I thought I'd post this in here so that it gets archived as part of
Stellalist...
Can someone explain WHY this circuit is important to these games? I'm
assuming it has something to do with how the 2600 reacts to attempts to
bankswitch and do RAM stuff with the Supercharger.
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.
An expansion port 7800 has the following circuit:
_____
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 | |
|_____|
7402 NOR gate (ALL THREE)
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
At 06:38 PM 8/23/2001 +0200, you wrote:
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
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tjentzsch at web dot de |
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