Brown notes that the predecessor Mac LC and LC II had the correct connections, as did the LC 475, which uses the same power supply scheme. This makes him “confident that Apple made a boo-boo on the LC III,” or “basically the hardware equivalent of a copy/paste error when you’re writing code.”
Making sure rehabbers don’t make the same mistake
Why was this not noticed earlier, other than a couple forum threads seen by dedicated board rehabbers? There are a few reasons. For one thing, the rail was only used for a serial port or certain expansion card needs, so a capacitor failure, or out-of-spec power output, may not have been noticed. The other bit is that the original capacitor was rated for 16V, so even with -5V across it, it might not have failed, at least while it was relatively fresh. And it would not have failed in quite so spectacular a fashion as to generate stories and myths.
As to whether Apple knew about this but decided against acting on a somewhat obscure fault, one that might never cause real problems? By all means, let us know if you worked at Apple during that time and can clue us in. Ars has emailed Apple with this tremendously relevant question, the day before Thanksgiving, and will update this post with any comment.
By posting his analysis, Brown hopes to provide anyone else re-capping one of these devices with a bright, reflective warning sign to ignore Apple’s markings and install C22 the electrically correct way. Brown, reached by email, said that he heard from another hobbyist that the reverse voltage “would explain why the replacement cap” they installed “blew up.” Some restoration types, like Retro Viator, noticed the problem and fixed it pre-detonation.
Modern rehabbers tend to use tantalum capacitors to replace the fluid-filled kind that probably damaged the board they’re working on. Tantalum tends to react more violently to too much or reverse voltage, Brown wrote me.
Should C22 or other faulty capacitors destroy your LC III board entirely, Brown notes that 68kMLA member max1zzz has made a reverse-engineered full logic board schematic.