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Bluebass

Atari Jr Schematic

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Does anyone have a schematic of the Atari Jr. I'm trying to figure out what is wrong with my atari Jr. I just bought it on EBay with no RF switch, so I wasn't able to check it before the mod. Now, the trigger doesn't work from either control port.

 

 

Also, I can't seem to get the color adjusted right.

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Fire button problems on the 6-switch and junior models are usually caused by a bad 4050 buffer, so start by replacing it. A bit of a pain the first time you do it since they're usually not socketed, so be sure to install one after removing the bad chip.

 

If the colors change when the chroma adjustment pot is turned, but will not become correct no matter what position it's in, there are a couple of possibilities: PAL/NTSC mismatch (try other cartridges), or a bad TIA IC.

Edited by A.J. Franzman

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BTW, the TIA makes sense. I don't have B&W at all no matter how the pot is adjusted. The TIA controls that too from what I've read. Right?

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I just noticed you mentioned a "mod", are you trying to build a portable? If so, both of your problems could be explained by construction mistakes.

 

Turning the color adjust pot may not be able to give a B&W picture in every unit, but the Color/B&W switch should do so when using an early Atari-made game (just about any title with a text label should work for a test).

Edited by A.J. Franzman

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I just noticed you mentioned a "mod", are you trying to build a portable? If so, both of your problems could be explained by construction mistakes.

 

Turning the color adjust pot may not be able to give a B&W picture in every unit, but the Color/B&W switch should do so when using an early Atari-made game (just about any title with a text label should work for a test).

 

No, the mod I did was Ben Heck's composite output mod. Just to make things simpler I'll put all the symptoms in one place:

 

1. trigger buttons don't work from either port. Tried two sticks w/ same results.

2. B&W picture is none existant. I flip the switch to B&W and the screen turns completely black.

3. While in color mode, the colors are really messed up. I can get blue red green and black. I have yet to seen any lighter colors, i.e. white, or lighter variations of blue or green. Color is affected by the pot, but is never even close to the correct colors.

 

I've never worked on a Atari, and I'm quite the novice when it comes to electronics. I've fiddled around with it as a hobby. I am sure I had the mod hooked up correctly, though. I believe I may have discharged a capacitor on the right side of the board opposite the RF circuit. about midway between the top and bottom of the board. I'm not sure though. I didn't smell any ozone, and nothing is visibly fried.

 

P.S. as I understand it, you can't chop the atari Jr into the 4 inch square to do the portable version. At least, I think that's what Ben wrote. right or wrong??

Edited by Bluebass

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Ben Heckendorn did eventually succeed in miniaturizing a Junior motherboard (the rare single-chip version) but then accidentally fried it. The proper method for a Junior will be different than for a 4-switch, and I don't think you can get a Junior down to a neat 4-inch square without a few extra circuit sections patched back onto it.

 

If you're using a Junior, you're gonna be almost on your own, so at the very least you should have some good close-up photos of both sides of the board before you hack it, and save everything you cut off, in case you have problems so you can trace out the circuits to see what you broke.

 

Your color problem sounds like you have the resistors connected wrong, and are not getting the proper luma levels. Are you using Ben's first composite mod, or the second one (with a transistor)?

 

Another possibility is that you're using the wrong mod. Where do you live? Most of the half-dozen or so mods on the net, including both of Ben's, are for NTSC consoles only. The PAL TIA has a different pinout. If your Junior is PAL, then an NTSC mod will not work.

 

The fire button signals on the Junior go to a 4050 buffer IC, then to the TIA IC (the 40-pin farthest from the cart port). Are you sure you didn't cut their tracks, or the tracks to their pullup resistors?

Edited by A.J. Franzman

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Ben Heckendorn did eventually succeed in miniaturizing a Junior motherboard (the rare single-chip version) but then accidentally fried it. The proper method for a Junior will be different than for a 4-switch, and I don't think you can get a Junior down to a neat 4-inch square without a few extra circuit sections patched back onto it.

 

If you're using a Junior, you're gonna be almost on your own, so at the very least you should have some good close-up photos of both sides of the board before you hack it, and save everything you cut off, in case you have problems so you can trace out the circuits to see what you broke.

 

Your color problem sounds like you have the resistors connected wrong, and are not getting the proper luma levels. Are you using Ben's first composite mod, or the second one (with a transistor)?

 

Another possibility is that you're using the wrong mod. Where do you live? Most of the half-dozen or so mods on the net, including both of Ben's, are for NTSC consoles only. The PAL TIA has a different pinout. If your Junior is PAL, then an NTSC mod will not work.

 

The fire button signals on the Junior go to a 4050 buffer IC, then to the TIA IC (the 40-pin farthest from the cart port). Are you sure you didn't cut their tracks, or the tracks to their pullup resistors?

 

I'm not making a portable. I don't think it is the mod either. I pulled out the Ben Heck's (2 pot version) and tried the one listed in the FAQ for the Atari Jr. That didn't change anything. It still looks exactly the same.

 

As far as PAL or NTSC, I live in Tennessee, but I bought the Atari online. I can't remember where the seller was exactly, but he was state-side. I would assume the system is from the US also but who really knows. He claims the system was working perfectly, but then again, he didn't send me anything to connect it to a TV. I wonder if he even tested it.

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You've probably got a bad 4050 then. Try replacing that chip or bypassing the ciurcut traces for the two firing buttons. As far as the composite mod, you'll be much better served with Ben Heck's second mod using a transistor. This mod is compatable with more TV's than the first one ever was.

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You've probably got a bad 4050 then. Try replacing that chip or bypassing the ciurcut traces for the two firing buttons. As far as the composite mod, you'll be much better served with Ben Heck's second mod using a transistor. This mod is compatable with more TV's than the first one ever was.

 

 

Do you have a link to his second mod? And where can I get a 4050. I've looked at some electronic dealers and can't find one.

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4050 is just buffers, you could completely bypass them.

 

 

How?

 

EDIT: Also, what can I do about the TIA IC?

Edited by Bluebass

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4050 is just buffers, you could completely bypass them.

 

 

How?

 

EDIT: Also, what can I do about the TIA IC?

The 4050 (or CD4050; CMOS part) has six buffers, you just connect each input to its output to bypass it. This should tell you if you've got a completely hosed 4050. Usually, when they go bad, it's only 1 or 2 of the buffers that die. But it's possible that yours has lost most of them; if so, it could be causing all of your problems. Replace the 4050 before you worry about the TIA. 4050s are cheap and plentiful; TIAs are hard to find and expensive, or must be cannibalized from another console. If changing the 4050 fixes all of the problems, you're done. If you still have a color problem afterward, then just maybe you still need a new TIA.

Edited by A.J. Franzman

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Would you send me the bad one? :D

 

My dad works on Mainframe computers, and used to test circuit boards for his company. I'm giving it to him. He screws around with stuff like this when he's bored.

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On 12/6/2006 at 5:10 PM, alex_79 said:

The 4050 buffer was removed in 4-switch models (woody and vader), but was restored in the junior design.

 

Schematics for pal jr. are on Atariage archive, while here you can find schematics for NTSC version.

The link for the NTSC version isn't working anymore. What difference is there between the NTSC version and the PAL version?

Edited by PaultheRoman
Added a question

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