Amiga A3640 CPU Board Repair

One question that I had when I discovered the capacitors backwards in the circuit was "How did this happen?"

At first, I also feared that if these were backwards, other capacitors in the A4000 may be in backwards.

After researching the issue, however, I came to realize that, no, none of the other electrolytic capacitors were installed backwards.

I did not have a schematic for the A3640 (although I have now created one), but was able to discern that of the five installed, only three were backwards. As near as I can tell, it looks like, at first, there were only the two electrolytics on the 68040 Daughterboard (C102B & C102E).

Apparently, C105, C106 & C107 were added as an afterthought. I can only theorize, but I'll bet that the engineer in charge of the A3640 passed the schematics by his senior engineer for approval. The senior engineer may have then suggested that a few more electrolytic capacitors be added for good measure.

These capacitors were then added to the schematic - backwards! It was never caught and went through manufacturing like that.

Of course, the board would pass all QA testing, and it would be very difficult to discover this problem... until much later by you and me when our Amiga 4000 mysteriously fails.