Any chance one bulb is burnt out? The MG500 using two bulb/filter combinations one for green (low grade) and one blue (high). The contrast is controlled by varying the brightness of the bulbs. Controlled by the keypad unit via triacs located in the power supply unit. Take a look at the user manual linked above to start. Schematics are around, search the threads here. I have a couple variations, I believe I found them here. The electronics in the power supply are mounted on the right hand end cover. A scope would be usefull to troubleshoot, you might get by with a meter if need be. The electronics in the power supply are pretty basic, the control pad not so basic.
Varitions to look for are the control pad: older type was all touch pads the later had a large bar for the start switch. On the power supply look at the cable connector to the head, one has all flat pins, the later ones have a single round pin with the rest flat.
Let us know what you find, these are too nice to let them die without a fight.