Mac Classic

The iMac G3 and it's cooling issues

The iMac G3 is an easy favorite for many Apple computer owners. It was originally released in 1998 and had several variations until it was discontinued in 2003.

One of the biggest issues with the iMac G3 is that it was designed to be convection cooled, meaning no internal fans were used and the computer relied on heat rising out through it's top vent. Since there was no active cooling some components within the iMac are prone to damage and failure over time. The PAV board (Power, analog, video) and flyback transformer are the biggest points of failure due to heat damage, the GPU is also known to suffer from some of these issues.

When the PAV board or flyback transformer start to fail you may notice the CRT arcing - a thin line appears quickly across the screen accompanied with a static popping sound. Your iMac may also shut off unexpectedly, or refuse to power on. These symptoms are serious, arcing issues can become fire hazards and should be taken seriously.

Cooling modifications

I recently came into posession of an almost new condition iMac G3 "Flower Power" model and wanted to take care of it since it had been so well kept. I decided to install some internal fans to help the machine keep cool and avoid any serious component failures in the future.

The case design of the iMac makes it difficult to install standard sized computer fans. I didn't want to open the top of the case because since I'm not comfortable working near the CRT. I also didn't want to risk breaking any of the plastic case tabs which become brittle from years of heat and UV damage.

I purchased a variety of fans and a Molex fan adapter from AliExpress and local trading websites. I also installed a solid state hard drive (SSD) which made more free space where the HDD would sit which I made use of with the fan cables etc.

WARNING!

Extreme caution must be taken when working near CRTs. CRT power supply units can store enormous amounts of energy even after being unplugged and unused for a long period of time. An electric shock from one of these can kill you due to the extremely high buildup of current stored in the machine. Do not touch any of the power supply internals unless you know what you are doing and have correctly discharged the machine!

Gear used

Approximate total cost: ~$27 NZD

2x Winsinn 30mm 24v

2x Winsinn 30mm 24v

2x NMB-MAT 12v

2x NMB-MAT 12v

1x Molex 4-Pin to 4x 3-Pin Fan Cable

1x Molex 4-Pin to 4x 3-Pin Fan Cable

Installation

Fan #1

Several of the fans I purchased wouldn't fit anywhere useful since I didn't want to open the top case, so I began with the smallest fans I received. I placed one in front of the flyback transformer inside the case. Be very careful not to adjust the CRT dials on the transformer. I also placed packing foam inside the fan's screw holes and folded it through to reduce vibration when placing it inside the case. I used a hot glue gun to secure the fans because it's transparent and easy to remove if needed. It also meant that I didn't have to make any alterations to the case. This fan pulls air from the side of the case into the flyback transformer and across the PAV board. There's not much of a gap for airflow inwards but it does seem to pull enough air through. This fan runs at 12v.

Foam placed to wrap around the fan to reduce vibration

Foam placed to wrap around the fan to reduce vibration

Fan 1 installed. Foam was folded across the top of the fan so the fan plastic wasn't touching the case

Fan 1 installed. Foam was folded across the top of the fan so the fan plastic wasn't touching the case

Fan #2

The second fan I installed was under the processor, and secured to the EMI shield. I drilled three holes into the shield because I have a spare if I ever wanted to replace it. This fan pulls in air from both of the bottom vents and runs at 12v. The air is directed at the CPU + GPU and spreads across the logic board and up the sides of the case.

Fan #2 mounted inside the EMI shield creating airflow inwards from the bottom of the case

Fan #2 mounted inside the EMI shield creating airflow inwards from the bottom of the case

Both sides of the drilled holes were glued to ensure any fragments of the shield couldn't come loose

Both sides of the drilled holes were glued to ensure any fragments of the shield couldn't come loose

Fan #3

After running the iMac for a while with the two fans installed I noticed that it still became hot at the bottom left side of the case (flyback transformer, PAV heat sink area) after running for +1hr. I opened the machine again and mounted another of the smaller fans to the case running at 12v blowing air directly onto the larger PAV heat sink. This pulls air from the side of the case and directs it straight onto the heat sink, which is then deflected upwards towards the top vent. After installing this fan there was a much more noticeable breeze coming from the top vent of the machine as it was running. The left side of the case is still warm with general use, but it's cooler than it was without the fans.
This fan is the noisiest in terms of vibration. I couldn't dampen the vibration with foam because I didn't want to risk it melting onto the heat sink. Instead I held the fan in place while it wasn't touching anything and hot glued it in place.

I didn't plan on using LED fans here, I'll probably replace them in the future.

Fan #4

I placed a fourth fan above the top end of the logic board PSU near the heat sinks. This also pulls air from the vents at the bottom of the case and directs it onto the PSU. This fan runs at 5v.

Ideally the machine should have a large fan above the PAV board, near the top vent increasing airflow out of the case. I wanted to do this but I couldn't find a way of doing this without removing the top case.

This was a tight fit when the EMF shield was put back in place but there was just enough clearance for the EMF shield

This was a tight fit when the EMF shield was put back in place but there was just enough clearance for the EMF shield

Before securing the EMF shield make sure your fan cables are protected with something so they don't get cut. I used folded insulation tape.

Before securing the EMF shield make sure your fan cables are protected with something so they don't get cut. I used folded insulation tape.

Results

Noise

There is of course increased noise after installing the fans. This is why Apple opted for convection cooling rather than fan cooling. The noise is comparable to the sound of running an external mechanical HDD on top of a wooden desk. This of course could be improved with better quality fans.

Temperatures

There is only one temperature sensor present within the iMac G3 which is on the CPU. The maximum temperature I have noticed after vigorous testing was 48°C (which is very good) reported by G4 Strip CSM, and 46°C reported by Gauge Pro. Testing included running the games SimCity 3000 and Tropico for two hours, then general use of the machine for another 2 hours.

There is a good breeze coming from the top vent of the machine now. The does case still gets warm inside, but feels a lot cooler than what it did without any cooling.

Possible Issues

When this article was originally posted I mentioned that I noticed the CPU and Cache were running at faster clock speeds for ~5mins after booting before returning to normal values. After testing this on several other machines I found that this is either a quirk of Mac OS 9 or the Newer Technology Gauge application. I have noticed this happening on an iMac G3, PowerBook Pismo and PowerBook 1400 regardless of any modifications.

Conclusion

To me the cooling additions were worth it and I believe they make a big difference with the internal temperatures. The added noise isn't that bad, I felt it was more annoying when I had a mechanical HDD installed (without cooling) and the constant seeking noises it would make. I plan on getting a temperature sensor to check the internal case temps and will update this page when I get the results.

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