7800 compatiblity fix

[jsoper]

I sent this to RGVC last night, but really wanted to post it here. If the graphics are too scrambled, save as a TXT file and look at it with a fixed font editor.

 

 

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:

 

code:

---

_____

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:

 

code:

---

,---------------<-------------,

| |

| _____ |

| | | |

P10 ----+-| | |

74174 | | |---, |

|_| | | |

| | | ______ |

|_____| | | | |

| | | |

`-->--| | 4 |

_____ | |-->-+-->--

| | | | 7408

Phi2 -----| | ,-->--| | P10

(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

 

[ 08-23-2001: Message edited by: Albert ]

[Albert]

I edited your post and put the diagrams into code blocks so they'd use a fixed-width font. They look much better now.

 

Thanks for taking the time to post it here at AtariAge. I think this would make a great addition to the 7800 FAQ, and I'll add it as time permits.

 

..Al

[cschell]

I think this modification is more than is required. This circuit doesn't need to exist, so there's no need for a switch. As John mentioned it's not on the early 7800s, and I have later 7800s that have the pads and circuit on the board, but the 74LS02 isn't installed and instead shorting jumper is installed in it's place.

 

I believe this circuit can be removed by simply removing a single capacitor. I know it can be removed by shorting a single resistor. When I get my digital camera back I'll post photos of what I'm talking about.

 

Chad

[cschell]

Ok,

 

I've put instructions up for the simplest modification that should make your 7800 work with the Supercharger and the other stubborn games. So far I've tried it on exactly ONE 7800, so it's definitely still in the experimental stage.

 

If you'd like to give it a try, you can see the instructions and photos at

http://www.schells.com/7800mod.shtml

 

There are lots of 7800 variants out there, so I don't know to how many this modification applies. But if you have the extra circuit, and it's connected via the highpass filter formed by C64 and R66, this should do the trick.

 

Chad

[jsoper]

Albert,

 

Thanks for the edits, gonna start a webpage sometime and get more net-savvy. Another item on a long to-do list.

 

It was nice talking to you at CGE as well. I was the guy in the Secret Agent line with the portable 2600 in a dreamcast controller.

John

[cschell]

quote

Chad, I will send you the pics of my mod before and after to show you the resistor in the same place you had the cap, if you would like.  

 

Sure, I'd like to see them. I'm curious if they updated the silkscreen, or if it still says C64 where your resistor was.

 

Email links are all over my website.

 

Chad

[Scott]

quote:

---

Originally posted by C. Schell:

Ok,

 

I've put instructions up for the simplest modification that should make your 7800 work with the Supercharger and the other stubborn games. So far I've tried it on exactly ONE 7800, so it's definitely still in the experimental stage.

 

If you'd like to give it a try, you can see the instructions and photos at

http://www.schells.com/7800mod.shtml

 

There are lots of 7800 variants out there, so I don't know to how many this modification applies. But if you have the extra circuit, and it's connected via the highpass filter formed by C64 and R66, this should do the trick.

 

Chad

---

 

YES! IT WORKS! I have the 86' version of the 7800 with the hole/opening for the port on the side, but the port electronics removed. I opened it up and saw the same layout that Chad has on his site. Instead of a Cap, I had a resistor there. I removed the resistor, screwed it back together popped in the Super Charger, and it worked! The cuttle cart works in at as well. I do not have Robot Tank or Decathlon to test, but as I mentioned the Cuttle Cart and Super Charger work fine now! 7800 history in the making. Hopefully this will fix the issue I have been seeing with my High Score Cart. Will know after 2 weeks of use.

 

Chad, I will send you the pics of my mod before and after to show you the resistor in the same place you had the cap, if you would like.

[jsoper]

I just checked my 3 incompatible 7800's and they all have color coded caps for C64. They actually look like green resistors. Kinda unfair for the colorblind I'd say.

John

[cschell]

I've seen capacitors like that used before in other Atari products. I suspect that's what was on Scott's board as well. A resistor doesn't make any sense. (Gee - let's take our logic levels and divide them down.)

 

John also informed me that his notes had the capacitor in the circuit, it just didn't get copied into his ascii diagram. (This is good, as otherwise I think there would be 7800s out there that had problems running even simple first generation 2K games.)

 

Chad

[Albert]

quote:

---

Originally posted by C. Schell:

I've put instructions up for the simplest modification that should make your 7800 work with the Supercharger and the other stubborn games. So far I've tried it on exactly ONE 7800, so it's definitely still in the experimental stage.

---

 

Hi Chad! I just added a link to your instructions from our 7800 Archives page. Thanks!!

 

..Al

[Albert]

quote:

---

Originally posted by John Soper:

Albert,

 

Thanks for the edits, gonna start a webpage sometime and get more net-savvy. Another item on a long to-do list.

 

It was nice talking to you at CGE as well. I was the guy in the Secret Agent line with the portable 2600 in a dreamcast controller.

John

---

 

Hi John! It was great meeting you at CGE as well. I remember talking to you, the hacked-up Dreamcast controller caught my eye pretty quickly.. It was cool seeing all the different 2600 portables people are working on.

 

Good luck when you start working on a web page, the more classic gaming sites the better. At least I assume you're talking about some sort of classic gaming website. And as for long to-do lists, you should see the AtariAge to-do list. Maybe starting a website isn't such a great idea after all!!

 

..Al