This is a lot easier than you probably thought it was a minute ago. There are already holes on the motherboard to mount a DIP switch! Apple probably had DIP switches installed on the motherboards when they were designing them, and then didn't include them in the production model in order to save money.

Apple put tiny 0 ohm resistors on the under side of the motherboard instead. So the motherboard needs to be removed, and the resistors need to be removed. Removing the resistors is not difficult. I melted and removed some solder, so that the resistor wasn't held on as well. I then used a razor blade to pop the resistor off. You can also do it the standard way and melt the solder, and remove it with a sucker or de-soldering braid. Just make sure you don't lose that resistor. You don't want it to have attached itself to another part of the motherboard. Also, don't splash solder on the motherboard. Its a ridiculously small amount of solder, but could potentially damage or destroy your motherboard.

Installing the DIP switch is straightforward. The board uses a standard sized 4 switch DIP switch, so you simply place the pins through the holes, then fill the holes with solder. Install the DIP switch so that looking at the computer from the front, with the computer open, 1 is on the left and 4 is on the right. Here is what it looks like when done:

Note that 70MHz, 105MHz, and 133MHz are in bold. These settings are apparently not supposed to be used under normal conditions. So that leaves the only overclocking option to be 120MHz. I have tried both 105MHz and 133MHz. 133MHz results in 3 beeps with the power LED blinking 3 times. I believe that indicates a memory error. All of my memory is PC133. So I'm not sure why exactly it doesn't work. 105MHz booted, but seems to have resulted in loss of function of the USB ports, since my mouse and keyboard became unusable. I felt no reason to try 70MHz.

It is also quite important to know that you will most likely need to change the multiplier on your CPU card. Those settings can be found at http://www.Power-Mac-G4.com. Otherwise, a 350MHz card will try to run at 420MHz, a 400MHz card at 480MHz, a 450MHz card at 540MHz, and a 500MHz card at 600MHz. My 533MHz G4 would not run at 600MHz until I boosted the L2 cache voltage. Any Apple OEM processor faster than 533MHz (installed in a Sawtooth) requires the CD ATA connector to be removed, modification to the heatsink or modification to the case to use the newer heatsink, and 12V to be spliced to one of the mount points of the new processor. That is why I chose the 533MHz processor. You will need PC133 memory, as PC100 will most likely not work any more. Also, PCI speed is decreased from 33MHz to 30MHz. This should have no effect unless you are using a RAID setup, Ultra320 SCSI, or some other PCI card that is actually faster than the PCI bus. Even then, it may have little effect.

Now onto what to expect. When I first did this mod, my processor card had a 4x multiplier, and the processor is rated for 533MHz. So at the 100MHz bus, it ran at 400MHz, and at the 120MHz bus, it ran at 480MHz. I am using Mac OS 10.3.5. After I installed the DIP switch and changed the settings, Apple System Profiler reported a 100MHz bus and 400MHz processor. I tried many different settings and Apple System Profiler always reported 100MHz bus and 400MHz processor. I then installed Linux. I tried many different settings and Linux did the same thing, it always reported a 100MHz bus and a 400MHz processor. I was extremely disappointed. However, I benchmarked 4 different settings using XBench. Here are the results:

I was ecstatic. All this time I thought this had failed, when in reality, it was OS X and Linux which had failed to report the actual speeds. Plus, the data sheet where I got the info from had "0" represent a set jumper, and "1" represent no jumper set (beware I think this is common). So at first I was trying all of the settings backwards. It is unfortunate that you can't go into Apple System Profiler and view the 120MHz bus and actual speed of the processor. But the bus overclock has successfully increased memory speed.

It would be nice to get 133MHz working. But I am happy with 120MHz. This modification will allow you to set Apple OEM processor cards (with an 8x multiplier) to run at up to 960MHz. My system is extremely stable. If I ever learn more about the 133MHz setting, or why Linux and Mac OS X always report a 100MHz bus speed, I will update this article.

All questions or comments are welcome. (remove NOSPAM from address when sending)
-Chris

Related Articles:
For other articles on System Mods, Case Conversions, PS swaps, etc. see the Systems page (organized by mac model/series).