iMac G3 Slot-Load FSB Mod

Discussion in 'PowerPC Macs' started by Arkious, Jun 29, 2014.

  1. Arkious macrumors 6502a

    Arkious

    Joined:
    Mar 14, 2011
    Location:
    Newcastle, UK
    #1
    All,

    I am attempting an FSB mod on the iMac G3 600-700MHz (750CXe) board. My aim is to get above 100MHz, maybe 133 that the processor is rated to. Obviously i may still not get above 700MHz but i may still get some improvement.

    Please feel free to offer any information or technical knowledge you may have on the matter. I will post information up as i find it

    So far i have found that the SYSCLK is set on pin H9 on the CPU. I will attempt to trace this from the CPU to the area of interest relating to the bus speed.

    Thanks for looking
     
  2. Arkious thread starter macrumors 6502a

    Arkious

    Joined:
    Mar 14, 2011
    Location:
    Newcastle, UK
    #2
    OK, i have my machine running (but not very stable) at 750 (from my old 600 board), if anyone knows how to give it more volts id appreciate it.

    tracking the FSB from the chip isn't so easy, but I'm keeping at it
     
  3. Arkious, Jun 30, 2014
    Last edited: Jun 30, 2014

    Arkious thread starter macrumors 6502a

    Arkious

    Joined:
    Mar 14, 2011
    Location:
    Newcastle, UK
    #3
    Voltage Controller

    Here is the voltage controller chip, and its location on the board;

    Here is the Datasheet for the chip, which if you scroll down has resistance pin outs to select voltage. Reading the sheet states when the Pin 6 is high it is 2.050V+ and when it is low it is 2.050V- and I'm not quite sure how to check it. anyone got any electrical experience?

    The 750CXe cpu requires 2V+

    http://www.onsemi.com/pub_link/Collateral/CS5160-D.PDF

    [​IMG]
     
  4. Arkious thread starter macrumors 6502a

    Arkious

    Joined:
    Mar 14, 2011
    Location:
    Newcastle, UK
    #4
    OK! some progress today. First of all... I have read in many places that IT IS IMPOSSIBLE TO GO PAST 700MHz! when actually as you will see below, no, no its not impossible at all.

    I now have my iMac running at 800MHz, i haven't tried anything further yet but will be fiddling with the cooling when i go higher than this. I have also read that people were under the impression apple simply over clocked the 600mhz processor on the 700mhz models, again this is incorrect as both have different part numbers as again you will see below. Finally, with regard to the CPU voltages, i can only find 2 resistors on the board and one is missing on the 700 compared with the 600 which i have took a picture of.

    Here is a screenshot of my 700MHz G3 over clocked to 800MHz running 10.4

    [​IMG]

    Here is the two different CPUs, take note of the different part numbers (700 first, 600 second)

    [​IMG]
    [​IMG]

    Finally, here is the resistors that are set out different, i still need to prove that these actually adjust the voltage but its the only difference i can see. Im going to remove the resistor from the 600 board to see if it allows boot clocked at 750.

    [​IMG]
    [​IMG]
     
  5. Hughmac macrumors demi-god

    Hughmac

    Joined:
    Feb 4, 2012
    Location:
    Kent, UK
    #5
    Interesting, let us know how you get on

    Cheers :)

    Hugh
     
  6. Zotaccian macrumors 6502a

    Zotaccian

    Joined:
    Apr 25, 2012
    #6
    It's great when modders take the time to report and describe methods, hopefully these pictures / messages will also stay in these pages. Very often when you click on links where mod instructions are supposed to be you get dead link, 404.
     
  7. Arkious thread starter macrumors 6502a

    Arkious

    Joined:
    Mar 14, 2011
    Location:
    Newcastle, UK
    #7
    I accidentally killed my photo bucket account and lost all my previous thread photos. But I'm using Imgur this time with separate albums so hopefully it keeps everything!

    On a little update, it's stable at 800mhz but any higher it's unstable. I'm going to change the thermal pads on the CPU to ensure it's being cooled properly.

    I will still be investigating the FSB
     

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