Tweaking vertical sync

[gibbous]

I just got a Megalo 410. web page It's a 41" rear projection japanese sit down cabinet. It's very cool.

So I plugged in my NBA Jam board.  Problem is, when I turned it on, the video just kept scrolling vertically, fast. Adjusting the V-hold changed the speed from fast to mearly "rapid", and does affect direction, but it never stops it completely.

A little google-ing brought up the fact that Midway boards like NBA Jam put out a 56k v-sync, but most monitors are expecting 60k. Hooking them up causes the symptoms I talked about.

Question is: is there something I can do to tweak the V-synch signal so the image displays properly? I really want to play this game on this screen. I'm very handy with my Hakko soldering iron and I frequent a couple of electronics shops with most anything I'd imagine I'd need, problem is, I don't know what I need. Any help is appreciated.

Thanks!

P.S. - if I need a semester in whatever class to figure it out, let me know.  In other words, if what I want to do is too complicated for a guy who barely knows what ohm's law is, just tell me.  I can take it.

[grantspain]

perhaps a different value hold pot may help,but from time to time i have seen this issue with various mismatches between boards and monitors and i just swap into a different cab,unfortunately you cant do that.
is your picture perfect apart from the rolling?and what is the rptv(toshiba,mitsubishi etc)?and do any other pcb's work o.k?

[gibbous]

I should correct my numbers, it's 54Hz that's out put by the NBA Jam and 60Hz that the monitor is expecting.

The picture appears to be OK besides the scrolling.  I wouldn't call it "perfect."  The contrast isn't very good, but it's still very playable.  I'm sure it need some adjustment.

I still haven't taken the back off to confirm, but I will when I get home.  Previous research I've done has indicated that Megalo 410's come with a Mitsu RPTV.

Other boards work fine.  I've tried my Main Event, Ninja Gaiden, Neo 1 slot, and a time soldiers (just for testing, not playing).  I haven't tried my MKs, because I'm so sure the v-sync signal is the problem, but I could try just to confirm it.

A new pot, I can handle.  Are you thinking of replacing the pot in the cab or putting another pot inline?

[grantspain]

my personal feeling on this is-don't mess with the monitor because its obvious there is nothing wrong with it and you don't want to screw up a rptv because they a sods to fix
best way forward with this is try some resistors on the sinc output of the nba board and see if that makes a difference
i don't want to be responsible for you blowing the crap outta your pride and joy cab
btw there are a couple of switches on the rptv control panel have you tried them yet

[MonMotha]

In some cases, using separate sync (rather than composite sync) can help with monitors that expect exactly 60 or 59.94, but are being fed something a little different.  On fully analog monitors this is a little less likely to be a problem than a digital chassis, but it's simple and worth a shot.  The Midway T unit (I think, whatever MK2, late MK, and NBA Jam run on) has separate sync available on the auxiliary video output towards the edge of the board, as it isn't on JAMMA.  It's a 6 pin 0.156" header that probably won't be populated (but you can solder down to it).  Check the schematic for the board.

[gibbous]

grantspain - I won't hold anyone responsible for anything that I decide to do to my cabinet.

I didn't realize resistors have an affect on frequency.  Wouldn't I also be lowering the voltage, having an adverse affect on the image?  Forgive me, but I don't know a whole lot about this.  My knowledge extends to the first chapter of Radio Shack's "Intro to Electronics".

MonMotha - Yeah, according to http://www.crazykong.com/manuals/NBAJam.man.pdf, it is the Midway T-Unit.   I will take a closer look at the actual board when I get home.  It would be the easiest fix, if it worked.  I might try it.

[SirPeale]

I didn't realize resistors have an affect on frequency. 

What do you think a potentiometer is?    If you look at electronic schematics, you'll note that pots are denoted by the prefix "VR" as in "VR201" and whatnot.  The "VR" stands for variable resistor. 

[gibbous]

*light bulb*  So if I've got a pot laying around, I'm probably better off hooking that up in line and seeing what happens?  Is it really that easy?

[gibbous]

Wait, wouldn't adding resistance only slow down the timing?  If I want it to fire faster, I have to remove resistance, wouldn't I?  Or am I