Vintage Mercury Marine Tachometer

I was sent a vintage Mercury Marine Tachometer to analyze.  It has a circuit that is unlike any other tachometers I have studied to date.  Rather than using an ammeter to move the needle, it uses an air-core gauge.  Most tachometers use PWM (Pulse Width Modulation) systems to control the needle movement while this one uses a modulated constant current circuit.

You can read the reverse engineering document I wrote about this tachometer here.