled or modulated.
B7 is therefore an Amplitude- Control or
Amplitude Modulation input.
As previously described, the Varicap biasing is provided through R25 which delivers the output, duly filtered and smoothed of U2, an Op Amp that is configured as an inverting summing amplifier.
One of the inputs of the adder is a DC voltage manually set by P7.
The others come from another portion of the circuit that will be disregarded for now as it will be studied later.
For now it is enough to note that sockets B8 and B9 allow external signals be applied to control the Varicap biasing.
B8 and B9 are therefore Frequency -Control or Frequency-Modulation inputs.
Proceed with the experimental work at the aid of a dual-trace scope and frequency meter.
Investigate the voltages and signals at various points of the circuit to reach a Good understanding of each aspect of the circuit operation.