Editing Atari 2600 Jr Composite Video Mod

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[[File:Atari-2600-Jr-Motherboard-Flat.jpg|thumb|right|340px|NTSC 2600 Jr. board; the RF section with A5 is at the right, the 4050 (A4) below it]]
The '''[[Atari 2600 Jr]]''' has RF output only.<ref name="manual" /> Two composite and S-video modifications are documented for the Jr.: the UAV board and home-made transistor mixers. The TIA's video signals and the mods for the six-switch and four-switch consoles are on the [[Atari 2600 Composite Video Mod]] page. TBA's UAV manual calls the Jr. "one of the more difficult installs" because there is little room inside.<ref name="tbaman" />

== How the Jr. builds its picture ==

On Jerzy Sobola's redrawn PAL schematic, the TIA's luminance and sync outputs are each buffered by a section of the 4050 at A4 (A4A, A4D, A4E and A4F) and summed through resistors into the modulator; another section of the same 4050 (A4B) is part of the reset circuit. The TIA's colour output reaches the modulator through a separate resistor and capacitor network, and the modulator itself is built round an LM3086 at A5.<ref name="sobola" /> The NTSC board in the photograph uses the same designators, with a CA3086 at A5.<ref name="photo" />

Because the reset circuit uses the 4050, it cannot simply be pulled out to make room for a video board as it can on some other consoles.<ref name="tbaman" />

== UAV board (NTSC three-chip board) ==

TBA's instructions, originally written by Bryan Edewaard, are for the NTSC three-chip Jr.; TBA had not seen the one-chip or PAL versions. They recommend the "Kit" version of the board.<ref name="tbaman" />

# Open the case carefully: TBA says it is brittle and its screw posts snap. Lift the left side of the top case first and unplug the flex cable to the switches. Release the snap levers at each edge to lift the board out, and straighten the tabs underneath to remove the shield.
# Fit the UAV's configuration jumpers and its round socket pins, but not the green terminal strip.
# Desolder the 4050 and keep it. TBA suggests clipping its legs and clearing the holes one at a time.
# If the RF output will be kept, solder the UAV directly into the 4050's holes so that the shield still fits. If RF will not be used, the shield can be left off and the UAV fitted in a socket.
# Fit the 4050 on the underside of the UAV: trim its leads flush, place it upside down between the UAV pins with pin 1 towards the middle of the main board, and solder each pin. Make sure nothing can touch the bottom shield.
# Lift the back leg of R54 and solder the colour pick-up wire to the exposed pad; connect it to the UAV's Color In pad.
# If the RF jack is not needed, it can carry composite video or audio instead: lift the right side of R19 and solder the chosen signal to the exposed pad.
# Set the jumpers as TBA's figure 6 shows and wire the outputs to the jacks.

Elsewhere in the same manual TBA notes that on some Jr. boards the plug-in UAV can be used, with Color In picked up from a via just under R28.<ref name="tbaman" />

The UAV has no audio circuit. TBA's Internal Audio Board (IAB) takes its supply from the two sides of C12 and its input from the front pad of R34.<ref name="tbaman" />

TBA's notes on the 2600's saturation-boost resistor (an 820 Ω resistor between TIA pins 6 and 9 on some boards) do not apply to the Jr.: its table lists the Jr. as having no saturation boost circuit.<ref name="tbaman" />

== Transistor and resistor mods (UK PAL Jr.) ==

David Savery's R3UK article documents home-made composite mods on a UK PAL "Revision A" Jr.:<ref name="r3uk" />

* To avoid overheating the TIA, he soldered to the resistors that the TIA pins connect to instead of to the chip pins: sync (pin 2), luminance (pins 5, 7 and 8), colour (pin 9, taken from the far end of its resistor) and audio (pin 13).
* A resistor ladder feeding a transistor amplifier, and Ben Heckendorn's simpler two-potentiometer mixer, both worked but gave a picture that was too dark.
* Following an AtariAge suggestion, taking the lowest luminance bit from TIA pin 6 instead of pin 8 on the PAL console, and using 1 kΩ as the ladder's lowest value, gave a clear, bright picture with no adjustment of the console's colour trimmer.
* He found audio only on TIA pin 13, and took it from the resistor connected to that pin.
* The finished stripboard fitted in the upper left corner of the Jr.'s case, with the shield refitted and two phono sockets cut into the rear.

His conclusion was that no single circuit worked for every console and that some tuning should be expected.<ref name="r3uk" />

== Related pages ==

* [[Atari 2600 Jr]]
* [[Atari 2600 Jr General Maintenance]]
* [[Atari 2600 Composite Video Mod]]
* [[Atari 7800 Composite Mod]]

== References ==

<references>
<ref name="manual">Atari Corp., Atari 2600 owner's leaflet for the 2600 Jr., C026260-001, 1986: connection to the television through the switch box. Hosted here as [[:File:Atari 2600 Jr Owner's Manual C026260-001.pdf]].</ref>
<ref name="sobola">Jerzy Sobola, redrawn schematic "Motherboard C012283 rev. B, Atari 2600 Game Console", sheet 1 of 2, 8 September 2000 (PAL 2600 Jr.). Hosted here as [[:File:Atari 2600 Jr PAL motherboard schematic (Jerzy Sobola).png]].</ref>
<ref name="photo">Evan-Amos, photograph of an NTSC Atari 2600 Jr. motherboard, Wikimedia Commons, public domain. Hosted here as [[:File:Atari-2600-Jr-Motherboard-Flat.jpg]].</ref>
<ref name="tbaman">The Brewing Academy, ''Bryan's Ultimate Atari Video Rev. D Instruction Manual'', revision 1.94, 23 March 2021: "2600 installation" (pp. 6, 7) and "Atari 2600 Jr." (pp. 12, 13). [https://cdn.shopify.com/s/files/1/0079/4807/5108/files/TBA_UAV_Manual_for_digital_20mb.pdf PDF from thebrewingacademy.com].</ref>
<ref name="r3uk">David Savery, [https://www.r3uk.com/index.php/tech-tips/39-cool-projects/71-atari-2600-composite-video-mods "Atari 2600 composite video mods"], R3UK, created 25 January 2004, updated 15 February 2008. Community project write-up on a UK PAL Atari Jr.</ref>
</references>

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