AAR Radio Magnetic Indicator Servoed 1985 avionics teardown 

Time to post it to Labo de Michael to use. Light is external, a separate small box on top that provides a red light to illuminate the scales, with brightness controlled by the instrument brightness controller. Either a 5V, 12V or 28V lamp, depending on aircraft type and the actual instrument type as to which type they use. Note you often have a mix of 28VDC lamps and 5VAC lamps, with separate brightness for them, with generally the 28VDC being for instruments that are there for normal and emergency use, and 5VAC for the rest, powered off the instrument lighting variac, with the panel being 28VDC powered off the resistor for instrument panel lighting. One a variable transformer fed with 115/26/6VAC 400Hz, and the other with DC battery bus voltage.