Atari 1200XL complicated video

[reifsnyderb]

I just picked up an Atari 1200XL and have jail bars really bad.  (It's not the cable.)  I lifted the end of c51 and c53 and still no joy.  Looking at the schematic, Atari has a ton of circuitry for the 1200XL.  Has anyone ever pulled out everything they didn't like and reduced it to that of a good old reliable 800XL video circuit?  I am thinking about doing this.

 

Thanks!

 

Brian

 

[+mytek]

UAV worked like a charm for me on my 1200XL.

[+kheller2]

UAV or SV2.1 

 

 

 

[ACML]

I've upgraded a lot of 1200XL motherboards and I've noticed that the older REV 13 boards have milder jail bars than the newer REV A 1200XL motherboards.  Don't know why, but I'm noticing a trend.  The jail bars already exist in the stock LUMA signal going to the monitor jack.  Plug a stock 1200XL into the s-video of your LCD/LED flat screen and you'll see jail bars in the B&W s-video (i.e. no CHROMA connected on the jack).  Point being that the jail bars effect is already there before modifications.

Edited February 24 by ACML

[reifsnyderb]

Thanks for the feedback! 

 

I read through the 8-bit Alchemist article here:   https://archive.org/details/Atari_Classics_v2n6_December_1993/page/n3/mode/2up

 

While I liked the article, I really don't like the lack of a schematic.  The other gripe I have is instructions like piggy-back R42 with a 120k ohm resistor.  Why not just install the right resistor in the first place?  I do wonder how many Atari's ended trashed by somebody badly following those instructions.  But, on the other hand, because of the article I decided not to mess with the color circuit.  In the article was also a description of some changes to the Luma circuit I was already thinking about.  Finally, I made the following changes:

 

Remove R162, R163, CR20, C115, L15, R21, R23, R25, C61, C63

Install as follows:

1.6k ohm resistor at R21

Jumper at L15

Jumper at C115

Jumper at R23

75 ohm resistor at R25

100 ohm resistor and jumper from Monitor pin 5 to emitter output of Q7 (I installed it at a pin of either C61 or C63....I forgot which.)

 

Also, note that the schematic was wrong as "Comp Lum" is really RF and "Video RF" is really LUM.  The big tip-off is that L2 goes to RF.

 

 

I still have faint jail bars on the blue BASIC screen.  But they are very faint.  The S-Video color is great and no jail bars are noticeable when playing games.

 

Now to see if the keyboard works....

 

 

 

 

 

 

 

 

 

 

[Ricky Spanish]

1 hour ago, reifsnyderb said:

I still have faint jail bars on the blue BASIC screen.  But they are very faint

You'll always have 'jail bars' to some extend. The UAV is still the best mod.

[reifsnyderb]

2 hours ago, Ricky Spanish said:

The UAV is still the best mod.

I've taken to liking it better when the change is to swap out some components and there isn't any spaghetti wire.

 

Turns out the keyboard doesn't work unless I take it apart and press on the mylar.

[+mytek]

If you totally want to bypass all of the 1200XL's video circuitry, but still want analog video output, then my UGV upgrade would probably be the best bet.

 

https://ataribits.weebly.com/ugv.html

 

You'll get UAV quality and also VGATE capability by piggybacking the GTIA chip with this upgrade video board.

[+mytek]

BTW, one of the things that often gets overlooked is whether the S-Video cable is double shielded or not. Often times people make the mistake of buying something like the 8-bit Classics cable thinking it's a high quality cable with individually shielded Luma and Chroma, which it isn't. I once made this mistake and chased my tail around and around trying to solve a jail bar issue after I installed a UAV in my 1200XL only to discover it was the cable all along. Once I made my own Atari version from a high quality S-Video cable that I verified had individual shielded wires, the jail bars completely disappeared.

 

This cable Does Not Have Individually Shielded Wires...

[reifsnyderb]

23 minutes ago, mytek said:

BTW, one of the things that often gets overlooked is whether the S-Video cable is double shielded or not. Often times people make the mistake of buying something like the 8-bit Classics cable thinking it's a high quality cable with individually shielded Luma and Chroma, which it isn't. I once made this mistake and chased my tail around and around trying to solve a jail bar issue after I installed a UAV in my 1200XL only to discover it was the cable all along. Once I made my own Atari version from a high quality S-Video cable that I verified had individual shielded wires, the jail bars completely disappeared.

 

This cable Does Not Have Individually Shielded Wires...

I think that's the cable I have.  I've always assumed the minimal jail bar issue I have is the cable as I know it's not double shielded.