Editing Atari 2600 Composite Video Mod

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[[File:Atari 2600 four-switch motherboard (Rev 16).jpg|thumb|right|340px|A Rev 16 CX2600A board; the TIA is the lower 40-pin chip and the RF modulator is at the lower right]]
Every '''[[Atari 2600]]''' leaves the factory with RF output only. On NTSC consoles the modulator is tuned to channel 3, 61.25 MHz.<ref name="fsm" /> A composite video mod takes the picture and sound from the TIA before they reach the modulator. Methods range from capacitor-coupling the modulator input, through one-transistor buffers and resistor mixers, to the Ultimate Atari Video (UAV) board.<ref name="tyne" /><ref name="r3uk" /><ref name="tbaman" />

The procedures below are for the six-switch CX2600 and the four-switch CX2600A. The [[Atari 2600 Jr]] has a different board; its mods are on [[Atari 2600 Jr Composite Video Mod]].

== The TIA's video outputs ==

The TIA produces the picture as separate signals, which the console mixes for the modulator.<ref name="r3uk" /><ref name="tbaman" />

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{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+'''TIA pins used by composite mods'''
! TIA pin !! Signal
|-
| 2 || Sync
|-
| 5 || Luminance 1
|-
| 6 || Blank
|-
| 7 || Luminance 2
|-
| 8 || Luminance 0
|-
| 9 || Colour
|-
| 12, 13 || Audio
|}

Pins 2, 5, 7, 8, 9, 12 and 13 are as listed by R3UK; pin 6 is the blanking output named in TBA's manual and in Atari's Tech Tip 4.<ref name="r3uk" /><ref name="tbaman" /><ref name="fsm" /> TBA describes the colour output as open collector with a 1 kΩ pull-up, and blanking as an open-collector output.<ref name="tbaman" />

=== The saturation-boost resistor ===

Atari's Tech Tip 4 (November 1982) adds an 820 Ω resistor between TIA pins 6 and 9 of the CX2600A to improve colour saturation, and it is built into boards from Rev 16. Console5 notes that Rev 14 and 15 boards lack it and some earlier ones have 680 Ω there.<ref name="fsm" /><ref name="c5" /> TBA's manual explains that during blanking, including the colour burst, the resistor halves the colour signal and adds a DC step. The circuit lowers the colour burst so that the television turns its colour gain up, but it distorts the colour waveform too much for the UAV. TBA lifts TIA pin 6 (if the TIA is socketed) or one end of the resistor, and says the boost can be added back at another point without affecting the UAV.<ref name="tbaman" />

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{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Board features that matter for a video mod'''
! Board !! 4050 buffer !! Saturation boost (TBA) !! Notes
|-
| CX2600 six-switch ("Heavy Sixer", "Light Sixer") || Yes, at A203; also buffers the triggers<ref name="fsm" /> || Yes || The 4050 is usually soldered in, but there is room under the shield for a socket on top<ref name="tbaman" />
|-
| CX2600A Rev 1 to 13 || No<ref name="fsm" /> || No, unless added under Tech Tip 4 || Some have a 680 Ω resistor between pins 6 and 9<ref name="c5" />
|-
| CX2600A Rev 14 and 15 || No || No || The blanking resistor is missing<ref name="c5" />
|-
| CX2600A Rev 16 and up || No || Yes || 820 Ω built in<ref name="fsm" />
|}

The saturation-boost column is TBA's table.<ref name="tbaman" />

== Simple buffer mods ==

Tynemouth Software's 2015 mod on a UK PAL 2600 is a typical simple version:<ref name="tyne" />

* The signal fed to the modulator is video and audio combined, so the first step is to stop them being combined. Tynemouth removed Q202, the main part of the mixing on that board, and found the picture improved with L201 removed as well.
* Audio was taken from a spare pad beside C120, with ground from C238, and wired to both audio sockets.
* With the modulator removed, its pins carry 0 V, 5 V and video. A single NPN emitter follower (a 2N3904) on padboard, fitted where the modulator had been, buffers the video. Tynemouth stresses the 75 Ω output resistor that some circuits omit, because it matches the television input.
* The colour timing needed a small adjustment afterwards.

