2600 JR composite mod

[Osgeld]

I recently got my fist Atari system up and running after getting a new old stock R6507 CPU, for inital tests I jumped a wire to TP5 as guided in this Atariage text file

 

the video was quite dim, and quite unacceptable, so I set about futzing around.

 

when I first got the unit I disabled the RF modulator portion of the circuit, later on I completely removed all components from that section, there is no chance I or anyone else would use it, it causes great interference, and it sucks extra power.

 

Even if I had a "dead" Atari I was still able to see the video generation signals with my scope, though there was nothing in the active portion of the signal. I removed C53 and C54, two axial lead capacitors on the lower right hand of the PCB, instantly my video signal went from slightly out of phase (I have an old scope with special settings just for NTSC video signals) to a nice crisp ultra sharp sync signals and color burst signal.

 

once operational I set about making a clean composite mod using the 4050 buffer IC already present on the Atari motherboard, swapping resistors in place of adding additional circuitry

 

farting around poking various points and referring to the Jr's rough schematics with a trimpot I have the base of what I would like to see out of a composite output.

 

Its still a work in progress, and some more tweaking needs to be done, but I feel its a pretty decent first result

 

mod details, using component markings on my A2600 Jr's motherboard

 

REMOVE:

R49

R50

R53 (no pop on my motherboard, but if its there remove it)

R54

R41

C53

C54

every component under the RF modulator shield

 

the resistors I used were just standard crappy radio shack 5%'s

 

R49 populate with 1.5k

R50 populate with 10k

R53 populate with 15k (without R53 you will get "scuzz" on the picture, adding it cleans the picture up quite nice)

R54 populate with 120ohm

connect the center of the RCA jack to TP5

 

results at this moment, still need to do a little tweaking

 

Venture:

 

Fishing Derby:

 

(the only games I have ATM for the VCS, just starting)

Edited September 7, 2013 by Osgeld

[Osgeld]

after spending a few days fiddling with this, its not the way to go unless you have no other means