Is it possible to replace the TIA chip on the 7800? Fixing compatibility?

[+-^CrossBow^-]

It is roughly located dead center along the right hand side of the main board. The capacitor is a green colored banded ceramic that you would mistaken for a resistor. 

[Tempest]

So in an interesting twist, after clipping the capacitor to disable the timing circuit, my 7800 is still able to play 2600 Dark Chambers.  It appears that right now the only thing it won't play is the early version of Decathlon (it plays the later one just fine from the Harmony cart though).   How strange is that?  I wonder if it has just enough 84 parts in it to make it nearly 100% compatible even though it's an 87 model.  Maybe the expansion port being there does something with the timing?  It's all really strange to me.

[+-^CrossBow^-]

Your 7800 is basically using the '84 design components into a later designed mainboard. As an example. Q4 on yours actually has the 3904 tranny there when most 7800s actually have a vertically mounted resistor there instead. I'm sure there are other changes as well if it was gone through more fine comparing and checking each component you have in this 7800.

 

But that is why the 7800 is @CPUWIZ's favorite console! ?

[+DrVenkman]

6 hours ago, -^CrossBow^- said:

But that is why the 7800 is @CPUWIZ's favorite console! ?

Now that I've seen pics and know what to look like, my AT-84 (fully socketed) 7800 has no C64 and no place to put one. My A1 7800 does have vias and a silkscreen for it, but they're empty and pads are wave-soldered through from the factory so it was never installed. 

 

Fun, fun ...

[+-^CrossBow^-]

That is what I mean. Tempest's is basically designed using the components from the AT-84 earlier systems but also has the rest of the stuff filled in used on later units. Tempest already mentioned this...but even with his C64 capacitor removed from the circuit. His 7800 is still actually able to no only play Stargate without any issues that I could find, but get this...

 

2600 Dark Chambers still works as well?!

 

So his would actually be the first one that has passed through my hands that is essentially 99% compatible with the entire library at this point that really shouldn't be given the revision and other components the way it was made. But...here we are. The only game that I was still not able to get to come up was my picture labeled Decathlon. It just produces random bars when powering up his system and that cart inserted. But Space Shuttle works just fine and it also an early picture label variant. Darrel Spice's bus stuffing demos work on it now as well and they didn't before so that is another set of tests I run on 7800s as part of the 2600 compatibility testing. His Parrot bus stuffing demo being one of the more fun to use.

[Lynxpro]

On 10/1/2019 at 9:40 AM, Tempest said:

G=that's what's confusing.  Given how tight fisted Atari was with money at the time, it boggles my mind that they decided to go through all the extra expense to engineer and add this circuit to fix one game.  I can only assume they thought that future games would also have this problem.

 

That being said, why does the original 84 Atari 7800 play Dark Chambers correctly?  I wonder if there's a way to fix other 7800's to play it AND disable that circuit to fix other games?  Probably no worth exploring as DC is only so-so on the 2600 anyway (although I enjoyed it back in the day).

 

Normally, I'd say "just get 7800 Dark Chambers and forget about playing the 2600 version", but someone made a hacked version of 2600 Dark Chambers into Gauntlet. And that's friggin' sweet. I know plenty of people here and elsewhere have expressed support for the idea of hacking the A8/XEGS and 7800 versions of Dark Chambers into Gauntlet*.

 

Maybe if a 7800 version was done, someone would investigate the possibility of making a Team Tap Adapter for Joystick Port 2 on the 7800. Since that mofo port was capable of using the 7800 Keyboard on it, the 2nd Joystick, and devices on the SIO Port built into the keyboard, it should be able to accommodate multiple joysticks...possibly even 3 of 'em.

 

 

*Yes, I know, Domark/US Gold/Mindscape released an officially-licensed version of Gauntlet for Atari 8-bit but it is slowwwwwwwwwwwwww....