Side project - MV864A meter restoration - simulating the DC Modulator Driver that produces 94Hz chopper signal

TRYING TO TEST SECTION BY SECTION USING LTSPICE SIMULATION

I started with the circuit that should be oscillating at 94Hz, producing square waves on LEDs inside the photoisolator component. This part of the circuit is called the DC Modulator Driver.

The light from the LEDs falls on two variable resistance photocells in that component, which are biased by the input signal to the meter and then amplified. Ultimately, the meter will compare the output of the oscillator with the detected but biased signals to capture the results of the input. 

I found an error in my schematic but after correcting it, I was still unable to see oscillation of the circuit. The manual's schematic has a different design using silicon transistors rather than the germanium ones in the actual meter I am restoring. I captured the DC Modulator Driver circuit from the schematic in the manual but that too did not oscillate. If it had worked but my reverse engineered version did not, it would focus me on possible errors or inadequate modeling of the germanium semiconductor devices. 

Since neither works, I have to find the root cause of the failure. It may be some missing components inside the photoisolator component, which is a sealed can that the manual insists should only be serviced by Millivac. I do know that the meters work in real life. 

GETTING THE MORE MODERN VERSION IN THE MANUAL TO WORK

A mistake in drawing the circuit of the manual's version of the DC Modulator Driver caused it to produce the 94 Hz output I expected. I tied on side of the oscillator output to the bias voltage source, which caused the opposite leg to produce 94Hz. However, when I corrected the drawing the simulation once again flat-lined. 

That was a clue. Turns out the value of the inductance, the four resistors in the photoisolator component and the bias voltage setting on the pot are all inter-related and must be in certain ranges to achieve oscillation. 

GETTING MY REVERSE ENGINEERED SCHEMATIC TO WORK

I discovered that the signal transformer was quenching the oscillation - when I removed the transformer I got my 94Hz square waves but oscillation didn't happen with the coil in place. I don't have specifications for the coil that I had to take wild stabs at inductance to find a value that allowed the transistors to oscillate. 

It could be that the transistor model I used didn't have enough oomph to oscillate, since I also don't have specifications for the 2N1373 germanium transistors I found in my meter. Also lacking were the resistance values in the photoisolator and the LED characteristics. 

PASSED REASONABLENESS TEST

Since I could achieve oscillation with generation of the square waves with certain component values, plus knowing that the real meter works so it is definitely oscillating, I knew that the schematic I derived was now representative and correct.