I’ve been using this iMac for a few weeks, and I’ve noticed an odd melty-plastic smell from time to time. I didn’t associate it with the iMac, though; the top of the iMac was hot, but not unbearably so.
After the weekend, though, the smell was more intense. Bruce found a particular spot on the back panel of it that was uncomfortably hot. Time to investigate.
I downloaded Temperature Monitor and found the power supply was at 81° Celsius (about 178° Fahrenheit). That’s far too hot! I found smcFanControl and turned up the fan speeds, and my iMac’s power supply is runs between 55-58° Celsius (131-136° Fahrenheit). That’s much healthier, and the plasticy smell has almost (but not completely) disappeared. The downside is that the iMac is no longer silent, though I’m pretty sure I can turn the fans down slightly over the next few weeks.
I appreciate that Apple was trying to make my workspace more enjoyable by keeping the volume down, but it turns out the smell of burning plastic is more distracting than the fan noise. I can only imagine being without a computer while the power supply and back panel were replaced would have been even more distracting.