Breaking The Speed Limit! - Instructions To Hotrod Your iMac To 300Mhz


by Kevin I. Glattfelder

iMac Overclock

The iMac is a wonderful machine that has opened up an entirely new group of people to the computer age. The iMac was designed for those who wish to enjoy the benefits of owning a computer and access to the internet without the hassles involved when dealing with PCs. While a large number of the machines were purchased by newbies a large number were snatched up by weekend hackers. The following procedure will show you how to overclock a Revision A iMac.

Warnings

The actual procedure is fairly difficult. It requires extreme skill and knowledge of soldering techniques. If you have never soldered on PC boards and chips I would not suggest attempting the procedure. The devices that need to be moved are very delicate and are approximately 1/16" X 1/32" of an inch in size. The PC board is also very delicate, too much heat and force and the solder pad will lift. Don't attempt this procedure if you have any reservations about your abilities. PLEASE NOTE THAT BY MODIFYING THE CLOCK SPEED OF THE MACHINE YOU WILL VOID YOUR WARRANTY

Helpful Links

Before performing the procedure point you browser at the following link.

http://www.theimac.com/ram_under.shtml

The first link has a table that shows the various positions of the four surface mount resistors and what clock speed they set. The second link gives a very detailed pictorial of how to dismantle an iMac and remove the CPU daughtercard. Study them and print a hard copy if possible.

Tools

You will need the following tools to perform the modification:

-Work table with a clean surface preferable white (so you can see the resistor(s) when you drop them!)
-Standard wattage soldering iron (15-20 watts) with a small tip, a tip cleaning sponge and a base would help.
-Solder
-Fine tip forceps (plastic would work well)
-Anti-static wrist strap (grounding strap)
-Well lit room
-Steady nerves

Procedure

First decide what speed you want the CPU to operate at. I have been overclocked to 300 MHz for nearly three months without any problems (it would not run any faster). Use link one to find the proper resistor combination for the chosen clock speed. Next follow the steps shown in link two in order to remove the CPU daughtercard. Flip the board over and locate the four resistors. Place the board on the work surface and secure it so that it will not move around. Plug in the soldering iron and allow it to fully heat. Make sure the tip is clean and test the temp by touching solder to it. The solder should melt instantly. When the iron is ready touch the tip to the lead of the first resistor that requires moving. Heat the resistor (don't apply any pressure and don't slip the device around) until the solder on the board melts. Use the forceps to remove the resistor (take your time and be careful not to pull too hard on the resistor). Place it in the adjacent position and use the forceps (I used my finger nail because metal forceps sink the heat away) to hold the resistor down. Solder each side of the resistor. Make sure that solder does not bridge to any other pad while performing the procedure. Once all of the required resistors have been moved double check your work. Make sure that the resistors are in the correct position and nothing is shorted. Reassemble the machine making sure that the CPU daughtercard is fully seated. Don't put the cosmetic back panel on yet. Plug in the power cord and the keyboard. Press the power button and wait for the machine to boot. If it bongs then that is a very good sign!! Allow the machine to boot and check for proper operation. BTW if you have the PowerLogix G3 Cache control program make sure it is turned off before performing the procedure. The machine may lock up while loading the control panel if the ratio is set too high. My cache runs at 150 MHz not any higher. The CPU will run hotter than before. One way to cool the chip is to remove the thermal sticker from the top of the processor and replace it with a thin film of heat sink compound.


MacBench 5.0 results expressed as the percentage of improvement over the base machine

 

 

Actual MacBench 5.0 Scores

.

Processor Score

Floating Point Score

iMac G3/233

719

790

iMac Overclocked to 300Mhz

908

933

Power Macintosh G3/300

1000

1000

 

 

Good Luck

If you take care when performing the procedure you can have a very fast machine. Macbench 5.0 scores rival and in some areas exceed those of the benchmark G3 300 machine.

Kevin is a recent convert to the Mac having worked on PCs most of his life. On his way to buy a Dell he was sidetracked by an iMac and has been a reformed Wintel user ever since. Much more than an iMac "new user" statistic Kevin works in the semiconductor electronics industry. If you have any questions or hacks you are welcome to send him e-mail.

Internal Links

  • Links to other sites with benchmark information
  • MacSpeedZone's CPU Upgrade Page
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