7800 with flickering interference

[shear_xear]

The two I have used while troubleshooting I received about two weeks ago.  I did just get a few more yesterday, but they could be from the same batch as well.  I will try one of them out just to be sure.  I do know that the UAV I used to check if the problem carried over to a second board worked perfectly with a later revision board, so there is definitely something about this revision that is causing this. 

[shear_xear]

Well the same problem occurred with the third UAV board.  I know I have installed the UAV on three 7800’s with the expansion port, but these are the first two that were fully socketed.  

[shear_xear]

Ok, so is there an actual difference between the white label pack in version of Pole Position II and the retail version with the picture of the cars?  I tested several new games, and none exhibited the flickering problem I had with Pole Position II and Ballblazer.  I think the problem is with these specific game cartridges.  

[+-^CrossBow^-]

Well...let me check my copy of PPII but mine is the white label pack in version. I also on a BB cart and I literally just modded up a 7800 about 30min with one of the new UAVs I received and at least on the diagnostics and initial games I was playing, it was working fine.

 

[shear_xear]

I think it’s the cartridges themselves. I can’t imagine Atari doing more than just sticking a new label on the retail version of Pole Position II.  Everything else worked as it should, including a couple homebrews.  No issues whatsoever.

[+-^CrossBow^-]

Sent you a PM on something to look into. Let me know if it helps or if yours is something similar to what I found.

[Mynameis7nad]

I'm having the same issue with pole position 2. I think it may be something about the game. Did you ever figure out more about this?

It's a fully socketed rev A board with an expansion port and for some reason has Blue pots compared to the usual black ones. Also I'm not using a uav mod. I'm using a basic 2600 av mod that vintagegameingandmore makes. No s-video but they work good and are cheap enough to be worth not making one on a breadboard when buying bulk. I use them on all 2600s, 7800s, 5200, and intellivisions after adding a voltage Regulator to them and this is the first time running into an issue like this.

Edited April 16, 2021 by Mynameis7nad

[shear_xear]

It doesn’t have anything to do with the mod, as I noticed the flicker (although greatly diminished) through the RF on one board.   I never figured out what was causing the flickering, but it only seemed to happen on the early boards with the expansion port.  My guess is that they resolved the issue with a board revision, or with a revision of the game.  

[+-^CrossBow^-]

3 hours ago, Mynameis7nad said:

I'm having the same issue with pole position 2. I think it may be something about the game. Did you ever figure out more about this?

It's a fully socketed rev A board with an expansion port and for some reason has Blue pots compared to the usual black ones. Also I'm not using a uav mod. I'm using a basic 2600 av mod that vintagegameingandmore makes. No s-video but they work good and are cheap enough to be worth not making one on a breadboard when buying bulk. I use them on all 2600s, 7800s, 5200, and intellivisions after adding a voltage Regulator to them and this is the first time running into an issue like this.

 

48 minutes ago, shear_xear said:

It doesn’t have anything to do with the mod, as I noticed the flicker (although greatly diminished) through the RF on one board.   I never figured out what was causing the flickering, but it only seemed to happen on the early boards with the expansion port.  My guess is that they resolved the issue with a board revision, or with a revision of the game.  

I thought I gave @shear_xear the details. I was able to replicate the issue more or less on an A3 revision 7800. And I've been checking for this (hot fix) on the boards on the last few I've done. In the newer schematics that were redone for the NTSC units in 2003, it shows a small .1µf cap that is attached along the A15 address line from the 6502 to ground. It can actually be attached anywhere along the same signal path as it connects to the Maria, the hex inverter above that and back to the BIOS itself. Anyway, this appears to be a deglitching cap that was added in later. Newer revisions either don't require it or have it embedded within the board layout and I've just not located it. But look on the bottom of the board where the 6502 is and see if you have what looks like a green or blue colored resistor attached off pin 25 on the 6502 with the other end attached to ground. That is actually a banded ceramic capacitor that was missing on these boards and added in the later schematics but I've found it not actually present in a few 7800s. 

 

Also, there is at LEAST two different version of Pole Position II and Ball Blazer. I have two different versions of each. And at least with ballblazer the differences is that one of them has an extra ceramic disc cap on the cart that isn't present on the other. My guess is that Atari was also including this deglitch cap within the games themselves that were known to have issues. But as you have no way to know this, it was also added at the factory onto the bottom of the 7800s themselves. 

 

So, that is what I would look for and if you don't see the cap there, try adding it back in. I did a video on this very issue because of this thread 6 months back:

 

 

[shear_xear]

It is definitely possible that you passed that along.  I have early 1984 copy write version of Pole Position, and I never saw the flicker with that version.

[+DrVenkman]

I just tested my 1984 white label Atari, Inc. PPII cart in my serial A1 daily driver machine, modded with a UAV for composite and S-video. I don't see the same effect you are getting, which @-^CrossBow^- has confirmed. I don't seem to have any photos of the underside of this board that I can find anywhere, so I can't confirm or deny the existence of the "de-glitching" cap he has ID'd. I'll check next time I have this machine apart. 

[+-^CrossBow^-]

To be clear, the original '84 release doesn't seem to have any issue with the glitching I was showing. It was only Ballblazer and the later released versions of PPII that would do this. On most of the 7800s, I've noticed a factory added ceramic cap off pin 25 to ground on the 6502 CPU on the bottom side of the PCB. So it was definitely something that Atari was correcting on early model 7800s before selling them.