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Overclocking Blue and White Power Mac G4/400

Overclocking Blue and White Power Mac G4/400

Sometimes cooling a system without overclocking can increase system stability
When my friend Ben got a 450 MHz G4 Cube, I got jealous. I have a B&W “Yosemite” G3 with a 400 MHz processor, which is a pretty good machine. However, when one of my best friends became the owner of a better Mac than mine, I began to want a G4 to be able to take advantage of AltiVec (aka Velocity Engine), and an extra 50 MHz or more would also be cool.

Well, a G4 ZIF upgrade was way out of my price range at the time, and a G3 ZIF upgrade costs almost as much but doesn’t have AltiVec. (Frankly, ZIF upgrades aren’t all that cheap even as of late 2005, when you consider the price/performance ratio compared to buying a new computer.) A while back I’d read a bit about overclocking, and I remembered a site called “Accelerate Your Mac” (www.xlr8yourmac.com). I checked it out again and went to the pages on G3 overclocking. Following the instructions outlined there, I decided that it was worth voiding my warranty (if it hadn’t expired already) in order to get some extra speed. All it would take was changing a few jumpers, and I should be able to overclock it at least 50 MHz, no problem. Well, that’s what I thought, anyway.

With jumpers instead of the jumper block, here’s the original 400 MHz config.

I pulled off the OEM jumper block and added my own jumpers (under $1.50*, Radio Shack #276-1512A or 276-1512, AKA “DIP Programming Shunts”), and decided to try my luck and clock my G3 at 450 MHz instead of the original 400 MHz (jumpers 4, 5, 6, 8, and 9 instead of 1, 3, 5, 6, 8, and 9; a table of jumper settings can be found at xlr8yourmac.com, partially mirrored at lowendmac.com). It worked!! Well, for short periods of time, anyway. After leaving it on for a while, or bringing my distributed.net client to the front, or especially when playing the demo of Unreal Tournament, my poor G3 would drop into MacsBug with a Bus Error or freeze up altogether.



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*Any prices herein are approximate U.S. dollar prices at the time of purchase and may not accurately reflect current prices.

One time when I happened to have the Powerlogix G3/G4 Cache Profiler open, I saw that the processor temperature was reported as 40 degrees Celsius at the time of the crash, higher than I’d ever seen it before. Normally it stayed between 35 and 39 degrees Celsius when clocked at 400 MHz.

I went again to Radio Shack within about an hour of trying to overclock my G3, and picked up a Pentium CPU Cooling Fan with heat sink (about $15, RS #273-248, no longer being sold as far as I can tell). I took it home and disassembled it, screwing the fan onto my G3’s heat sink and plugging it in where the hard drive was getting its power, and then piggybacking the hard drive onto it.

Then I had to decide what to do with the heat sink. It didn’t seem like it would cool my processor any better than the standard one, so I removed the mounting clip and placed the sink next to my G3’s heat sink, touching what was exposed of two chips on the processor board. (According to Mike from Accelerate Your Mac, these are the “cache chips”—L2 cache I believe—and he warns, “make sure no metal contact [sic] with the leads.”) I then bent the mounting clip and held the new sink in place by bridging the clip somewhat tightly between the G3’s sink and the Pentium sink.

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