I've bought a few boardsets over the years which would not sync up correctly on my monitor, although they were hooked up correctly (don't just assume that if your game doesn't sync up, this tip will fix it; make sure the board is putting out the right sync, and you have it hooked up to the monitor correctly). I just wrote it off as a difference of monitor circuitry, since the game would run fine on other monitors.
However, I found an article on www.arcadecollecting.com which talks about how to "Strengthen" a sync. The first article mentioned putting the sync through a couple of inverters to boost it. I tried that first. Although it did help a little bit, it was not enough. I then tried the recommendation in the second article and it worked great! The solution was this:
Taken from Rodge Boots article, re-written and formatted for clarity:
Take a 555 timer and make the following connections:
1 - Gnd
2 - sync input from game (tied to pin 6)
3 - output to monitor
4 - +5VDC
5 - no connection
6 - sync input from game (tied to pin 2)
7 - no connection
8 - +5VDC
I made a small adapter using a finger board and JAMMA edge connector. I soldered the fingerboard to the solder eyelets of the JAMMA edge connector, cut the trace for the sync pin, and wired this circuit in. Worked out great!
Most monitors like the sync levels to be in the range acceptable by TTL ICs where a low is
below 1 volt and a high is above 2 volts but not exceeding 5 volts. There is sometimes the
other extreme where the sync inputs are overdriven. This can be the case with the Gorf/WOW
type boards as well as the Nintendo boards.
From Rick Schieve: