jremington:
Two options: reverse engineer the "internal circuitry" and use it as intended, or remove the internal circuitry, and drive the indicator motor directly.The more I think about it, the more I like the idea of using a standard motor driver to drive the motor directly. I have done that to drive laser light show galvanometer coils and it works fine (but rather slow).
However, the chips you list are a voltage regulator and a beefy op amp, which suggests that voltage inputs are all you need (i.e. the "sine/sine return" and "cosine/cosine return"). Two bipolar DACs connected to an Arduino could drive such voltage inputs.
Test the instrument by applying +1.5V or -1.5V to the sine and cosine inputs, using the return connection as ground. You will need in addition a bipolar 15V power supply capable of several hundred mA output on each polarity.
it doesn't have to be fast.... idk about you but I cant turn a 50 foot school bus around in a circle very fast! Im down for what ever is easier in the long run. Unfortunately I don't have a bipolar power supply to test with at the moment. ill see if I cant rig something up here shortly. you wouldn't happen to have a link to a sutable DAC would you? im looking and seeing all kinds and some are super cheap and some aren't.