I'm sorry I should have been more descriptive. The unit is based
arround a 68HC705 micro and has feedback for both the stirrer motor
(simple encoder) and the heater (thermistor). The heater is
proportionally controlled using triac.



As "stable" it is presumably controlled proportionally not thermostat
(either on /off)
so some control circuit and triac ?


electronic hints and repair briefs , schematics/manuals list on
http://homepages.tcp.co.uk/~diverse