From recent experience, my bet would be the Bourns filter. The external floppy port has power pins on it, and if those get fed back onto a signal pin they will blow one or more channels in the filter. It can happen from a fault in the drive, but also wrong cable pinouts - say the drive was configured for a different model Mac. Those external ports are treacherous, because they serve different functions on different models, but they look the same. You need to check compatibility before firing up an untested (says the one who didn't do that, last time...)
Fortunately, the Bourns filter usually acts as a fuse and protects the IWM/SWIM controller chip. The controllers are much harder to replace.
Rick