There can be few among those of us who produce printed circuit boards, who have not at some point placed a component the wrong way round, or with the wrong footprint. Usually this can be rectified with a bit of rework and a fresh board spin, but just occasionally these mishaps make it into the wild undetected. It seems nobody is immune, as [Doug Brown] is here to tell us with a tale of an Apple product with a misplaced capacitor.
The LC series of Macs came out through the early 1990s, and their pizza-box style cases could be found slowly turning yellow in universities and schools throughout that decade. Of them there was a persistent rumor of the LCIII had a misplaced capacitor, so when he received an unmodified original machine he took a look. The investigation is quite simple, but revealing — there are three power supply rails and one of the capacitors does have a significant leak.
The explanation is simple enough, the designer had placed a capacitor on each rail, with its negative side to the ground plane, but one of the rails delivers -5 volts. Thus the capacitor is the wrong way round, and must have failed pretty early in the lifetime of each LCIII. We’re curious then since so many of them went through their lives without the component being replaced, how the circuit remained functional. We’re guessing that there were enough other capacitors in the -5 volt line to provide enough smoothing.
Ah, the joys of a cap designed in with polarity reversed. In the late 80’s I worked at SMC [Standard Micro Systems], and since they produced a chip that implemented the ArcNet protocol, they also made an interface card for the IBM PC. The circuit needed a one-shot chip to stretch the system reset pulse to the width that the ArcNet chip needed, and in tern, the one-shot needed an electrolytic cap to produce the pulse. These boards would fail after a year or so, and the cap was the cause. The design engineer swore up and down that the circuit was correct, and there was no reason for the cap to fail prematurely – he had copied the circuit directly from the one-shot’s spec sheet. Looking at the example circuit and the internal design of the chip, it was discovered that the negative side of the cap was connected to an external timing resistor that was connected to +5V. He had blindly followed the sample design, and ignored the obvious weirdness of the circuit. A +5, current, even thru a large resistor, with the positive of the cap connected thru an internal ground inside the one-shot chip, caused the cap to slowly fail.
This is why you should strive to always make the wrong configuration look wrong and the correct one look right.
Somebody saw three capacitors in a row with one inverted and blindly changed it assuming that because it looked wrong it must be wrong, without actually checking the circuit.