You probably have a bad power supply. The power supply makes
logic voltages and supplies 110 volts for the fuser. The "50 Service" can mean that the fuser is not warming up or that it has overheated. Is there any way you can get these fusers into another LJ 4 to test them?

If you have a meter you could check for 110VAC fuser voltage at the power supply. The connector is in the front end of the supply. Remove the plastic side cover and whatever sheet metal to get at the front end of the supply and check for 110VAC at J102. There must be a relay in the supply that turns the fuser on at a certain point in the power up cycle.

The diagram shows the fuser power going through the High Voltage supply but I think it is just a convient place to hang the fuser connector

Best bet might be to find a parts machine. I have seen working LJ4's at the surplus places for $20.

Al