Half the General Illumination (GI) lights were not working on my Diner machine. From what I had read online, GI on this generation of games frequently fails because the GI connectors take a fair bit of electrical load and heat up over time – eventually they burn themselves out, and need to be replaced. I fully expected this would be the task at hand here, and a connector rebuild would be in order.

To my surprise, after opening up the backbox I found the connectors to be in perfect order! Furthermore, I measured the output pins for the GI backbox circuits at 6.3v AC, exactly what they should be. That eliminates everything up to the interconnect board pins as a potential problem, which only left the wires going up to the backbox lamps… and a small mysterious PCB between the lamps and the interconnect board. I checked continuity on the wires connecting the different lamps in each circuit, and they looked fine….

It turns out there is a relay that controls power to the general illumination circuits, which is what the small PCB board is all about. The GI is connected to the normally-closed (NC) pins on the relay, which means that with no power applied to it, current can flow normally to the GI lamps. Under normal circumstances, power flows through the relay to the GI lamps and they provide light to the backbox and playfield. When the game activates the coil side of the relay though, the GI circuit opens and the lamps turn off. This is why you’ll hear clicking noises when you’re playing a game and the entire game flashes dark – it’s the relays activating and deactivating.

Luckily, there’s another relay PCB exactly like the backbox one under the playfield. It was a simple matter of swapping the latter one into the backbox to test if that was the problem – sure enough, the backbox GI worked fine with that relay board! Clearly something wrong with the backbox relay PCB… When re-installing the bad one, I also noticed the GI would sometimes come on simply by twisting the bad relay PCB. Ah ha, an intermittent connection! I was glad to see this, actually, as it meant it wasn’t a bad relay but rather just a bad connection on the PCB.

So, why was only half of the GI out? The GI is split into two different circuits, both of which  pass through the relay, which actually is like two different relays in one as it has two independent circuit switches (but only one coil). Using my multimeter, it became clear that although both sets of normally-closed pins had continuity (and thus were OK), only one of the two circuits had continuity to the molex header pins that interface the board. It could only be a bad trace on the board, so I soldered on a jumper cable to restore the circuit.

Follow the trace between the two pins connected by the white wire – see how that white wire should theoretically be electrically redundant and not needed? Well, not if the trace is bad 🙂 I replaced the board in the game, and boom! Backbox GI back in working order! 🙂