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  1. #1


    Member Since
    Feb 19, 2014
    Location
    France
    Posts
    3
    Unhappy iMac G3 (slot) mod - Video Connector
    Hi everyone,

    I have been trying for the last few days to include a mini ITX-PC board in an iMac G3 slot-loading while keeping its PSU and CRT (sort of nostalgic 'vintage' thing , I guess ....).

    I have made a short custom cable to internally connect the VGA output of the PC board to the HE20-pin video connector but it looks like I am still missing some pins.



    Even when the +5V-trickle (T5V) and PFW signals are shorted, I cannot hear the "PONK" sound of the CRT charging up ... (needless to say that the CRT works quite right with the original iMac logic board).

    Has anyone made such a mod and did you find out if pins 4, 5, 7, 8, 12, 19 and 20 have to be connected and where to ?

    Thanks for any help you can share here or by PM.

  2. #2

    cradom's Avatar
    Member Since
    Feb 14, 2004
    Location
    Groves, Texas
    Posts
    4,551
    Specs:
    21in. iMac 10.11 --- HP win 10
    I used to have all kinds of links about putting PC motherboards in older iMacs but they seem to have vanished.
    You might have some luck with info here: Low End Mac: Make the most of your Apple gear
    I don't know about the slot load but my tray load had a regular vga connector going to the crt. Were they changed?

    Edit: might take a look here https://www.google.com/#q=pc+motherboard+in+imac+g3
    Delusions are often functional. A motherís opinions about her childrenís beauty, intelligence, goodness, et cetera ad nauseam, keep her from drowning them at birth.
    -Lazarus Long

  3. #3


    Member Since
    Feb 19, 2014
    Location
    France
    Posts
    3
    Hi Cradom,
    Thanks for your answer.
    I have spent the last hour to go through the "abundant" iMac literature at the lowendmac.com site.
    So far to no avail, maybe I should go more deeply inside ...
    Should you find again those links you used to have, please don't hesitate to share coz I'm stuck here !
    Cheerio

  4. #4

    cradom's Avatar
    Member Since
    Feb 14, 2004
    Location
    Groves, Texas
    Posts
    4,551
    Specs:
    21in. iMac 10.11 --- HP win 10
    See my second link...
    Delusions are often functional. A motherís opinions about her childrenís beauty, intelligence, goodness, et cetera ad nauseam, keep her from drowning them at birth.
    -Lazarus Long

  5. #5


    Member Since
    Feb 19, 2014
    Location
    France
    Posts
    3
    Hi Craig

    Unfortunately, I could not find any relevant info on the internet despite the numerous hours of google search that I made ... :-(

    So I have decided to make a test board to parse every single signal that comes out of the mother board and connects to the video card :



    What I found is that, besides the "classical" red/blue/green, hsync/vsync and ground signals, most of the other signals are NOT used BUT the two SDA and SDC signals (respectively pins 1 and 2).

    When connected to an oscilloscope DURING THE BOOT, these two signals show that the motherboard raises them to +5V then real quick ( after a few 100th of msec ) applies a particular pattern sequence (logical states +5V and 0V) then finally leaves them to +5V all through out.
    From this point on, the "PONK" of the CRT is heard, and everything works out right !

    Then, you can even drop the two signals (SDA and SDC) to 0V, no matter what : the iMac is definitely ON and so is the CRT.

    My deal now is to find how to replicate those TWO short sequences (they are both different, I mean SDA seq is not the same as SDC seq !) WITHOUT the main board.
    And then, I should be ready to get rid of the iMac motherboard and install a PC board in the iMac G3 !

    Any hint, idea, suggestion, reflection whatsover is VERY welcome ...
    ym58

  6. #6

    cradom's Avatar
    Member Since
    Feb 14, 2004
    Location
    Groves, Texas
    Posts
    4,551
    Specs:
    21in. iMac 10.11 --- HP win 10
    You may be chasing a lost cause. As old as those imacs are you might be better off scrapping the crt and psu and replacing it with a flat panel and atx power supply.
    It can be done. Those old crts wont last long and are power hungry compared to today's flat screens.
    However, it's your time and money.
    Delusions are often functional. A motherís opinions about her childrenís beauty, intelligence, goodness, et cetera ad nauseam, keep her from drowning them at birth.
    -Lazarus Long

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