The basics

There are two important issues to consider in this upgrade:

1) Soldering of small parts. In order to change the processor PLL, you are going to have to desolder and resolder some very small resistors. This is tricky to say the least. If you don't feel up to this part, but are still interested, you might be able to have this portion of the modification done at an electronics shop.

2) Heat. The G3 chip doesn't put off a lot of heat compared to fast Pentium II's, but it's going to put out more when running at 300 MHz (or higher) than it does at 233. The stock heatsink in the iMac is pretty poor; not a lot of surface area to it. Chances are good that you will have to replace it with one a bit larger (I did). Space is cramped, though, so you may have to rearrange or remove the iMac's stock cooling fan to make room for a larger heatsink (I did). There's a longer discussion of cooling techniques on this page.

Prep work

Be sure to make a backup of anything critical on your hard drive, just in case you can't get your iMac running again. Also, be sure to follow anti-static procedures throughout.

Use a utility like PowerLogix' SpeedMeter to get an idea of the normal processor temperature for your stock iMac; this can be helpful later when you're trying to determine whether it's running too hot. My iMac ran at about 64 C / 147 F from the factory, and about 56 C / 132 F after I removed the foam from the stock heatsink (still clocked at 233 MHz). Be sure to let the iMac run for a few hours to see how warm the processor really gets.

Also install PowerLogix' G3 Cache Control (there are some alternative software items listed later, on page 5), set the cache speed to the lowest possible ratio, and restart your iMac to make sure the ratio is set. This may prevent startup problems once the iMac is accelerated; you can experiment with different ratios later to see what the highest your iMac can achieve is.

You will need a clean, well-lighted working area and some basic tools: a #2 Phillips screwdriver, a small flathead electronics screwdriver, a set of very small and long needlenose pliers, a good soldering iron with a pencil tip (two soldering irons would be even better), and if possible a "project glass"; a magnifying glass on a flexible arm. I did not have one of those last items, and I wish I had. Having an assistant would be great.