iMac PLL configurations

What the heck is a PLL? I don't remember exactly that the TLA stands for (that's Three-Letter-Acronym, something we all see too much of), but what it boils down to is this: the iMac system bus is clocked at 66.7 MHz; the processor is clocked at 233 MHz. The processor-to-bus ratio, or PLL, is 3.5:1 (233 / 66.7). Using the little resistor positions, numbered R117 through R124, it is possible to set the iMac processor speed all the way up to 533 MHz, or a processor-to-bus ratio of 8:1. It is seriously doubtful that the standard iMac processor could ever run that fast for more than a couple of minutes without a liquefied gas cooling system, but Apple was probably looking at the CPU board as a long-term product; G3 / PPC750 chips which can run at speeds close to that may soon be available (there are already 466 MHz chips out there). All they would have to do is have the robots solder in a different chip at the factory.

I am not certain what the G3 chip in the Rev. A iMac is rated (it appears to be 250 MHz, though I could be mistaken) but overclocking it is possible. I have heard from many people who have achieved 266 Mhz and 300 MHz with their Rev. A & B machines. A couple of people noted that their machines were not stable at 300 MHz, and they consequently lowered them to 266 MHz. Mine is stable at 300 MHz after installing a large heatsink & fan. Lars Hansen in Norway has a Rev. A running at a stable 400 MHz after installing an overclocked processor card out of a Rev. D iMac!

Enough about that, though! Here is a photo of the CPU daughtercard; the resistors you're going to need to move are in the lower left corner of the board. This shot is of the "bottom" of the board, the side you won't see until you remove it from the processor & drive chassis.

Note that the type above is standard newspaper type, i.e. small! These resistors are tiny. Below is a close-up (sorry it's a little blurry). This was taken prior to modification, so the configuration below is what you should see on yours. The red bracket indicates the four resistors you need to work with.

And below is a chart of what position you'll need to put the resistors in to achieve a given speed. The orientation is identical to the resistors indicated by the red bracket above.

All of the information for this chart was garnered from the tables on Takashi Imai's iMac modification pages. It appears to apply to all of the various iMac models: A (original), B (better graphics chip and 6 MB video memory), C (266 MHz and 6 GB drive) and D (333 MHz). No guarantees on that, but it seems to be the case.