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The one capacitor that almost always causes problems is the small blue dipped tantalum bypass capacitor in HP67 card readers (also some '65 readers). If your reader writes but won't read, this beastie is the problem. It will look and test good, but it develops a high internal ESR (equivalent series resistance) and stops doing its thing. When rebuilding a reader in these machines, I always replace this cap even if it seems OK. Eventually it won't be.
I have also seen bad tantalum power supply filter caps in Woodstock series machine power supplies (particularly HP29C units).
Very occasionally the main power cap in an HP41 will develop a leak and drain your batteries quickly.