Atari 7800 plays 2600 games but almost never 7800 games

[Aussie2B]

As I said in the finds topic, I recently found a few 80s systems. The 7800 looked to be in the best shape, but unfortunately, it hasn't been smooth sailing getting it to work. It turns on just fine, with the power light coming on, and it can play 2600 games without much issue, but when I popped a 7800 cart in, it was nothing but solid color screens, screens with various colored vertical bars, rolling, black and white scrambled screens, etc. The problem is somewhat similar as in this video:

https://www.youtube.com/watch?v=XNOQk1nQaEQ

Even though the old ones looked fine, my fiance replaced the capacitors for the heck of it, but that changed nothing. Unlike in the video, letting the system warm up didn't solve the problem, but it did gets us from no graphics to somewhat getting the logo screen. Also unlike the video, keeping the board away from the metal shielding makes no difference. There was one single time that a 7800 game actually started up, and I played for a while with no issue. I don't know what was unique about that one lucky time, but there were countless tries before that and countless times after. It didn't work again after that one time even when we touched absolutely nothing except pressing the power switch off and back on.

Does anyone have any idea what could be the problem and how it could be solved?

[jb143]

Just out of curiosity, since you mentioned in the other thread how dirty everything was, did you clean the contacts on the games and systems? With alcohol. And really clean the heck out of them? And clean the system inside in out in general? Since one 7800 game did work, that's where I'd start. Especially since dust can get in there so easily.

Internally, it's actually a 7800 and a 2600 built into one with a shared CPU(that's slowed down for 2600 mode), and since 2600 games work, that should narrow things down a bit.

All I can really say, is your lucky you had the power supply for the thing.

[Aussie2B]

Yeah, it slipped my mind to point that out. I scrubbed down the contacts of the games with q-tips and isopropyl like I do with all my games, and since we opened up the system, we cleaned not only the cartridge slot but the whole board (though the board was pretty clean to begin with). This stuff is about as clean as it can get now. If it were a matter of cleanliness, I would expect the 2600 games to be just as finicky about working, but they're pretty reliable now.

I've read that different chips are responsible for playing 2600 and 7800 games and that, in cases where 2600 games play but not 7800 games, it could be a bad MARIA chip, which is responsible for playing 7800 games. But since I got a 7800 game to play one single time, I don't know if that's confirmation that the MARIA chip is fine. The 7800 I found is apparently among the earliest models, so I'm also lucky in that many of the chips are socketed rather than soldered directly to the board. It's easy to pop them off and back on, so we carefully looked for any signs of damage. But there's just nothing jumping out as the culprit.

I still have some more 7800 games I need to clean, so I suppose I should see if there's any game among the bunch that can work better than the rest.

[Gameguy]

If the chips are socketed, try removing and reseating them. It could be a poor connection with one of them.

[Cornelius]

Could it be a finicky power switch? I know, unlikely since the 2600 works, but maybe that aspect isn't as touchy about power. Just a hunch.

[AdamAnt316]

Could it be a finicky power switch? I know, unlikely since the 2600 works, but maybe that aspect isn't as touchy about power. Just a hunch.

Doubt it, since the 7800 uses a 'soft' power button rather than a physical on/off switch as with the 2600. My guess is either an issue with the MARIA chip, or bad/dirty contacts on the pins of the cartridge slot associated with it detecting 7800 games.
-Adam

[jb143]

Hmmm...I wonder if it could be a problem with the power supply, or something in the power section of the board, not generating quite enough power for the 7800 portion?

I would still give the contacts another go at cleaning them though just to be sure.

[Aussie2B]

Okay, so I've cleaned every single one of the 2600 and 7800 games I found, all with the same cleaning method, and all ten of the 2600 games work perfectly, always loading up on the very first try. Meanwhile, none of the seven 7800 games are working. I don't know why I was able to get Centipede to work that one single time before, but I haven't been able to replicate that, not with Centipede or any of the others. The best I can get with the 7800 games is a scrambled Atari logo screen. If I let the system sit on for awhile, it gets less scrambled, but never enough to actually work properly.

I tried reseating all of the socketed chips, but that didn't make any difference. I did notice a spot of corrosion around a pin of the MARIA chip, which I cleaned up, but I don't know if that's a sign that the MARIA chip is failing.

The power switch seems to be working fine. The light always comes on in conjunction with it, and I've also done a lot of testing with the board outside of the system shell, pushing the switch directly rather than via the plastic button.

We also tried messing with the pots. The color pot works as expected, but the pot closer to the cartridge slot seems like it's doing absolutely nothing.

[AdamAnt316]

Okay, so I've cleaned every single one of the 2600 and 7800 games I found, all with the same cleaning method, and all ten of the 2600 games work perfectly, always loading up on the very first try. Meanwhile, none of the seven 7800 games are working. I don't know why I was able to get Centipede to work that one single time before, but I haven't been able to replicate that, not with Centipede or any of the others. The best I can get with the 7800 games is a scrambled Atari logo screen. If I let the system sit on for awhile, it gets less scrambled, but never enough to actually work properly.

I tried reseating all of the socketed chips, but that didn't make any difference. I did notice a spot of corrosion around a pin of the MARIA chip, which I cleaned up, but I don't know if that's a sign that the MARIA chip is failing.

The power switch seems to be working fine. The light always comes on in conjunction with it, and I've also done a lot of testing with the board outside of the system shell, pushing the switch directly rather than via the plastic button.

We also tried messing with the pots. The color pot works as expected, but the pot closer to the cartridge slot seems like it's doing absolutely nothing.
What about the contacts within the cartridge slot itself? I believe the outer sets of pins are involved with telling the system to engage 7800 mode, though I could be wrong. I don't think corrosion is a direct sign of a failing chip, though cleaning it off and ensuring that each pin is making a good connection with each contact in the socket would be a good idea (if the pin contacts within the socket show signs of corrosion, it might be a good idea to replace the socket). Some info on troubleshooting a 7800, as well as a theory of operations, can be found here (http://atariage.com/forums/topic/216763-7800-theory-of-ops-ts-guide/). Technical specs, as well as info on how MARIA does her thing, can be found here (https://en.wikipedia.org/wiki/Atari_7800#Technical_specifications). Good luck!
-Adam