Keeping your iMac cool

The G3 / PPC750 chips don't give off a lot of heat for as fast as they are, partly because their internal wiring, if you will, is so small, and partly because they are very efficient chips without as much internal wiring as Pentium or Pentium II chips of comparable speed. Still, the G3 chip in the iMac, even at 233 MHz, can build up some heat, and the stock heatsink is barely adequate. I have to wonder if a lot of iMacs will have trouble over the long run because of that small heatsink.

Speeding it up to 300 MHz will cause it to build up even more heat, and in my experience at least, the stock heatsink has to go if you want a machine that is usable for more than an hour at a time. Replacing it presents two problems:

Problem #1: where to find a heatsink that will fit into the odd shape on the top of the processor card.

Problem #2: the internal cooling fan of the iMac is immediately above the processor card, and will not allow a heatsink which is more than about 7-8mm taller than stock.

To solve these problems, I wound up using a heatsink designed for an AMD K6 processor and cutting it up to fit in the oblong space around the G3 chip, as well as relocating the internal cooling fan to the top of the case to make room for the height of the heatsink. I installed a Pentium fan on the top of my heatsink as well; this makes for a very tight fit when putting the processor & drive chassis back into the iMac case, but it can be done. Note that a heatsink fan installed the traditional way, where it is pulling air down onto the heatsink, will actually make the heat problem worse as it pulls hot air from the monitor area onto the heatsink. I installed my fan upside-down so that it pulls air across the heatsink and then pushes it up into the monitor area. I also removed the blue panel from the side of the processor & drive chassis to promote airflow through the case.

The specific product I used was an AOC heatsink & fan, item # EC-K6-5B. I had to replace the included 50mm fan shortly after I installed it into my iMac, probably due to rough handling of the fan on my part. I now have a 60mm x 60mm x 13mm AOC fan installed, still upside down. It was a tight fit! Now the 60mm fan sounds like it's failing; I have to recommend against this particular brand (AOC).

The iMac generates more noise with the extra fan installed; if you're like Steve Jobs and fan noise really bugs you, you may not want to do this.

Here are some photos of my final product. Note that the stock heatsink would not have been visible at all in the first photo; here, it sticks up perhaps an inch and a quarter above the metal case sides which fit flush into the iMac case.

To find or create your own heatsink, start checking out computer supply stores; you're looking for one with a lot of fins (they provide more surface area to dissipate heat) and a fairly thin plate at the bottom (makes it easier to cut through). One which is bare aluminum, i.e. not painted, will be more effective, though almost all seem to painted. The dimensions are approximately 1 3/8" (35mm) by 2 3/8" (60.3mm). Note that the length is actually less than the stock heatsink; if your heatsink is going to be taller, it needs to be able to poke into the off-center hole where the case fan was -- and to clear it on the side, it has be smaller in the long axis than the original heatsink. Height is up to you; you should have no problems fitting a Pentium or K6 heatsink of normal height, even with a fan on top of it, into your iMac.

To cut it up, I used a Dremel-type tool with little "whizzer" cutting blades. The Dremel-type tools are $40 on up, and the little blades are a few bucks a package. Borrow the tool if you can.

Be sure to use some heatsink grease, also known as heat transfer paste or heat transfer compound, between the processor and the heatsink. This should be available in a good electronics or computer store; probably at the same place you find your heatsink. CompUSA and Computer Renaissance don't seem to carry items like these; it may save you a lot of time if you call some places first to find out what they carry, rather than simply driving around to them all. Radio Shack also supposedly has this product (Cat. No. 276-1372), but none of my local stores seem to stock it.

Don't even bother asking for an "iMac heatsink"; you are almost certainly going to have to make your own from one designed for a different computer model.

The fan is another issue. A 50mm x 50mm fan should work fairly well, and it's a size commonly used for Pentium, Pentium II and AMD K6 heatsinks. Be sure to buy a ball bearing fan rather than a sleeve bearing fan; it will last much longer and run more quietly. They're only a few dollars more. Also, note that some fans, when installed upside down, don't have the necessary clearance between the blades and the heatsink. You may have to purchase some small washers to use as spacers, as well as some longer screws, at your local hardware store.

A pair of smaller fans might also work well, and would make the installation of the chassis easier. You could probably use two 35mm x 35mm fans side by side on top of the heatsink. Two 30mm x 30mm fans would probably also fit, but that might be pushing the low end of the airflow necessary to keep the heatsink cool.

Be certain that your fan has the necessary clearance between the blades & the heatsink before installing the chassis back into your iMac; try powering it off a 9-volt battery or something similar to make sure it spins freely.

