I recommend that you have a copy of the 1200XL Computer Field Service Manual before you start this or any other modification to your computer. I find that is goes quicker if you can compare the 2 schematics and make your mods from there. This schematic is what the modified circuit looks. It is not the same as the one appearing in AC.
Got a 1200XL with bad video? With a few simple changes under the hood, you can get rid of that tearing, fuzzy screen and add separate CHROMA operation.
Unlike the movies, the sequel to the CLRPIC modification is much "better" than the original. If you were disappointed that the CLRPIC mod did not allow for a upgrade, then you are at the right place. This version only requires you remove 10 components (16 if you won't be using the RF modulator) and add 4 wires and 4 components! IT IS STILL A GOOD IDEA TO HAVE SOME ELECTRONICS SOLDERING SKILLS. Also, these changes were developed empirically, which means, I just diddled with the values until they worked (NOTE: I have further diddled with the video circuit of the 1200XL and this is the result cd.). The video output is very good - better than an 800XL and depending on your preference for overdriven characters, better than a 130XE or 800 in separate CHROMA or COMPOSITE video.
On with the changes. As in the original CLRPIC article, I will list the existing component in the left column and what to put in place of it in the right column. A "--" indicates that there is nothing to be connected and "wire" calls for a jumper, shorting out the part.
| Remove | Replace With: | |
| C115 | 10uf | wire |
| C60 | 100pf | -- (nothing) |
| R187 | 1meg | 2.2k |
| L15 | 820uh | wire |
| R188 | 750 Ohm | 680 Ohm |
| CR19 | 1N4148 | -- (nothing) |
| C119 | 3.9pf | -- (nothing) |
| C62 | 10pf | 1uf to10uf |
| R23 | 180 Ohm | 47 Ohm |
| R24 | 180 Ohm | 47 Ohm |
| C116 | .0068uf | -- (nothing) |
| CR20 | 1N4148 | -- (nothing) |
Now add a wire from either side of R44 to pin 5 of J2.
That's all!! Works fine on a number of monitors that I have.
Below is the parts placement for the 1200XL video portion of the circuit board.
The parts that are either replaced or eliminated are the ones with asterisks (*) next to them.
I added new lables for the parts so it should be easier to find them.
