The capacitor that Apple soldered incorrectly at the factory
Downtown Doug Brown digs into the internet rumors that the Apple Mac LC III, a “pizza box” Mac model produced from early 1993 to early 1994, should have had a recall due to incorrect assembly:
Here’s the affected section of the board (above) before I removed the original capacitors. You can see that all three of these caps (C19, C21, and C22) have their negative side pointing upward, matching the PCB silkscreen that has the + sign at the bottom.
Yikes, C21 leaked really badly and damaged the solder mask and the copper underneath. When I removed it, there was a big puddle of goo sitting there.
What’s happening here is there is one bulk capacitor for each of the three power rails provided by the power supply. C19 is for +5V, C21 is for +12V, and C22 is for -5V. You can see the trace from C22’s positive terminal going directly to the -5V pin on the power supply connector.
This arrangement makes sense for the two positive power rails, but it’s backwards for the -5V rail.
The bottom line is that Apple’s silkscreen markings and factory placement for C22 on the LC III are flat out wrong. There’s no other correct answer. Please, if you are recapping one of these machines, install C22 backwards from what the silkscreen on the PCB says.
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