How to get rid of the rainbow effect on composite modded ColecoVision?

Discussion in 'Modding and Hacking - Consoles and Electronics' started by bacteria, Jan 13, 2010.

  1. bacteria

    bacteria I am the Bacman

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    Took video off pin 36 of the ColecoVision graphics chip; got composite, but there is a rainbow effect on the image - I believe this is usual from this old chip.

    What, if anything can I do to get the image normal and stable, no rainbow effect?

    This is what I mean:

    [​IMG]

    Example from someone else's post:

    ColecoVision composite:

    [​IMG]

    What it should look like:

    [​IMG]
     
  2. Dr.Wily

    Dr.Wily Peppy Member

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    Don't know, but I have the same issue in RGB.
     
  3. bacteria

    bacteria I am the Bacman

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    You have the French ColecoVision then? (the French version can RGB output via a plug) as detailed here or did you manage to take the R-Y, B-Y, Y channels and make a circuit to make that work?
     
  4. Jamtex

    Jamtex Adult Orientated Mahjong Connoisseur

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    Why are you running it via a cheap LCD screen which is likely to make the output 10 times worse?
     
  5. Dr.Wily

    Dr.Wily Peppy Member

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    Yes I have the french with RGB. I have some colors issue like rainbow effect but also missing colors (no bad contact on cable or video connector) or dark image. Maybe defect caps ?

    +1 best image on CRT only.
     
    Last edited: Jan 13, 2010
  6. Dr.Wily

    Dr.Wily Peppy Member

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    sorry double post...
     
    Last edited: Jan 13, 2010
  7. mooseblaster

    mooseblaster Bleep. Site Supporter 2012, 2014

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    Probably because, being bacteria and all, he's probably modding the Coleco into some sort of handheld gaming device. :p
     
  8. bacteria

    bacteria I am the Bacman

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    Indeed, it is indeed going to be a modding project, part of a proper Multi-in-one system I am going to make.

    Tried the composite out on my large television, same result with the rainbow effect.

    Hoping there was a way to get image without the rainbow effect??
     
  9. Jamtex

    Jamtex Adult Orientated Mahjong Connoisseur

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    As it seems to be the American Colecovision using the TMS9918A chip have you pulled down the signal from Pin 36 to ground using a 330 (you could also try 470 and 680 ones too) ohm resistor?
     
    Last edited: Jan 13, 2010
  10. bacteria

    bacteria I am the Bacman

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    It's a PAL ColecoVision which is why I couldn't do what BenHeck did as the RF circuit is entirely different (his solution to composite was easier than others I found).

    I tried a few resistors, 470 Ohm, 560 Ohm, 75 Ohm; also tried a capacitor there too to see if that smoothed things a bit, lost picture so didn't work.

    From what I read, appears the resistor works on American units not PAL. In saying that though, I have seen a lot of conflicting information on different aspects from research.
     
  11. Jamtex

    Jamtex Adult Orientated Mahjong Connoisseur

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    Solution 1 is to buy an NTSC colecovision mainly as the PAL one runs at 50Hz and is 17.5% slower then what the games were written for... it's easier to mod for composite video.

    The 9929A outputs colour difference signals, however it outputs a near PAL composite signal from the RF modulator.. all you need is to amplify it.

    There will be a board with the RF modulator (an silver box marked ASTEC) on the underside it will tell you what the pins are, but there should be three marked 5V, AUD and VID, two of these go into the RF modulator...

    Cut the two wires going into the RF modulator and take the wire that did go into the RF modulator from the VID part of the board and feed it to the base a BC547 or BC548 transistor, pull this up using a 4k7 resistor. Feed +5V to the collector and connect a 1K resistor from the emitter to ground and a 75ohm resistor from the emitter to a cable for video.

    The audio pull down with a 1K resistor.
     
  12. bacteria

    bacteria I am the Bacman

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    Thanks for the information, however I replaced the graphics chip so too late to try this - old chip is destroyed.

    The rainbow effect is a bit annoying in some regards, in others it adds a bit of charm to the old games. Just spent an hour and a half or so playing Gorf, Space Panic, Pepper 2 and Donkey Kong.

    Interesting thing though - I have 4 screen types and got the following results:

    PSone NTSC screen - get scrolling monochrome image, as if signal is PAL
    PS2 screen (PAL) - get scrolling monochrome image
    8" TFT screen - get proper image, all be it with the famous rainbow effect
    PSone PAL screen - not tested yet
     
    Last edited: Jan 14, 2010
  13. Jamtex

    Jamtex Adult Orientated Mahjong Connoisseur

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    What the hell have you done then? It's like you're giving us small bits of important information on every post....

