Atari 7800 Chroma/Luma/Audio Outputs

Getting chroma/luma/audio outputs from a 7800 is somewhat tricky due to its hybrid nature. It is helpful to think of it not as a single machine that plays both 7800 and 2600 games, but as a distinct 2600 and 7800 machines mounted in a single case. This means there are two of every signal--two sets of luma signals, two chroma signals and two audio signals (even though most 7800 games use the 2600 for their audio, Ballblazer has its own audio chip whose output is fed into an extra pin on the cart slot.)


Difference Between Consoles

There are two basic versions of the 7800 console that I know of. The earlier version is the one designed by Atari Inc. and sold later by Atari Corp. It's the one that will play the notorious cranky Activison carts and the Starpath Supercharger. The later, the more common version is purely Atari Corp. (read "cheap and shoddy design"). The most conspicuous difference between the two is the earlier version has four 14-pin ICs on the right side of the board under the shielding, while the while the earlier version has only three. The extra chip is the 74LS02 second from the bottom

This article details the modification to the later version of the 7800. Chances are the earlier version is practically identical, but I haven't verified that yet.

Note:

Lee Krueger notes that he has done successful A/V modifications on several different 7800s using these same instructions. Some were the earlier version of the 7800 mentioned here.


Where to get Signals

Most of the components do not have visible labels on the board's silk-screen, so giving component numbers is not useful.

On the left side of the board, there's a horizontal row of mostly resistors above two ICs.

 
 o   o   o   o   o   o   o   o   o   o   o   o   o   o   o    o
 |   |   |   |   |   |   |   |   |   |   |   |   |   |   |    |
8k2 39k 20k 10k 7k5 82k 4k3 4k7 3k3 3k3 3k3 3k3 1k0 1k0 10uf 1k0
 |   |   |   |   |   |   |   |   |   |   |   |   |   |   |    |
 o   o   o   o   o   o   o   o   o   o   o   o   o   o   o    o
 |   |   |   |       |   |   |
 |   |   |   |       |   |   +--2600 Chroma
 |   |   |   |       |   +------7800 Chroma
 |   |   |   |       +----------Luma 0
 |   |   |   +------------------Luma 3 (2600 Luma 2)
 |   |   +----------------------Luma 2 (2600 Luma 1)
 |   +--------------------------Luma 1 (2600 Luma 0)
 +------------------------------Sync  

There is another, shorter row of components above that, just to the right of the RF modulator.

 o   o    o    o   o    o   o   o
 |   |    |    |   |    |   |   |
400 47pF 47pF 1k0 47pF 9k1 6k8 18k
 |   |    |    |   |    |   |   |
 o   o    o    o   o    o   o   o
                            |   |
                            |   +--Audio
                            +------External Audio (from cart slot)

 


Where to get Power

The circuit needs +5VDC source and a ground. IMHO, since they both snake all over the board and can be tapped into at any number of points, it's easier to tell the user how to find the appropriate places to get them than tell the user exactly where. Keeps things flexible, ya know.

Ground can found at any point continuous with the following:

  • The metal shielding
  • The center pin of the voltage regulator (7805, located at the top of the unit, above the cart slot attached to a bigass heat sink).
  • Pin 8 of either controller port.
  • Pin 7 of any of the 14-pin TTL chip (i.e., 74SLxxx).

+5V can be found at any point continuous with the following:

  • The right pin of the voltage regulator, as orientated with the pins pointing downward and the side with the printing facing you.
  • Pin 7 of either controller port.
  • Pin 14 of any 14-pin TTL chip (i.e., 74LSxxx).


Luma Output

This is the same circuit you have seen before.

