There's some exposed pins on the coin mech wiring harness...can we see that better?
It's very possible one of the exposed pins shorted out something...esp.if they could touch metal.
Also, the top of that ribbon cable...it needs to be firmly inserted into the display panel's socket.
It does look like someone was mucking around on your machine.
Up on the other photo, towards the left side of the panel, there's some duct tape covering a screw??
I'm going to bet that whoever had this machine before you, bent a pin somewhere and it's shorting out against another pin.
They may have already done the damage by removing or installing something with the power switch [ON].
Turn the power [OFF], carefully inspect circuit board pins underneath them...the pins should be perfectly straight.
For starters, there's one right behind that colored ribbon cable Tilt is talking about.
It's called an I/O card and it helps the deck buttons communicate with the machine's main computer called an MPU.
You can see part of the handle sticking out. There should be a harness connected to the top of it.
Grasp the handle and slide it out towards 11 o'clock.
Take a good look at the pins underneath it.
If they look good, slide it back down slowly until it locks in....you'll feel the resistance as you push it onto the pins.
The main MPU board is located on the left wall of your cabinet under the reel shelf.
There's a bent handle on the front of it, grasp that and slide out the whole tray.
Inspect the pins under that.
If the pins look straight and good, take some pictures of the MPU and show us, we might spot something else.
Especially, around the battery areas.
To re-install the MPU, carefully slide it back into the metal housing the same you removed it.
At the end, firmly push it in until the end of the tray is flush with the metal housing.
Below I've attached a photo of the exposed wiring - we need to see what's going on there.
See if you can take a better photo of that.
Click on photo below to enlarge if needed...>>>