NeoGeo MV4F - Jailbars on foreground and background

[srarcade]

Hello- first time poster and new to pcb repair (at least more advanced stuff). I've been on KLOV forums for a number of years and collecting games for about 15. Slowing down the buying and focusing on repairing some of the broken gems I have.

Anyway- I purchased this non working neo geo 4 slot for the reasonable price of about $20. I quickly discovered the battery had corroded a bunch of traces which I then repaired. All pins have continuity now between boards and traces. The board now starts up and all 4 slots work (victory!) but there are jailbars on the background and foreground sprites for all 4 slots. The overlay sprites like life bars, timers etc are fine. The neo-geo bios screen is fine. I discovered when i "touch" (not squeeze together) the bottom side of CN8, the jailbars seem to shift around. I can even get my finger in a certain spot and they almost completely disappear. I checked the pins on CN8 and even re-flowed the solder on both ends, but it didn't help. I have a logic probe wired up to the board, and i've been probing around some of the multiplex chips on the top but i'm not finding anything stuck. I'm a newbie at debugging with the probe so I probably don't know what I'm doing. I don't have a schematic to go by to tell what the outputs are on each component, but i've been reading a ton of logs on similar fixes and going by that so far. Any ideas to help steer me in the direction of my problem? I seem to have a general area locked down, just need to pinpoint the trouble spot.

Thanks!

[channelmaniac]

When touching things and the screen goes crazy you most likely have a bad chip or a bad trace.

First step: Look for gouged/broken traces on the top board and on the bottom board.

Second step: inspect the pins on the connector to see if any are bent/broken then check continuity between the upper and lower board through the connector.

Dirty slots and cartridge edge connections are a HUGE cause of problems but the screen shouldn't change when touching the connectors... unless you are wiggling the board when you do it. That can cause the carts to shift ever so slightly in the slots.

[srarcade]

I tested out all the pins on CN8 for continuity from bottom board to top board, all check out. I have really gone over all the traces on the bottom board, they all are showing continuity on the dmm. I then went on each pin and tested the trace to chip on the top board, all checked out. So it's likely I have a bad TTL chip somewhere. I'm probably going to need a schematic to see the outputs of these? Since it's occurring on all slots, would that limit the area on the board it would be coming from?

[channelmaniac]

If it changes on the screen when you touch a pin on the connector the follow that back to the chip driving it.