Atari 7800 Composite Mod
Every NTSC and standard PAL Atari 7800 has RF output only.[1] A composite mod takes the video and sound from inside the console instead. ConsoleMods describes the range: simple composite mods that tap the video going to the RF modulator, and S-video mods that use the separate chroma and luma signals from MARIA and the TIA.[1] The widely documented modern route is the Ultimate Atari Video (UAV) board.
French 7800s are an exception. They have a 13-pin DIN connector with RGB in place of the RF output, made by a Sony V7021 decoder board that converts the console's composite video. ConsoleMods says this is not true RGB and compares the picture with composite from one of the better video mods.[1]
The procedures below summarise the cited guides. Read the full guide before cutting anything.
Where the video comes from
[edit | edit source]On Atari's schematic the video is built from two sets of outputs, MARIA's for 7800 games and the TIA's for 2600 games, and only one set is active at a time.[2][3]
| Signal | MARIA / TIA source | Path |
|---|---|---|
| Sync | U1 pin 48 / U8 pin 2 | Gate U3, then R10 (8.2 kΩ) |
| Luminance (most significant) | MLUM3 U1 pin 44 / TLUM2 U8 pin 7 | U3, then R13 (10 kΩ) |
| Luminance | MLUM2 U1 pin 45 / TLUM1 U8 pin 5 | U3, then R12 (20 kΩ) |
| Luminance | MLUM1 U1 pin 47 / TLUM0 U8 pin 8 | U3, then R11 (39 kΩ) |
| Luminance (least significant, MARIA only) | MLUM0 U1 pin 42 | R15 (82 kΩ) |
| Colour | MCOL U1 pin 43 / TCOL U8 pin 9 | R16 (4.3 kΩ) / R17 (4.7 kΩ), R2 (470 Ω) to ground, L1 (47 µH), C8 (47 pF) |
The summed signal, CVIDEO, goes to pin 3 of the RF modulator RF1. C7 (47 pF) runs from that node to ground.[2] On consoles with the expansion port, the same CVIDEO node is brought out on pin 3 of J2, with +5 V on pin 2 and ground on pins 1, 9 and 18.[2] The repair flowchart expects "triangled pulses" on CVIDEO with a colour-bar cartridge running.[3]
TBA's UAV manual says that many earlier composite buffers deviated from the 75 Ω standard, which made the picture unreliable, with shadows, smearing and ghosting, especially on long cables.[4]
The UAV board
[edit | edit source]The UAV was designed by Bryan Edewaard and is now made in its revision D form by The Brewing Academy (TBA). TBA lists an on-board regulator, a three-channel video amplifier for 75 Ω loads, re-clocking of the luminance lines and a chroma-shaping circuit with a phase adjustment pot.[5] The revision D board has a spare pad for the TIA's colour signal (Tcol), which the 7800 needs because it has two colour sources.[4]
Installing it
[edit | edit source]Ivory Tower Collections' 2017 guide for the 7800:[6]
- Remove the five screws in the base and lift off the top case.
- Straighten the RF shield tabs with needle-nose pliers and lift off the upper shield.
- Find the video resistors to the right of and below the RF modulator.
- Choose a place for the UAV. It fits on top of any chip, but the guide advises against the 6502 and MARIA, which run hot.
- Solder the wires to the UAV first: the resistor connections, +5 V and ground, then the composite, chroma and luma outputs. Using composite and S-video together takes 15 connections.
- Connect +5 V and ground to the rails on the board, and each numbered UAV input to the south leg of the matching resistor.
- Drill the case for the jacks. The guide suggests pilot holes and Forstner bits, because the old plastic cracks easily; most installers put the jacks along the lower sides, where there is most room.
- Wire the outputs to the jacks, check for shorts and solder bridges, and test before closing the case.
TBA's manual adds that on an NTSC "Rev A" board the UAV can be fitted as shown in its photograph without cutting or removing anything, working in both 2600 and 7800 modes and leaving the RF output working.[4]
TBA also sells a mounting board designed by Ivory Tower Collections. It carries a CD4050 "to correct for color errors on some 7800 Maria chips" and a pad for taking audio out.[5]
Audio
[edit | edit source]The UAV has no audio circuit. The Ivory Tower guide takes audio from R5 and R6: cut their north legs, join the two cut ends, and connect the positive leg of a 10 µF 16 V capacitor there, with its negative leg to the centre pins of the audio jacks.[6] On the schematic, R5 (6.8 kΩ) takes the cartridge audio line (J1 pin 18) through C10, and R6 (18 kΩ) takes the TIA's audio through C14. Their far ends join at the audio line, which is also brought out on J2 pin 17.[2]
Board revisions
[edit | edit source]Atari's schematic is the only drawing of the NTSC console. ConsoleMods describes the change from C025233-001 to C025233-002 as an added timing circuit around U13 for 2600 compatibility.[2][7] ConsoleMods says standard PAL 7800s (C300633-001) can be modified in the same way as NTSC ones.[1] Atari's schematic covers only the NTSC console, so check the resistor positions on a PAL board against the board itself.
Related pages
[edit | edit source]- Atari 7800
- Atari 7800 General Maintenance
- Atari 7800 Troubleshooting Guide
- Atari 2600 Composite Video Mod
- Atari 5200 AV Output Upgrade
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 ConsoleMods Wiki, "Atari 7800: Video Output Notes" (revision of 26 February 2024). Community reference, CC BY 4.0.
- ↑ 2.0 2.1 2.2 2.3 2.4 Atari, Inc., Schematic Diagram 7800, drawing C025231-001 Rev. A, 1984: video summing network, RF1, J2 peripheral port and audio. Hosted here as File:Atari 7800 Schematic Diagram C025231-001.pdf.
- ↑ 3.0 3.1 7800 repair flowchart, sheet E (video). Hosted here as File:Atari 7800 Repair Flowchart.pdf.
- ↑ 4.0 4.1 4.2 The Brewing Academy, Bryan's Ultimate Atari Video Rev. D Instruction Manual, revision 1.94, 23 March 2021: introduction (p. 3) and "Atari 7800 Installation" (p. 25). PDF from thebrewingacademy.com.
- ↑ 5.0 5.1 The Brewing Academy, "TBA's Ultimate Atari Video (UAV) board for Atari 7800", product page. Manufacturer's description.
- ↑ 6.0 6.1 Cro§Bow, Ivory Tower Collections, UAV – Ultimate Atari Video – A7800, installation guide, 2017. thebrewingacademy.net. Community guide.
- ↑ ConsoleMods Wiki, "Atari 7800: Atari 7800 Model Differences" (revision of 5 October 2025). Community reference, CC BY 4.0.