If you do not have fast fuses in 12V DC wire you may end up with loose and broken PCB traces in RF board, also the connectors can be damaged.
I bought a used K3 #6913, the seller said that he had messed the polarity with car battery. That caused a major circuit board problem in the trace that lead to KPA3 in RF board. The seller had fixed the problem temporarily but it really need better fixup to keep it working. He hadn't put fuses in the DC wires as the manual suggest, maybe he just trust the 20A circuit breaker of the KPA3. Obviously that is not adequate protection. Maybe also Elecraft engineers can do something to the reverse polarity protection to make it more robust, to my mind the protection is not adequate if it results loose PCB tracks, eventough support so claims. At least Elecraft should supply fast fuses in the 12V DC line to prevent other damages, maybe a humble manual note is not enough.
Here is a picture of what is the result if you don't have the fuses in place.
https://plus.google.com/photos/103676169039107946672/photo/5889028521722277378
73 de OH9VD/Jarmo
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