NES repair (corrupt graphics, same spot on 1 game, rest plays OK?)

Got a top loading NES, when running a game (will get game details when near it) the intro seems fine, all except for 1 row on one of the screens - get garbled graphics.

Game seems to play OK though.

Tested mission impossible - similar thing, intro is fine except the text on the licenses is completely garbled. Game also seems to run fine with no other corruption I can see.

Tested carts in another nes - both work OK.


I have already replaced the 74LS373 with a 74HC373, but still the same.

Board looks clean, no visible damage. Cart slot also looks clean, but used some fine sand paper to clean the contacts.

Any suggestions where I should start looking?

 

Clutching at straws, but if the games are PAL and the console is NTSC, could there be odd issues? I know they list the top loader as region free, but isn't there subtle differences? My front loader (PAL-lockout disabled) started to play a particular NTSC game (Sky shark) but then froze. Worked fine on a US console though.

 

Thats not a bad idea, I think the test games are pal and so was the other console I tested it on.

I will see if I can find a NTSC copy of the game that displays problems and check that.

 

Could be a bad VRAM 6116. This doesn't sound like a PPU address line issue but have you checked them all? From the PPU to the 373 and onto VRAM and CHR ROM. A broken VRAM A10 used for mirroring or broken high CHR address line could be the culprit but it seems unlikely.

 

I have checked all traces from the cart slot to their respective destinations. I have not checked the chr/prg roms legs to the pins yet - intend to do that this weekend.

I also ran the nes test cart, which reported vram etc as OK

 

To the slot is enough, but it's important to get the address lines to the 373, then from the 373 to the slot. Also PPU A13's propagation through the 368 inverter should be checked, but that shouldn't be the problem.

I'm not sure how well the VRAM was tested; it's possible but unlikely for a failure to slip by it. It doesn't sound like a broken trace/373 problem though because you'd see more corruption. Is the corruption consistent across games? Have you found any more? The PPU's counter->multiplexer logic could be damaged which could wipe out a row of BG tiles but it also doesn't seem likely.

 

Update:

Game that is giving issues is Cobra Triangle. I tested mission impossible on a PAL console and the unreadable part I was describing is normal. Cobra triangle appears to be a game that doesnt work well PAL -> NTSC and vice versa.

Source:
http://www.retrones.net/Como era la NES/Especificaciones/FRECREFRNES.htm

So turns out, I just had bad luck on my test cart (Because of corrupt graphics on Cobra Triangle, I wrongly assumed the mission impossible one was too - rather than intentionally being like that)

So all in all, there is actually nothing wrong - except having a PAL game that is known to cause issues on NTSC consoles.

Appreciate the help, turns out to be bad luck and inexperience with nes hardware/games to know this was an issue.

Thanks for the replies/help - its appreciated

 

Ah yes I had rad racer ntsc and yes, not fun on the pal console. There would be rectangle lines on the screen that would flash.

 

Can this happen vice versa? NTSC glitches on a PAL system?

 

Yes some ntsc games will glitch on a pal console as mentioned above. The ntsc games I had that didn't work, glitch or had doubles of (bought a few xx in 1 carts) I traded to game traders when I did a ntsc nes repair for them. I got a talking jawa (little star wars guy) for my son, much enjoyment!

 

Reminds me when i had a colecovision that would only display black & white. I assumed it was the console and tried to troubleshoot it for hours, only to find it was an incompatibility with the TV i was using.

 

Just to completely confirm - I burnt a eprom with the USA/NTSC version of Cobra Triangle and swapped it into the cart - works perfectly.