edited Apr. 21, 2009
L5 & new power lead
The same trace that brings 5V to the video circuits also powers the clock oscillator circuits. 1 clock circuit for NTSC, and 2 for PAL. The shared power route can cause faint vertical bars to appear in the picture. The purpose of inductor L5 was to deal with this noise, but it doesn't really do enough.
Remove L5 (or at least lift one leg of it out of the board). This will break the old 5V path.
Solder a wire from the front lead of C3 (electrolytic to the right of the power jack) to the front hole of L5 (or to the front lead of C51 next to it). If you run this wire along the RF modulator, it should clear the shield. 20 or 24 gauge wire should work well.
For some reason, Atari decided to add this cap which blurs the image. This capacitor is not present in all 800XL's. If it's not there, then great. If it is there, then remove it entirely.
The Q3 luma amp is an emitter-follower type. The large collector resistor reduces the gain of the amp a bit too much. Place a 10-ohm resistor across R116.
If you have a 390-ohm resistor (orange, white, brown) here, solder a 100-ohm resistor across it. If you have a 100-ohm resistor here (brown, black, brown) then you may leave it alone, or solder a 330 ohm resistor across it if for a small luminance improvement.
The ideal resistance here is 75 ohms. When video drivers deviate from the 75-ohm standard, it causes an impedance mismatch with the coax cabling and the monitor which degrades the picture and can cause ghosting (reflections) in longer cables.
Chroma Feedback Resistor
Solder a 1K resistor from the bottom of R58 to the right side of C55. This will improve color somewhat.
Missing Chroma Output
On the bottom of the board, solder a 100-ohm resistor to the junction of R67 and R68. These two adjacent resistors run left to right and their junction is on the left when looking at the top of the board. Put a piece of tape under the resistor to insulate it from the board. Solder a wire from the other end of the resistor to pin 5 of the DIN connector (bottom-left pin from the back).
the resistor can also be soldered on the top of the board and a wire run to the jack as in post #2
Optional: Power Filtering
I have removed this step. The current scheme of an electrolytic bypassed with a monolithic is sufficient once you run the new power wire.
I've attempted to show the improved picture, but it's hard to get a good digital snap of a monitor.
A picture of this modification is posted on page 2 of this topic.
Edited by Bryan, Tue Apr 21, 2009 9:41 AM.