Elfen said:

One of the reasons why I left the (Mac repair) company was because the company owner took a bunch of new jack techs and had them replace the diodes on the PSUs and then sell the repaired PSU as a new one.

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Back when Mac discussions like this took place on Usenet (comp.sys.mac.****) and there was no Ebay (comp.sys.mac.wanted) there was an outfit offering to fix "dead" Q605s and LC/P 475/6 for $100 with exchange of the old logic board.

They were taking the old logic board, replacing the battery, and sending it back out again.  $100 to change your battery.  Quite a racket.