I just noticed one more thing - if the NTC thermistor in the schematic
is the actual one that senses the oven temperature, then the oscillator
and bridge are one in the same. If this is the case, then the oscillator
amplitude could be proportional to bridge imbalance - if the NTC is
being balanced by the 1414 ohm resistor. I doubt that this is the case
though, since the DC resistance of the inductor would swamp the others,
so it would need to be AC-coupled into the bridge to isolate its
resistance, but the schematic shows direct connection. Also, you would
need the phase information to know which direction is needed to correct
it, so there would have to be another signal pickoff from this circuit
to get the demodulator phase info.
If the thermistor is just sensing ambient temperature of the control
circuit, then it's probably compensating for circuit drift and
oscillator amplitude. I think this is the more likely setup, given the
schematic info.
Ed
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