Side project - MV864a meter restoration - simulations and look at the non-board wiring

SIMULATED PREAMPLIFIER AND DRIVER OUTPUT SECTIONS

The photoisolator component has twin photoresistors that are driven by two LEDs. The LEDS oscillate at 94Hz, thus the resistors should pick up the 94Hz signal, with the input to the meter driving the photoresistors. 

The current flowing through the photoresistors is detected by a MOSFET and a string of amplification stages, giving the 94Hz signal at an amplitude based on the input voltage. That is then further amplified by the driver output circuit. The signal from the driver output circuit is rectified to turn a positive and negative cyclic signal into a stream of positive pulses. 

The positive pulses are mixed with the original 94Hz source in the synchronous demodulator to recreate a DC voltage that is an analog to the original input signal. This also detects whether the input was negative or positive and causes the output of the demodulator to take the same polarity. 

LTspice does not have a photoresistor model. In addition, I had to model the effect of the LEDs producing the 94Hz alternating positive and negative square wave. Since I have no specs for the LEDs, photoresistors nor the MOSFET detector, I had to guess a bit to produce a reasonable result from the circuit. 

I used a voltage source to generate what the photoresistors would pass into the MOSFET, consistent with a current from the input flowing through them modulated by the change in resistance as the 94Hz light from the LEDs hits them. The circuit simulated properly, thus I believe that everything except the synchronous demodulator checks out. When I simulate that part, I will feel confident in the schematic I captured by the reverse engineering. 

LOOKED AT DISCRETE CONNECTIONS ON THE METER

I began to trace out the wires coming from the main and power supply boards. For each wire, I noted the color and its destination. As I did this, I looked closer at the Range Attenuator and Function switches. My working assumption was that the range attenuation and function switching wouldn't have changed more than trivially between the version I have and the schematic shown in the manual. To my dismay I discovered the differences are more substantial.

CHANGES IN THE ROTARY SWITCHES FROM MY VERSION TO THE MANUAL VERSION

The Range Attenuator switch in the manual has eleven sections of 15 positions, but the meter I own has only nine sections. The Function switch has 7 sections, just like in the manual, and it matches. I am going to have to dig in and reverse engineer this to see how it has changed. 

The last two sections seem to match between the meter I own and the manual schematic. Wires run to the back panel that has all the calibration potentiometers to adjust each of the fifteen ranges so that the meter produces the correct reading. Other than changes in the component numbering between the versions I believe it is identical. 

Calibration board on right, wires to Range Attenuator switch

I began to label all the components from my photos of the actual meter, so that I know the parts number of each and can apply that to the reverse engineered schematic as I bring the copied over schematic part from the manual into agreement with the actual wiring.