The upside-down capacitor in mid-‘90s Macs

Ars Technica reports on The upside-down capacitor in mid-‘90s Macs, proven and documented by hobbyists:

But there’s a lot to suggest that user paul.gaastra, on the 68kMLA vintage Mac forum, has been right for more than a decade: One of the capacitors on the Apple mid-’90s Mac LC III was installed backward due to faulty silkscreen printing on the board.

The simple version of the story is that on the LC III motheboard, a model of Macintosh from the early 90s (introduced in 1993 and discontinued in 1994), an error on the printed circuit board made a capacitor (an electronic component) being mounted backwards. This error seems to have had no ill effect during the life of the machine, but this model of capacitors become a plague as they leak the electrolytic liquid over time. So they get replaced.

From Dowtnown Doug Brown The capacitor that Apple soldered incorrectly at the factory:

Although the computer seems to work okay in this configuration with a normal electrolytic capacitor like what was installed by the factory, tantalum capacitors are not quite as forgiving when installed in reverse.

Tantalum capacitors are a different type that doesn’t leak, but they happen do be much more sensitive when mounted backwards, and behave differently. They may emit the “magic smoke” or cause other problems. That’s where the investigation started and repair person realised it was an error from the manufacturing, even checking with model have similarities: they indeed have it properly printed and mounted.

Sometime repairing old computers involve fixing the mistakes made decades earlier.