[time-nuts] hammond 77589

[Ed Breya]

Since you measured the same R on opposite ends, it appears to be a 
center-tapped inductor. Since there's no input signal other than power 
in the schematic, it must be a low frequency oscillator. If you draw in 
the inductor on the schematic, it should make more sense. It could be 
that they used an AC bridge for the temperature control, so it was used 
as a clean, regulated-amplitude, low frequency excitation signal.

AC excitation would have been easier to process back then - easy to 
amplify up with very high gain, and synchronously demodulate to get the 
feedback/control signal. DC at high gain and low offset was trickier, 
and often would have used a chopper-stabilized amplifier - essentially 
making it an AC system. This just skips the chopper part on the input 
end. You should find that the signal from this circuit goes to the 
thermistor bridge, and also to some circuitry following the amplifier to 
provide the demodulation.

Ed