1943 board graphics problem

delroy666

Well-known member

Donor 2011
Joined
Jul 30, 2009
Messages
1,189
Reaction score
26
Location
Ottawa, Ontario, Canada
I'm working on fixing up a 1943 PCB. I've resolved most of the issues it had (controls not working, messed up scrolling background graphics) but there's still a problem remaining with the sprites that I just can't seem to solve and it's driving me nuts. As you can see from the photos, the sprites (planes) look fine until they go over top of any of the scrolling background objects (clouds). When the planes go on top of the clouds, it looks as if only every other line of them gets drawn. I've been over almost the entire bottom board with a logic probe, following the schematics from the graphics ROMs to where the data goes back out to the main board, but I can't find anything obvious.

Can anyone offer some suggestions for where to look? It seems to be a problem with whatever circuit is responsible for integrating / blitting the sprites on top of the background tiles, but I'm not sure where in the schematics that would be. Any help would be greatly appreciated!
 

Attachments

  • IMG_0806.jpg
    IMG_0806.jpg
    41 KB · Views: 36
  • IMG_0808.jpg
    IMG_0808.jpg
    48 KB · Views: 43
1943 Board issue

I have the exact same problem with my 1943. I wish I could answer this for you. I don't have a logic probe so I'm not much help but I've reseated all the chips and cleaned the board thoroughly but still have the problem. I'm also hoping someone can shed some light on this.

If you do find the answer, please post it.
 
The following reference these schematics: http://arcarc.xmission.com/PDF_Arca...3_The_Battle_of_Midway_Instruction_Manual.pdf

Maybe look at p19, the "COLOR MIXER" section:

Here, on the left side, you have video data entering from multiple sources (scrolling gfx, sprites, tiles). It get's buffered by a set of LS273s (you might check the OBJ ones for stuck lines), then some logic (3 LS20s, might probe them) is used to feed a PROM (for graphics priority, what's on top of what), the output of the PROM is used to select among the various inputs with an array of LS153s (might probe them too). Anything downstream of here (the LS273, and the color PROMs) would likely be an issue ALL OVER the screen, not just with sprites/objects.

Of course, the issue could also be upstream from here. Other pages that have potential include the "LINE BUFFER" sections, "OBJECT TIMING" sections, and maybe the "SCROLL" sections.
 
Last edited:
DarrenF, thanks for your response. I think you're right about that color mixer circuit being responsible. By shorting some signals together and watching the effect on the screen, I could tell that the CHARx signals represent character (text) data, OBJx are sprite data, SCRx are scrolling foreground (clouds) and SCR2 are scrolling background data (ocean/ships). The PROM at 12C seems to select which of these data sources gets passed through the muxes and rendered to the screen.

I probed all of those signals and unfortunately couldn't find anything floating or stuck though. I will dump the PROM and verify its checksum tomorrow, just to be sure it's good. Otherwise, I guess I'll poke around this circuit a bit more and maybe throw the logic analyzer on there to see if I can tell what's going on, since I'm pretty convinced the fault has to be in this area.
 
I took a break from this 1943 board to work on a few other things, but now I'm back at it. I've ruled out the color mixer completely. I checked out that circuit with a logic analyzer and it looked good. Just to be sure, I borrowed a known-working board set and by swapping in the working ROM board with my CPU board and vice-versa, determined the problem is for sure on the ROM board, not the CPU board where the color mixer is.

I've checked all the sprite, foreground and background graphics circuits, Line Buffers and Object timing circuits with a logic probe and found nothing wrong. I even checked every single IC on the ROM board with a logic comparator and everything looks good. I have no idea what to do next. This board's kicking my ass. Anyone have any suggestions before I give up and call this one a loss?
 
Back
Top Bottom