Brown notes that the predecessors Mac LC and LC II had the correct connections, as did the LC 475, which uses the same power scheme. This makes him “confident that Apple has made a boo-boo with the LC III,” or “basically the hardware equivalent of a copy/paste error when writing code.”
Ensuring that rehabbers don't make the same mistake
Why wasn't this noticed before, other than a few forum threads seen by dedicated board rehabbers? There are a few reasons. For starters, the rail was only used for a serial port or for certain expansion cards, so a capacitor failure or out-of-spec power output might go unnoticed. The other point is that the original capacitor was rated at 16V, so even with -5V across it it might not have failed, at least when it was relatively new. And it wouldn't have failed in such a spectacular way that stories and myths were created.
As for whether Apple was aware of this but decided not to act because of a somewhat obscure bug, one that might never cause any real problems? In any case, let us know if you worked at Apple during that time and can give us more information. Ars emailed Apple the day before Thanksgiving with this hugely relevant question and will update this post with any comments.
By publishing his analysis, Brown hopes to provide anyone else re-covering one of these devices with a clear, reflective warning sign to ignore Apple's markings and install C22 in the electrically correct manner. Reached by email, Brown said he had heard from another hobbyist that the reverse voltage “would explain why the replacement cap” they installed “exploded.” Some restoration types, like Retro Viator, noticed the problem and fixed it before the explosion.
Modern rehabbers tend to use tantalum capacitors to replace the liquid-filled capacitors that have likely damaged the board they are working on. Tantalum tends to react more violently to too much or reverse voltage, Brown wrote me.
Should C22 or other faulty capacitors completely destroy your LC III board, Brown notes that 68kMLA member max1zzz has created a completely reverse-engineered schematic of the circuit board.
