At this point (2022), the PowerBook 3400c rarely needs to be recapped, however, I have heard of failing caps on the DC Power Board preventing these units from booting. And like any old computer with electrolytic caps, it will need this done eventually, so why not now?
Disclaimer - After recapping my 3400c with this list, the sound can occasionally crackle as the system warms up. This only happens on occasion, but it did not happen at all before the recap. It always goes away completely after by the time it boots. It could be an issue with my PowerBook, or an issue with the caps I used. I just thought that was important to mention.
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