The designators are from that PAL board; NTSC boards number their parts differently. Tynemouth later made a PCB version, sold through The Future Was 8 bit.<ref name="tyne" />

== Resistor-mixer mods ==

R3UK's article compares two home-made mixers, built from the TIA pins listed above. One is a resistor ladder for the luminance bits feeding a transistor amplifier together with sync and colour; the other is Ben Heckendorn's mixer of two potentiometers. Both gave a usable but dark picture until the ladder was changed. The article warns against soldering to the TIA pins directly, and suggests the resistors on each line instead, on the TIA side.<ref name="r3uk" /> The details are on the [[Atari 2600 Jr Composite Video Mod]] page, because R3UK's console was a Jr.

== The UAV board ==

The UAV was designed by Bryan Edewaard; The Brewing Academy (TBA) now makes the revision D board. It has its own regulator, a three-channel video amplifier for 75 Ω loads, re-clocking of the luminance lines and a chroma-shaping circuit with a phase pot. It plugs into the 4050 socket where there is one and can run alongside the original video circuit.<ref name="tba2600" /><ref name="tbaman" /> TBA sells a plug-in version only for the six-switch 2600 and the Jr., and even then only some boards can use it.<ref name="tba2600" />

=== Six-switch CX2600 ===

TBA's steps:<ref name="tbaman" />

# With the cartridge slot towards the back, run a wire from the left side of R231 for the UAV's power. The 4050's own supply pin goes through an inline resistor and will not power the UAV properly.
# Run a wire from the right side of R212 for Color In.
# Cut pin 1 off a 16-pin socket, so that it does not connect the 4050's supply to the UAV, and solder the socket on top of the 4050. Only pins 3, 5, 8, 11 and 14 need soldering.
# Set the UAV jumpers for the 2600 and plug it in, green terminal towards you.
# Solder the power wire to the UAV's 5 V pad and put the Color In wire in the leftmost terminal.
# Pull the TIA (C010444) and bend pin 6 out of its socket, or lift one end of R213 if the TIA is soldered in.
# The remaining terminals are ground, chroma, luma, composite and a second ground.

=== Four-switch CX2600A ===

Four-switch boards have no 4050, so the UAV is wired in. TBA had to lift the chroma mixing capacitor on its four-switch console, because the heavy load on that signal distorted it too much for the UAV, which needs a clean square wave.<ref name="tbaman" /> Rev 16 boards also need the saturation-boost resistor lifted.

=== Audio ===

The UAV has no audio circuit. TBA's Internal Audio Board takes its input from TIA pin 12 on the six-switch console.<ref name="tbaman" />

== Related pages ==

* [[Atari 2600]]
* [[Atari 2600 General Maintenance]]
* [[Atari 2600 Troubleshooting Guide]]
* [[Atari 2600 Jr Composite Video Mod]]
* [[Atari 7800 Composite Mod]]
* [[Atari 5200 AV Output Upgrade]]

== References ==

<references>
<ref name="fsm">Atari, Inc., ''Video Computer System Field Service Manual, Domestic Model 2600/2600A'', FD100133 Rev. 02: RF module frequency (flowcharts, pp. 4-16, 4-18), 4050 buffer at A203 on the CX2600, board revision bands, and Tech Tip 4 (820 Ω blanking resistor). Hosted here as [[Atari 2600 and 2600A Field Service Manual]] ([[:File:Atari 2600 and 2600A Field Service Manual.pdf|PDF]]).</ref>
<ref name="c5">Console5 TechWiki, [https://wiki.console5.com/wiki/Atari_2600 "Atari 2600"], "Blanking resistor (Tech Tip 4, 11/17/82)". Secondary source.</ref>
<ref name="tyne">Tynemouth Software, [http://blog.tynemouthsoftware.co.uk/2015/02/atari-2600-composite-video-modification.html "Atari 2600 Composite Video Modification"], February 2015. Repair blog.</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.</ref>
<ref name="tbaman">The Brewing Academy, ''Bryan's Ultimate Atari Video Rev. D Instruction Manual'', revision 1.94, 23 March 2021: introduction (p. 3), "2600 installation" (pp. 6, 7), "Atari 2600 Six-switch model" (pp. 9, 10) and the Internal Audio Board (pp. 10, 11). [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="tba2600">The Brewing Academy, [https://thebrewingacademy.com/products/tba-ultimate-atari-video-uav-board-for-atari-2600 "TBA's Ultimate Atari Video (UAV) board for Atari 2600"], product page. Manufacturer's description.</ref>
</references>

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