    Are you saying you've ripped out the 9929 and replaced it with a 9918?
     
    Last edited: Jan 17, 2010
  14. madhatter256

    madhatter256 Illustrious Member

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    Looks like you need to amplify it...
     
  15. bacteria

    bacteria I am the Bacman

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    Jamtex - sorry for leaving reply so long, forgot:

    Was really fed up with the Dina system. Audio sucked, couldn't improve the composite, and to top it all, it was working ok and then stopped. All I did was try to run the RF plug into an old television, like it was meant to be. Even the cart plug is bad, grips onto the game cart like it's life depended on it!

    As the Dina fried (no idea how), used dremel to cut off the pins on the TMS9919A chip at the base to keep all the pin I could easily; and on the ColecoVision, used strong pliers to lever off the heatsink and then cut through the top of the chip pins on the TMS9929A chip.

    [​IMG]

    [​IMG]

    Placed the new chip over the old pins, pressed down to make them touch as best as could. Applied some flux, and tapped each pin with solder so the pins were tinned and at the same time filled with solder bridging the pins together. One pin didn't connect, so the graphics were jagged badly, found culprit and re-applied.

    Pic shows half-done chip.

    [​IMG]

    Sound is tapped off here:

    [​IMG]

    And video (composite) off pin 36 - lift off pin 36, wire a 460 Ohm resistor to it and connect to ground. At pin 36 wire to the composite connection. (result in above screenshot).

    UPDATE - actually, made no difference using a resistor like that as opposed to not using one at all and filling in the pin again and wiring straight to it. Comments?

    [​IMG]


    madhatter256 - from all the research I have done, looks rather like the ColecoVision can't output better composite than it does, someone with RGB (a French ColecoVision) gets the same issue, so RGB on the Coleco is no better.

    Solution might be to use a more up to date graphics chip and piggy-back it onto the Coleco one maybe; however I don't have the expertise to know which chip, if any, would work better or what to wire to what (and resistors to use, etc) to achieve that, to tap off better video. If someone does, I would be grateful for the information, and might make it worth the while to whomever could give me a working final solution..... :nod:
     
  16. Jamtex

    Jamtex Adult Orientated Mahjong Connoisseur

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    Pin 36 should be not be connected to the motherboard, just bent up and connected via a 470ohm resistor to ground and the to video, you could also try a 15pf cap to ground too.

    Pins 35 and 38, should also not be connected to the motherboard, bend these up and connect to ground.
     
  17. bacteria

    bacteria I am the Bacman

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    Thanks, did that, removed the connection to pins 35 and 38 and wired them to ground.

    Removed the connection for pin 36, any attempts to connect the video line to a capacitor; or a capacitor to ground in any combination gives no image. Wiring one end of the resistor to ground and the other ends together makes no difference, to one end of the resistor to the video wire and the other end of the resistor to the other end of the video wire and ground gives no picture. Basically, if the video line is connected to ground at all the picture goes.

    Best result I could get was using about 400ohm resistor to connect between the video line as 470ohm was a little too dark; so the colours on screen are about right, however there is still the rainbow effect. There is also a little background hiss on the audio line too, minimal, not noticable during a game only on the title screen and game select screen.

    Is pin 34 relevant - ideas?

    [​IMG]
     
    Last edited: Jan 24, 2010
  18. l_oliveira

    l_oliveira Officer at Arms

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    The MSX2 (V9938) video chip is fully compatible with the TMS9918 and outputs standard RGB along with NTSC CVBS video.
    Three problems with it are:

    -It's a 64PIN SDIP package so some sort of adapter board would need to be built.
    -It uses a 21mhz crystal (same as the FAMICOM/Super Famicom)
    -It's a "overkill" like killing a fly with a a bazooka.

    Also it might be somewhat difficult to obtain outside of Japan.
     
  19. Jamtex

    Jamtex Adult Orientated Mahjong Connoisseur

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    Okay try amplify the signal. Use the transistor configuration i suggested above.
     
  20. bacteria

    bacteria I am the Bacman

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    Just tried it, had to get the parts off ebay.

    Anything connected to grounding stops the video working.

    Tried using different resistors, no improvement over just using a resistor from pin 36 to output video.

    Tried cutting the line from the chip to the board, made no difference against being connected.

    In short, nothing seems to make any difference at all. Image is stable, which is important, however there is the rainbow effect as detailed before.

    Best I can get is using a 75 ohm resistor from pin 36 to video.

    This is a pain, spent a bit trying various people's suggestions, with no difference to image quality.
     
    Last edited: Feb 12, 2010

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