Materials Needed

  • 100uF capacitor
  • 10uF capacitor
  • 10 ohm
  • (2) 75ohm - an 82 ohm works well (RS 271-1107)
  • 750 ohm
  • 1.6 kohm
  • 2 kohm
  • 4.7 kohm
  • 9.1 kohm
  • 18 kohm
  • 36 kohm
  • CR - low power silicon diode (RS 276-1122)
  • Q - 3904 or equivalent (RS 276-2016)
  • RCA jack
                                 o+5V
                                 |
                                 |  +
                                 |---|(----,
                                 |  100uF  |
                                 |         \/ GND
                                 |
                                 |             10uf
                                 |---/\/\/---,--|(--,
            CR1  750       1.6K  |    10     | +    |
    Sync --|<]--/\/\/-,  ,-/\/\/-`           |      \/ GND
            4.7K      |  |        ___/-------`
  Luma 3 ---/\/\/-----|  |     Q /|/c\
            9.1K      |--|------(b|   )                   RCA jack
  Luma 2 ---/\/\/-----|  |       \|\e/          75        __
            18K       |  |           \-----,---/\/\/-----O__ LUMA
  Luma 1 ---/\/\/-----|  |  2K             |             |   OUTPUT
            36K       |  `-/\/\/----/\/\/--`         GND \/
  Luma 0 ---/\/\/-----`          |    75
                             GND \/


Chroma Output

Chroma output is a little tricky. Tying two chroma signals together into one doesn't work so well. The y need a bit of attenuation first. Rather than try to find specific resistance values, I just used a couple of 10K potentiometers and monkeyed them around a bit until I achieve a mix that gave good results. There are probably correct values to use here, but I haven't bothered finding them yet.

Materials Needed

  • (2) 10k Trimmer potentiometer
  • 1uF capacitor
  • RCA Jack
		10K Pot
2600 Chroma -----/\/\/-----+
		           |
		10K Pot    |	1uf	__ RCA Jack
7800 Chroma -----/\/\/-----+----|(-----o__ Chroma Output
					|
					GND

Tuning the chroma output

  • Turn the 2600 chroma trimmer down to its minimum value and the 7800 chroma trimmer up to its maximum.
  • Put a 7800 cart in the slot and turn the console on.
  • Turn the 7800 chroma trimmer down gradually until you get the best picture, i.e., until the image has lost its grainy appearance.
  • Turn the 2600 chroma trimmer up to its maximum. You'll probably see annoying wavy diagonal bands. Turn the 2600 chroma trimmer down until they go away.
  • Remove the 7800 cart and put in a 2600 cart. Check that the colors are acceptably bright.

Note:

Lee Krueger recommends using Pitfall II for the 2600 cart. It has several gradations of brown and green. The subtle differences can be seen if the adjustment is made correctly.


Audio Output

Like the chroma output, the two separate signals need proper attenuation before mixing. Doing otherwise will give very load normal audio, and very quiet external audio.

There are two approaches here, either one of which produces the same results.

Audio--Method 1

In this method, you use the resistors that already exist on the board, and disable the mixing oscillator (Q1) by pulling the base lead. With this approach, the unit's RF output will have no audio.

Materials Needed

  • 1uF capacitor
  • RCA Jack

Schematic

                  1uF             RCA Jack   
Audio -------------|(-------------0   Audio Output
                                   |
                                  GND

Audio--Method 2

All you are doing here is building a circuit identical to the one that exists on the board already, except it's not feeding the RF modulator.

Materials Needed

  • 6.8K
  • 18K
  • 1uF capacitor
  • RCA Jack

Schematic

            18K
Audio -----/\/\---+
                  |
                  |
external   6.8K   |           1uF            RCA Jack
audio------/\/\---+-----------|(-------------0   Audio Output
                                              |                           
                                              GND


Composite Video Output

To gain composite output, just tie the Luma and Chroma outputs together into a single output.


Where to Put it

I mounted the circuits on a small project board from Radio Shack, tapping into the appropriate points on the 7800 board with a length of ribbon cable. I then ran the ribbon cable out of the shielding via the hole for the expansion port, and tucked the board into the empty space in the "wing" there. I drilled three holes underside of the wing as far back as I could to mount the RCA jacks.

WARNING: Take care drilling the holes in the 7800 shell. It's mad of very brittle plastic. Start with a small hole and gradually enlarge it. Drilling a big hole first or trying to enlarge a smaller hole too much will put a bigass crack in the shell.


Jay Tilton
Last modified: 06/08/97


Disclaimer

This modification is the work of Jay Tilton. ResQsoft takes no responsibility for any damage done to your hardware. If you do this modification. You do so at your on risk. This article was transcribed from text that used to exist on Jay Tilton's webpage. All pre cautions were taken to ensure that nothing was lost in the transcription, but I make no guarantees that it is accurate. If you find typos or discrepancies, please let me know. I will check it against the hardcopy I have.

Good luck....

Check out Saundby'sAtari 7800 Mods for a similar modification


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