Sega Model 2 graphics problems- Dead or Alive

aclbandit

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Video of the problem: https://youtu.be/eSHvp64CozU

Had some trouble getting this behemoth enough power, but it's sitting at a happy 5.0V on the far chips now that I've run a second power line from the New Astro's NVS-400 power supply.

But it still isn't happy. This is the best I got it to look, and this was after running it for ~15 minutes. Initially, the textures are usually black and the colored lines across the screen are much worse:

It seems like the video RAMs going bad on this thing is a fairly common problem, and from what I can tell from the Google it matches what I'm seeing pretty well, but I'm not sure.

I've heard also that the video ROMs sometimes cause similar issues if they're not making contact with the main board.

I've tried pressing down on the top board, and that changes the problem, but doesn't fix it. The only 'fix' is to leave it running for about an hour without the fan so it gets nice and toasty, and then kind of tap the metal casing. After that, it will sometimes 'click in' and work fine until it gets powered off.

I checked continuity from the top board to the bottom board on the side where the three boards interconnect, and I've checked continuity between the pins that only touch two boards. It all checks out great.

While I was at it, I pulled and reseated all of the socketed chips. I've pulled the boards apart and put them back together.

The seller is willing to ship me another board to try out (sans top ROM board), but before I put them through that much trouble (and before I start checking individual traces)... am I missing something simple?
 
Pull all of the boards apart and re-seat them. When you screw the boards back in, leave all of the stack screws loose and secure/screw the filter board first, so all of its connections are tight to the filter board, next push the stack as close to the filter board as you can. Then go back and screw the stack to the cage and tighten the "stack" long screws.

Also, while you have it apart, you might want to ground out the "super cap" on the cpu board. In fact, this cap might be the one that needs to "warm up" before you get everything working happy (or one of the other many caps.)

If you have a second set of boards (check system16.com for other possible ones, yours could be either a 2A or a 2B), you can swap boards to find the one troubling you. I'd guess the cpu board, but that is just a guess. ;)
 
Pull all of the boards apart and re-seat them. When you screw the boards back in, leave all of the stack screws loose and secure/screw the filter board first, so all of its connections are tight to the filter board, next push the stack as close to the filter board as you can. Then go back and screw the stack to the cage and tighten the "stack" long screws.

Also, while you have it apart, you might want to ground out the "super cap" on the cpu board. In fact, this cap might be the one that needs to "warm up" before you get everything working happy (or one of the other many caps.)

If you have a second set of boards (check system16.com for other possible ones, yours could be either a 2A or a 2B), you can swap boards to find the one troubling you. I'd guess the cpu board, but that is just a guess. ;)

I will be doing this tonight or tomorrow.

This is definitely a 2A, sorry that I didn't mention it earlier.

I want to be sure I'm understanding completely before I start messing with the super cap. Two questions:

1) What, precisely, do you mean by 'ground out'? I think this can be achieved by removing the cap and jumping the solder points of the two leads, but I want to make certain first.

2) What is the supercap supposed to be doing, and why don't I need it in the circuit? Is this 'ground' a temporary thing to test whether that was the problem (which should be followed by replacing it), or is it a permanent recommended fix? Why?

Thanks -- I appreciate the help.
 
You don't need to remove the super cap.

You don't want to "permanently" ground it either.

It will just restore everything to factory defaults. It was a trick an operator wanted me to try when I had a problem with a 2A PCB. It didn't help me, but it is free & easy to try.

You just take a screw driver or wire and shorting the two leads on the cap when the game is off/pulled apart. (You can't get to it when assembled, but I want to be clear on off.) Short it for 30seconds or so.

I can't find a good picture to get you the part location, but the cap is light green (color may vary) and in my case a little shorter and wider/bigger than all of the other caps. You can see the cap on the cpu board when the ROM board is on. It looks different than all of the other caps on the cpu board.

To be honest, I think the re-seating of all of the PCBs in the stack is more likely to solve your problem. But, you have to pull it apart to get to the super cap, so doing both at once won't hurt.
 
Its not a super cap issue. The issues related to super cap would be viewpoint only. I have repaired over 5000 Model 2 and 2A and have never seen this issue before. Most common issue on 2A is IC27 SMD on Video Bd having opens. Check it out and see if its solid. Other than that you will need another system to swap pcbs to figure out where it is. I'm leaning towards Video Bd but could also be on ROM Bd.
 
Its not a super cap issue. The issues related to super cap would be viewpoint only. I have repaired over 5000 Model 2 and 2A and have never seen this issue before. Most common issue on 2A is IC27 SMD on Video Bd having opens. Check it out and see if its solid. Other than that you will need another system to swap pcbs to figure out where it is. I'm leaning towards Video Bd but could also be on ROM Bd.

The seller actually told me to contact you if I can't get it sorted, so glad you're here.

Good call on IC27 -- I checked it out and although you can't really see it in the photos well, there were some pins that were (and I'm sorry for making up terminology) "nearly-shorted." I'm assuming that the heat-up time is what caused the problem to eventually go away (and come back after cooling). I've cleaned the pins up on IC27 (and other chips nearby).

So, I think the reason it was so tough to figure out is that I'm diagnosing two problems, not one. The cleanup of IC27 fixed some of the stranger issues, and now it appears to be 100% a connection problem someplace.

See the following video (the image is a link to it) -- I can get it to 'snap in' to a functional place by pressing down on the front right of the board stack (as seen in the orientation in the video), but as soon as I let go it goes back to being wacky. I don't think this will be hard to diagnose, but if anyone's got good suggestions about where to check first, it will be a big help.

 

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Just make sure you have no opens on those two large SMD QFPs. From where you are flexing it, it still could be one of those. I'm also wondering if someone in trying to troubleshoot problem bent some pins up there on ROM Bd.
 
Just make sure you have no opens on those two large SMD QFPs. From where you are flexing it, it still could be one of those. I'm also wondering if someone in trying to troubleshoot problem bent some pins up there on ROM Bd.

I will check this, then.

Any chance anyone has a schematic? That will keep me from having to follow tiny traces, which is not fun. I can do it if I have to, but I won't like it.
 
For posterity:

Ended up sending this to irepairsega, and the issue was basically exactly what he stated earlier in the thread.

Thanks!
 
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