
The DJM 500 is more from the mid 90s. The successor was the DJM 600.
Your problem sounds weird indeed. I would make sure to put in the channel faders in the correct order, althouth there should be no difference (besides the connector is located slightly different to route the cable properly):
CH1: DWG1475
CH2: DWG1476
CH3: DWG1477
CH4: DWG1478
Master: DWG1474
From the service manual I cannot see that there is any other part number for the master fader. From your description it sound like you may have put the cable in the wrong direction in the master fader, make sure that pin 1 is aligned properly (for all cables). The crossfader is not connected to master fader directly, it is using a VCA circuit.
Maybe someone messed up something inside the mixer already? If you have technical/electronic knowledge, I would recommend to search for the DJM 500 service manual and have a look for yourself. For this old unit it is not worth to give it to a Pioneer Service (which I would recommend for every newer unit of course).
Regards
Markus