Hello, thank you very much for your quick, complete and very precise feedback.
From memory, two years ago now I bought chips from a member of the site because I was told to do so.

I played with these chips because today again by pressing everywhere between the test button and the reset key the machine started again and I have just played around fifty games with it without problems, sometimes it works for 6 hours immediately without problem and then impossible for three days it remains blocked.

For the lights, it is perhaps not very visible in the pictures but sometimes it is the bottom 3 which are lit, including the green one and sometimes only the orange one at the top.

Attached are pictures of the MPU, the location of the Varta battery which was not present on the board, the chips as well as the picture of the coin mechanism which works perfectly and which gives 10 credits for one token.
Thanks for your help

My backplane board does not have words on it either. 3902 13" GameKing cabinet. There is an 8-pin port labeled J12. Is this Netplex? I could not find my particular machine in the NLG File System (I am a contributing member.)

No.  J2 on the top of the motherboard is the Netplex connector. 

Come on guys, isn't there anyone on this form that can answer my questions?
Also looking for  SMI chips 9716 or 9717 for this machine with no replies. Do I have the one and only Bally Beach Party slot machine?
Help is desperately needed. Thanks to anyone that replies.

My guess is a couple of originals and the rest are chinese replacement buttons.

Sounds more like a problem with the 1st bill entryway optic sensors on the head, doesn't it?
If there's a problem with those, or the heads' circuit board, I'd imagine the motors won't turn on - to pull in the bill.

I'm not an expert with bill acceptors...I just kind of know how things work, but I've always used coins on S+ machines.
I would wait for some input from NLG members that may have more validator experience than I, before you rush out to get another one.
Member Jim has better knowledge about these old bill acceptors...esp. testing them... maybe you could invite him to check out your Topic?