I don't know a lot about Wurlitzers of this era. I'll start this off with, on the bench, applying 14vdc on the bench power supply to pins 3 ground and the plus to pin 4 of X5, in about 1/2 second, the leds K and Z light up and stay on. Then in another 1/2 second relay 2 energizes and stays on. I don't know if that is a normal power up routine? The SCC is a 2500 CD.
Now the really weird part. Inside the jukebox, with no cables plugged into the board, from pins 3 and 4 of the plug for X5, it measures about 14 volts DC, no AC, this is normal. But, when that plug is in the SCC on power up, the 14 volts jumps up to 30 volts! Thankfully the 7805 has over-voltage protection, in addition to a zener on the board so the +5vdc goes to zero.
I have tried this with all of the plugs in the SCC and with only plug X5 in. Either way the 14vdc jumps up to 30vdc and the +5vdc goes to zero.
On the SCC, it has a history of R3 running hot and turning the board very dark. Also someone cut into the plasic cover in what looks like an attempt to cool the area at and above the 7805 and R3.
Also, I don't know if it's common, the interface board top third is very dark from heat. This machine has been on 24-7 for most of its life.
Any help greatly appreciated.