I originally installed a 50mm x 50mm x 13mm Pentium fan, but I think I mishandled the fan while I was installing it because it began vibrating loudly shortly after I installed it. It did, however, keep my iMac running steadily at 55 C / 131 F for several hours before I found & installed a replacement for it.

Due to the dearth of electronics stores in Des Moines, the only replacement fan I could find was a 60mm x 60mm x 13mm fan. This is awfully big, and I was barely able to fit it into the case. It maintains a slightly lower operating temperature; 123 F to 131 F. If possible, experiment with a 50mm square fan first -- you will have a much easier time installing and removing the processor & drive chassis with the smaller fan.

To supply power to the fan, you have a few options; you can remove the CD-ROM drive and hard drive, as I did, and install the pass-through connector for the fan between the hard drive and the normal iMac power line. You could hunt around for a length of wire with the proper connectors to install between the power connector and the logic board, thus avoiding removal of the CD-ROM and hard drive. Or you could, if you're brave, snip the wires and splice in the wires to the fan.

For a very detailed guide on how to remove the CD-ROM and hard drive to get at the power connector, see theimac.com's guide to drive removal. I did not find it necessary to remove my logic board to get at the power connector on the drive, but that probably would have made the procedure easier.

Check out this page for other possibilities for iMac processor cooling.

Moving the iMac case fan

I suppose it would be possible to simply remove the internal cooling fan entirely, rather than go to the trouble of relocating it to the top of the case (though it really wasn't that bad). The heatsink fan would still provide some airflow through the case. This might, however, shorten the life of your monitor tube and even your power supply considerably, and is not a recommended solution.

This requires some determination, but if you are willing to take a soldering iron to your processor card, this is easily something you can do. The first step is to detach the fan cage from the main case; the second is to detach the fan from its cage. Undo the two screws holding the fan into the main case, and pull gently until the metal cage for the fan comes out. Now undo the four screws, one at each corner, holding the fan into the cage. Leave the pieces hanging loose; you have to disassemble the rest of the iMac shell to be able to unplug the other end.

Undo the screws holding on the bottom of the case, and remove it (lift in back, then push forward a little bit). You don't have to unattach any wires to do this. The red circles in the picture below show you the 8 screws you'll have to undo. The one under the expansion bay door is tricky; be careful. A very long #2 Phillips screwdriver would be helpful for this screw.

Next, you'll need to turn the iMac upright and remove the faceplate. There are two screws, one on either side of the hole for the CD-ROM tray. Remove these. The faceplate comes off before the Bondi Blue part, despite all appearances to the contrary. Take a small flathead screwdriver, start at the bottom (by the speakers), and lever gently out on the faceplate. It should begin popping out. Remove it by pulling forward and up -- note that there are tabs on the top, between the faceplate and the flavored plastic part, which might break if you're not careful.

Next, remove the two screw covers on the top of the machine, at the edge of the flavored plastic part, and then remove the two screws. The flavored plastic part is actually a little tricky -- in order to get it to separate from the fornt of the machine, you have to push it down. It should come off pretty easily, though.

Be very, very careful not to touch any parts of the monitor tube in this process. It carries enough voltage to kill, even if it's unplugged (and even if it's been unplugged for hours).

I apologize for not having any pictures of this process; I was out of film when I made this modification. I will repeat the steps and take some photos sometime soon.

See the little metal bridge across the back of your iMac, right above the monitor tube? This is where we'll attach your fan; just under the bridge.

First, you have to remove it from where it's at, though. Peek around a little, and you'll see that the red and black wires for the fan run through a little retaining clip; remove them from this clip. You may actually have to unseat the clip, remove all of the wires from it, extricate the fan wires, put the others back into it, and then put the clip back.

Now you need to unattach the end of the fan wires from the board on the right side as you're looking at the back of the iMac. There are three screws holding this board in; remove all three, and pull the board out just far enough that you can get to the fan plug (at the bottom, in about the middle of the board). It's a locking plug, so you'll need to get at it with a small screwdriver from each side. An assistant at this point would be very helpful.

After it's unattached, pull the fan out from the bottom so that the wires are free, pull the wire out of the hole in the fan cage, and then position the fan at the top of the case. Note that you have just the fan now, not the fan and its metal cage. It's ok to let it rest on the monitor tube while you plug the wires back in (just don't touch it yourself). The wires on mine were barely long enough, but they do reach from the fan down to the plug socket. Carefully drop the wires down through all of the parts to where you can reach in under the board and plug them back in.

Use two of the screws that held the fan into its cage to go through the metal bridge and into the holes in the fan. You may have to install it slightly off-center or tilted; that's OK. Be sure you've got it pointing in the proper direction so that it's pulling air up through the case. Now on to reassembly.