[Mondial]

Given how fussy these MPX circuits are regarding the lamp, it might be a good idea to use the proper incandescent at the beginning to at least check if the MPX decoder is functional. After confirming proper operation with the lamp, then proceed with the LED conversion.

Right now, we don't know if its an actual MPX problem or an incompatibility with the LED causing the problem. A further complication is that it seems each Fisher model used a different variation of the MPX circuit, so what worked for the 4400 may be incompatible with the 700-T.

Part of the difficulty with the LED conversion is that the characteristics of the incandescent vary depending on whether the filament is cold or lit. It is basically a nonlinear resistor. When lit up at normal operating current, it may exhibit a resistance of say 400 ohms. When cold however, the resistance drops to around 50 ohms or so, which presents an entirely different load to the MPX driver circuit. Unfortunately Fisher engineers decided to use the lamp as part of the circuit to switch the decoder to stereo mode, which is what makes it finicky. Instead of using a separate transistor to drive the lamp directly, they combined functions, saving the cost of a transistor, but making the circuit problematic regarding lamp replacement.