Atari 7800 troubleshooting question: screen garbage

[Nateo]

My brother-in-law bought me a used 7800 for my birthday this year! (that's good!)

But it didn't have a power supply. (that's bad)

So I found that Best Electronics makes new ones, and I bought one! (that's good!)

It cost $30. (that's bad)

But it came earlier than I thought! (that's good!)

 

But then I put everything together, and the darn thing doesn't work. All I get is a blank screen, or screen garbage. That's bad.

(can I go now?)

 

I went through the flow-chart found elsewhere in the forums, and it makes me suspect the Maria chip, but I can't be too sure. Is it as simple as replacing the Maria? Thanks for reading!

[+DrVenkman]

Couple questions ...

 

1. Have you thoroughly (and I mean THOROUGHLY!) cleaned the system cartridge slot? Spray electrical contact cleaner into the cart slot, then insert and remove a 7800 cartridge vigorously half a dozen times or so. Use a real contact cleaner, something like CRC brand available at any autoparts store or even Wal-Mart, or better yet a brand like Deoxit, which is sort of magic on dirt and corrosion. Then let the cartridge and cart slot on the console dry for a couple minutes before powering on the system.

 

2. Have you tried multiple games, both 7800 and 2600? If 2600 games work but 7800 games don't, or vice versa, it could help narrow down the potential culprits.

[Nateo]

Couple questions ...

 

1. Have you thoroughly (and I mean THOROUGHLY!) cleaned the system cartridge slot? Spray electrical contact cleaner into the cart slot, then insert and remove a 7800 cartridge vigorously half a dozen times or so. Use a real contact cleaner, something like CRC brand available at any autoparts store or even Wal-Mart, or better yet a brand like Deoxit, which is sort of magic on dirt and corrosion. Then let the cartridge and cart slot on the console dry for a couple minutes before powering on the system.

 

2. Have you tried multiple games, both 7800 and 2600? If 2600 games work but 7800 games don't, or vice versa, it could help narrow down the potential culprits.

 

1) I haven't thoroughly cleaned the cartridge slot, but I did use a credit card, some paper and isopropyl alcohol and that got a little gunk out.

2) I only have 2600 games to try - I suppose I could run to one of the local game stores and pick up a copy Pole Position II.

[+DrVenkman]

Okay, well the answer to #2 shows that MARIA is not to blame here - it's only active for 7800 games. So at least there's that.

 

So, I'd start by getting some good contact cleaner and giving things a really good cleaning, including the carts you're using. The method I described above manages to handle both cartridge and slot well enough for most people most of the time. If you try several carts and none of them display properly, you've likely got a hardware issue. It could be a bad TIA chip (the same main chip that's used in the 2600), a bad 6502 RIOT chip (also used in the 2600, used to read the position of console switches as well as RAM for 2600 games), or possibly some other component.

 

Honestly, if more rigorous cleaning doesn't help, figuring out more will probably require taking things apart, a close visual inspection and possibly some component replacement.

[Nateo]

I did take the opportunity to open it up, and everything looked pretty clean to me board-wise. I'll grab some Deoxit on the way home from work tomorrow and gun for the cartridge slot.

[+-^CrossBow^-]

If you get blank screen or garbage and you only have 2600 games to test, then first guess is the TIA not the Maria IC. Like DrVenkman state, the Maria only kicks in when the system detects you put a 7800 game into. 2600 games, the system literally switches into 2600 mode and so the TIA would be handling all the video processing at that point.

[Nateo]

It's fixed! At first I replaced the TIA, which only added garbage noise to the garbage picture. The only spare chip for the machine I had left lying around was the RIOT chip. So I figured "why the hell not", popped the old one out, clicked in the new one and voila! Working Atari 7800!

[+-^CrossBow^-]

That would have been the second thing to do as I had to replace the RIOT on one for a similar condition but since I changed out both the TIA and RIOT (For what I thought were unrelated control port#2 issues), I wasn't sure that the RIOT was part of the fix for the video.

 

I have had to replace both in 2600s to get them to work though.

 

But yeah I typically start with the TIA, then RIOT, and then the CPU.

[MightyNo0]

My console will load the game but wont start up. Like it will try and play then reset, can any one please help me

[+-^CrossBow^-]

So the games come up okay, but you are unable to actually start to play them? Which games, 2600 or 7800 games. Most 7800 games and few 2600 games will start just using the fire button on the controller, but many 2600 games required you to use the Reset switch to get them going. If the reset isn't responding, it could very well be a bad reset switch. The 7800 panel buttons are known to be problematic but are cheap and easy to replace if you have some moderate skill with a soldering iron.