VPGL 15 - Post Pass - Rules

Hi Pinballers,

Welcome to arcade gaming competition with VPGL 15. This tournament is the 3rd of the new decade and part of the overall VPGL 2020 and all gamers worldwide are welcome to play. You can join in at any time and play.

Competition:
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How game board components work

This will be a series of posts on how specific components work on a game board. Its purpose is to help you understand how the circuitry works. Feel free to add to the series if there's a topic you want to cover.

First up: Reset!

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Atari 7800 S-Video Mod

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  • Atari 7800 S-Video Mod

    Atari 7800 S-Video Mod

    This mod was performed on a PAL B Atari 7800.

    Building the Circuit

    Parts List
    • 18 column x 13 row Vero board
    • 1 x 4.7k resistor
    • 1 x 9.1k resistor
    • 1 x 18k resistor
    • 1 x 36k resistor
    • 1 x 750 ohm resistor
    • 1 x 2k resistor
    • 2 x 75 ohm resistor
    • 1 x 1.6k resistor
    • 1 x 10 ohm resistor
    • 2 x 10k trimpots
    • 1 x 1uF capacitor
    • 1 x 10uF capacitor
    • 1 x 2n3904 transistor
    • 1 x diode (any value)
    • 1 x S-Video socket
    • 1 x RCA socket (for audio)
    • Variety of hook up wire


    Creation of the Circuit

    The following diagram illustrates how to create the circuit on a piece of 18 column by 11 row Vero board.


    To cut the Vero board to size, use a metal ruler and a sharp stanley knife to score a deep groove along the board, allowing you to snap the excess off.



    Methodically solder the components onto the board as illustrated. Pay attention to the polarity of capacitors, and the transistors. For the components which have both legs in adjacent holes, orient the component vertically rather than horizontally across the board.
    Depending on the trimpots you are using, you may need to perform some modification to get them to fit on the Vero board as pictured in the diagram. See the picture below, a cut off resistor leg has been used to reposition the middle leg of the trimpot.


    Also ensure when you mount them that you are able to subsequently adjust them. For horizontally mounted trimpots as picture in the completed circuit, they are facing away from each other.
    See the picture of the completed circuit below:





    Console Modification

    !Before doing any work on the console, press the power button while it is not plugged into power to discharge it. Failure to do so is likely to damage components when you start to work on it!



    Flip the console upside down, and remove the 5 screws.



    Remove the motherboard by simply lifting it out.



    Using a set of needle nose pliers, carefully straighten the tabs in order to remove the upper and lower shielding.



    Now the shielding can be fully removed. For reference, in this mod the circuit will be placed as indicated by the red square. You can mount it in other locations, just make sure there is enough room and no shorts are caused.



    Now for attaching the S-Video circuit to the console. Refer to the picture below:



    Now attach all of the wires to the S-Video circuit. Use a different colour for each, and remember to note which colour you are using for each signal. The length to use for each wire will depend on how you route the wires.



    There are several potential ground points. Use a multimeter to identify alternative ground points if needed. Now carefully attach all of the wires to the console as indicated in the diagram. For the Audio, connect the AUDIO & EXT AUDIO points together and attach to the RCA socket. Connect the LUMA OUT & CHROMA OUT points to the S-Video socket (pictured below). Don?t forget to attach the ground wires to the S-Video & audio sockets.



    Double sided tape was used to insulate the bottom of the S-Video circuit (and later to attach in place).



    Now with the circuit all set up, some adjustment needs to be made to the trimpots. Without re-assembling the console, connect the power adapter, S-Video & Audio cables. Switch it on, and hopefully it works!
    • Turn the 2600 chroma trimpot (R11) down to its minimum value and the 7800 trimpot (R12) up to its maximum.
    • Put a 7800 cart in the slot and turn the console on.
    • Turn the 7800 chroma trimpot down gradually until you get the best picture, i.e., until the image has lost its grainy appearance.
    • Turn the 2600 chroma trimpot up to its maximum. You'll probably see annoying wavy diagonal bands. Turn the 2600 chroma trimpot down until they go away.
    • Remove the 7800 cart and put in a 2600 cart. Check that the colours are acceptably bright.


    Once the trimpot adjustment has been done, it is time to tidy the mod up.


    Using cable ties really helps neaten everything up. Also the S-Video circuit has been positioned on top of a couple of chips, using the double sided tape.

    With the motherboard positioned in the case, the appropriate spots to rote the wire and mount the sockets can be determined.

    A hole is cut in the side of the metal shielding using a hand nibbler for the wires.



    Now the metal shielding can be re-attached.




    Now the holes are drilled into the side of the case for the sockets.



    The wires for the S-video & RCA socket wire re-cut down to size, and mounted.



    Now you can re-assemble the console, congratulations you have completed your Atari 7800 S-Video mod!


  • #2
    Good write up once again. Seems you found the issue with the dark picture. My pcb picture is a little hard to see the red dot for where you need to cut the trace. Good work. I will uPdate the main thread with a link to this

    Comment


    • #3
      Just remembered this guide today and thought it would be worth mentioning how to adjust the brightness of the output for this mod.

      Replace R5 (750 ohm resistor) with a 10K Trimpot. Adjusting this trimpot will alter the brightness of the output. On my console I found the output to be too dull with the 750 ohm resistor, but other people who have performed this mod have had a good picture with the 750 ohm resistor at R5.

      Comment


      • #4
        I am working on a new version of this mod using a video encoder chip that provides true s-video and composite out so should give a better picture. If only I had 40 hours in the day I could get all my projects finished.

        Comment


        • #5
          Just a big thanks to posting this guide. So easy that a dumbo could do it (and I should know). I happened by a beat up Atari 7800 a few months back, and I have to say, it is a bit of a hidden gem in terms of games, plus all those 2600 games to play. With S-video out, it's even better.

          Comment


          • #6
            Sorry to wake the dead, but I tried this mod with mixed results. The luma signal is perfect, but I couldn't get good colour no matter what I set the pot to. It was there, but quite grainy and wavy.

            Comment


            • #7
              For what it's worth I handed this off to another modder with more experience than I, and he couldn't get it working either.

              Comment

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