Choplifter - 2 boards - different syncs

modessitt

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So, I built me a JAMMA-to-Sega System 8 adapter so I could test two Choplifter boards I have. Once I realized I forgot to run a jumper from the JAMMA video gnd to the regular gnd, everything was working okay. First board was running just fine, hooked up to a K7901 using composite negative sync.

Hooked up the 2nd board and had a scrambled picture. Adjusting the holds did no good, and I eventually figured out that it works fine, but only with composite POSITIVE sync!

According to the instruction manual I found online, there is no sync selection done via dip switch, and no mention of a board modification to change it. Also, the wiring diagram shows the normal sync output to be composite negative sync.

I looked at both board, and the only difference I can see is that the positive sync board has a disc capacitor bridging two legs of IC103, and the negative sync board does not. I tried removing one leg of the cap to see if that was what causing the sync difference, but it didn't change anything.

Anyone know anything about why two separate Choplifter boards would operate on different sync? They didn't release two versions with the kits for operators who may have a 4600 with only the positive-sync-only daughter card, did they?

Thoughts?
 
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