Why did the Apple III have more heating problems than the Altair?

Question

The Apple III was famously prone to overheating, to the extent that Apple advised customers to drop the machine a few inches onto the desk in the hope that this would knock chips back into their sockets, that had worked loose over several heating and cooling cycles.

This is sometimes attributed to Steve Jobs insisting on the absence of a cooling fan, and no doubt this was a factor.

However, the Altair 8800 had no cooling fan either. It was a computer of similar tech level, in similar configuration (horizontal, sometimes with monitor stacked on top, sometimes with a rather hefty, heat-producing double 8" disk drive on top), and it had 18 expansion slots compared to only four in the Apple II, so that a fully loaded machine would have had a lot more components in it, all generating heat.

If the Apple III had overheating problems, why did the Altair not?

Answer 1

TL;DR: Basic Design

Altair excels in thermal over the Apple III by two several basic points:

• Less Power to be Removed
• More Volume to Handle the Heat Transport
• Better Airflow to Remove Heat

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Power Requirement/Supply

The Apple III PSU is rated on input at ~130W (1A at 120V~ or 0.6A at 220V) while maximum power requirement for all components is defined on p154 of the Owner's Guide as 100W.

Being an open system the numbers for the Altair are a bit harder to come by. According to the manual (p.9) the basic PS is build of 3 transformers for a secondary total of ~85W.

• T1 8A at 8V ~= 64 W
• T2 1A at 8V + 0.8A at 16V ~= 20.8W
• T3 0.3A at 16V ~= .48W

More Volume

The Altair houses boards and PS but nothing else in a spacious square 18 cm x 43 cm x 46 cm ~= 30l case with no further subdivision. The outer dimensions of an Apple III come close (12 x 44 x 46), but the encloses volume is less than half and heavily comparted giving a volume less than 5l for all boards.

Better Airflow

All Altair boards are vertically orientated. This gives a nice unhindered upward airflow. In addition the case featured vents allowing hot air to directly escape.

The Apple III had it's main board, which accounted for up to 2/3rd of the power consumption orientated horizontally mounted with next to no space for free airflow as a massive metal plate was put right above. Also all heat was supposed to be transported only indirect via that metal frame.

Answer 2

The absence of a fan was only part of the problem. (By the way, the Apple II also did not have a fan). [1] describes it like this:

Those that did work initially often failed after minimal use thanks to Job's insistence that the Apple III not have a fan (a design demand he would make again on the Mac). He reasoned that in addition to reducing radio-frequency emissions (a severe problem with the Apple II), the internal aluminium chassis would conduct heat and keep the delicate components cool. He was wrong.

Compounding the problem was that Jobs dictated the size and shape of the case without concerns for the demands of the electrical engineers, who were then forced to cram boards into a small space with little or no ventilation. As the computer was used, the chips got hot, expanded slightly, and slowly worked their way out of the sockets, at which point the computer simply died.

So we have:

• an internal RF shield impending airflow
• a small space for the motherboard itself, with no airflow
• a higher clock frequency (2 MHz), which makes everything run hotter
• quite a few highly-integrated custom chips (more transistors in a small space = hotter)
• ignoring the engineers

I couldn't find a good picture of the Apple III interior, but if you look at the available ones, you can see a bit how a big part of the motherboard at the bottom is covered by the RF shield.

None of this is a problem in the Altair (or the Apple II): those motherboards don't have custom chips, only standard chips; the clock frequency is lower, and the air can easily circulate above the motherboard vertically between the cards.

[1] Linzmayer, Owen W. (2004). "Apple III Fiasco". Apple Confidential 2.0: The Definitive History of the World's Most Colorful Company. No Starch Press. p. 42.