I just bought a used AP snackshop 7600. Really don't know any of the history of the machine, as in whats working and whats not. I did test the unit out and the coin ops side of the machine seems to be functioning ok. The bill validator however, not so much. It didn't work at all, until I changed it out with the spare unit the guy gave me when I purchased the machine. The DBA will sometimes take a dollar, flash 1.00 on the display and then back to a blank display and not allow a vend. Other times it will keep spitting the dollar out 5, 6, 7 times. And yet other times it will accept the dollar, display 1.00 and let you make a selection. Sometimes it will just eat the dollar and not display anything. Its been driving me crazy trying to figure this out. Now I'm a computer guy, so I'm familiar with the boards, dip switches and connectors. These don't scare me. I just don't know if there is a problem with the control board, the DBA, the board on the side of the DBA??? Does the DBA need to be cleaned? (seems pretty clean)

I have tried numerous bills, both old and new, but still the same thing. Any suggestions of things to check would be great. Since this is my first machine, I do not have any spare parts or known good, working parts, besides a non-working DBA, to test. Thank you all in advance.