LM723 is a voltage regulator designed primarily for different power supply applications. But it can also be used as a temperature controller. I have these great McGraw-Hill Publications books "Circuits for Electronic Engineers" where from I found this "Using transistor arrays for temperature compensation" article by Mahendra J. Shah. With 15V power supply it worked right from the box very good indeed. With only one heater-transistor it could rise the temperature from 30°C to 50°C in 5 minutes and reach the final 50.5°C temperature in 10 minutes.
LM723 has also some kind of comparator inside it called Error Amplifier. It seems to work more smoothly than just an on/off switch. There is also a reference voltage generator inside this IC that generates a constant voltage of 7.15V from where the desired temperature CV is adjusted with trimpot R1. I modified this circuit a little to be used from 12V power supply. The pin numbers of LM723 in my schematic refers to PDIP version of this IC. The original article claims that with this heater circuit the LM3046 array exhibits a temperature coefficient of -3.1 ppm/°C from 33°C to 50°C. Peak current that this circuit draws from 12V is about 80mA where from it comes down to 52mA when the circuit stabilizes. In Moog Prodigy you need two of these with total current need for 104mA. I haven't compared this current consumption to Prodigys original heater circuit. But that's sure that it is a better circuit than the original. Next I must try this inside my Prodigy clone.
