Subject: Re: [stella] 2600 Jr Difference From: Glenn Saunders <cybpunks2@xxxxxxxxxxxxx> Date: Thu, 23 Aug 2001 08:13:41 -0700 |
The postings about 7800 compatibility got me interested in revisiting a fix I first made a couple years ago. Thought I'd write it up as a new post so everyone would see it.
_____ 11 | | 10 10 |} D2-----|D Q|-----------| } (6502) | | | } 8 | | 9 | }------A12 (cart) |> | A12----| } |_____| (6502) | } |}
74174 7408 Flip Flop AND
A later model 7800 has an extra flip-flopish circuit between the two pin 10's above:
,---------------<-------------, | | | _____ | | | | | P10 ----+-| | | 74174 | | |---, | |_| | | | | | | ______ | |_____| | | | | | | | | `-->--| | 4 | _____ | |-->-+--->---P10 | | | | 7408 Phi2 -----| | ,-->--| | (out) | | | | | | |---' |______| D1 via ---| | flop | | |_____|
The fix I did was as follows: 1. Cut trace coming from 7408 P10 with an exacto-knife 2. Verify cut trace with ohmmeter 3. Wire up a SPDT switch so that 7408 P10 can be connected to 74174 P10 (expansion circuit) or 7402 P4 (later circuit)
After I did this my space shuttle and robot tank carts started working, however decathlon still wouldn't. I then added a 1uf capacitor to ground onto the line and decathlon came up fine. In my final assembly, I used a second switch to connect the cap but decathlon worked even without the cap. Seems like the loading of the switch itself was enough.
If you do this mod, you do so at your own risk, yadda yadda yadda. If anyone wants to put this into a FAQ, go ahead.
I've seen posts that dark chambers sometimes doesn't work on the expansion 7800's (which prompted the circuit change) and that player paddles 3 and 4 are not connected in some units. Have not seen any evidence of this in the 4 7800's I own.
Had a supercharger go bad in my expansion model 7800, and wrecked two others modifying them for 8k games (even had one working for a night). Got so sick of buying new ones that I don't experiment with them anymore. You're on your own there.
That's about all I know. Wanted to fix the color problem in my new portable 2600 but this ate up the whole evening, grumble grumble :)
Sure hope these ascii drawings don't get garbled, John Soper
Hi,
maybe it doesn't exactly match this topic, but I think it might be interesting too:
The topic is: atari 7800 compatibility fix
http://groups.google.com/groups?hl=en&safe=off&th=8144a73fd15cd431,1
Have fun! Thomas _______________________________________________________ Thomas Jentzsch | *** Every bit is sacred ! *** tjentzsch at web dot de |
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