1980’s Yamaha CP70B electric piano with a tremolo circuit. This photocoupler converts the signal from the tremolo circuit to a resistance in the gain circuit.
MCD527 is the part in the schematic:
Here is the original data sheet:
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Assuming your description of the device as having a resistive output is correct, the available options are somewhat limited as it’s not a particularly popular use case these days.
MCD527 has a resistance when on (RON max) 1k ohm, and resistance off (ROFF min) of 2M ohm. If I’m reading the schematic correctly, when the tremolo is engaged in the circuit (switch) it inputs the signal from the op amp into the photocouple diode using the driver transistor. The photocoupler is in series with a variable resistor (depth pot to adjust the amount of modulation percentage) in the volume circuit coming out of the preamplifier going into the buffer amplifier for the output channel. I’m wondering if a phototransistor would accomplish this task by itself, or possibly with an added resistor in series? Thoughts? Thank you so much!!
It may be possible to do it that way, however due to the phototransistor behaving more like a switch I would expect the abruptness to have a noticeable impact.
The “NSL-32SR2S” looks to have the most similar RON/ROFF values to the values you gave.
NSL-32SR2S Advanced Photonix | Isolators | DigiKey
NSL-32SR2S, matches the circuit schematic of the MCD527, with the resistor photocell. I will do a deep dive on the specs, I think we’re on to something here! thank you!!