I recently picked up a S+ machine with the 16 battery error. I replaced the battery, then found one of the door optics was bad. One leg was broken off. Got a new set installed, got past the 61 and had it running. Great. Played it a few times then I got the 21 coin tilt error. I pulled off the optic unit, noticed that the grey 4 wire connector had a broken connection. I replaced that then also found that the harness connection had a broken wire. I repaired that, and verified the connections are good in the harness. Reassembled and still have the 21 error. I took it apart again and tried using my phone camera to see if the optics were working, works for verifying a remote control works, and I have nothing. I have fluctuation 9.5 vdc between the red and green wires, it also reads between 6.97 and 7.1 VAC. On the optic test, it will intermittently switch between 0 and 1. Jiggling the wires does nothing. I noticed in the test for 10, 11 and 12, it will flash to 1 when the voltage drops down to around 9.1. I figured the 10000 16 volt cap may be the issue so I pulled it and it tested it. Got nothing on the cap test setting. Ordered a new one amd installed it, no difference. I am unsure if a 4021 chip will work intermittently or not. Seems like most switching IC's either work or don't... Stuck and am looking for ideas. Thanks in advance!