The tray harness kit is not too difficult to install, it's just very time consuming. You will replace the tray boards as well as adding the boards on the rails and you'll bypass the connectors on the back of the cabinet. They will tell you it can be done in 1 1/2 hours, but that's only if you have done a bunch of them or don't take care to follow the directions. I took my time and it took me 3 hrs to install. I found a couple of confusing parts in the instructions but figured out what they meant or had left out pretty easily. I do remember installing the top rail board or bracket on the wrong rail and had to move it. I believe the top bracket is different than the rest and you may also have crummy screws included in the kit. I also remember that the screw hole template was off and not to scale. Just stupid stuff that I had to work around.
Why do you want to do MDB in these machines? If you want to install a card reader I don't believe the factory logic board can accept the input it will get from the kit. This board is the same as what was in the National 430 cold food and it wasn't able to accept credit from a card reader. If you want MDB for another reason, you will have to find 24V somewhere in the machine to connect to. The wiring diagram doesn't show if there are any 24V outputs from the transformer. You will have to call National tech support and ask them. The MDB kit is pretty straight forward; you have a power harness to 24V, a male 12 pin plug to go where the old changer plugged in and a female MDB connector for your new coin mech and validator. The coin mech needs to be a late model that uses a motor to pay change out, not solenoids. You also have to be able to count coins into the coin tubes so the coin mech won't ever pay out imaginary coins. A late model changer helps here too, as they have "float up" or "par" level capabilities built in.
