Virtua Fighter 1 Graphics error

[Das Muel]

Hi folks,
I have a sega model 1 Virtua Fighter PCB that's developed a fault recently. I've attached pics to demonstrate. As you can see it only affects the top portion of the screen, and only when polygons appear there. My memory test and TGP tests come back fine, and the board is working perfectly apart from this. I've tried using contact cleaner and reseating all the boards etc, even switching out the power supply for a high end Mean well unit, but it's still there unfortunately. Does anything obvious stand out to anyone?
Any help would be much appreciated.
Cheers,
Sam (Retro Muel)

 

[Ringo]

Could be a bad ROM socket?

I had a VF2 with a similar missing polygons issue, but it was a reduction in polygons on all charas rather then the issue being located in one section of the screen. The ROM's checked out and the problem followed the ROM board. As a last resort I tried pressing down on ROM chips whilst powered on, I got lucky and was able to locate the bad ROM socket causing my issues.

I don't recommend my low tech approach as your problem may be different, but dodgy sockets can cause all sorts of weird 3D issues.

 

[waiji]

Model 1 video board has a huge amount of 'HM65256' memory for frame buffer. When one or several of these memories die, the above symptoms occur.
Or, one of the 74F240Ns controlling HM65256 may be broken.