I offered to try to fix a Cathedral Reverb for a friend (though also own one myself). I found the ferrite bead FB3 to be burnt out. I traced the circuit and this ferrite bead is in series with the input signal both for bypassed and when on.

Since the ferrite bead is probably only there to reduce RF interference picked up at the input, and it was really not feasible to replace it (the solder pads were burnt up as well), I was able to solder a wire across where it was. Now the pedal works fine.

But I see others have posted about fried ferrite beads in other EHX effects, and they also seem to be the ones in the input path. Seems to be a common circuit across many EHX pedals.

So my question is what could be the cause of burning them out? When in bypass mode the ferrite bead is in series between input and output, so if there was a DC voltage present on either input or output, and not on the other side, a high current could flow through the bead burning it out, if the current wasn’t limited in some way. Could that be a reason? Wondering what could cause a DC voltage to be present on either input or output, though a pedal immediately before or after the EHX pedal, where the input or output coupling capacitor had broken down dielectric (i.e passing DC), could put 4.5V onto its input or output.

Since the Cathedral pedal works fine after my fix, seems unlikely that it could be something to do with the rest of the circuitry in the pedal. Though another speculation is if the bypass switch is also switching the power to the Status LED, maybe there is a fault, shorting or arcing inside the switch that briefly puts 9V onto the input?

I have several EHX pedals, Cathedral, Super-Ego, B9, that probably have this similar ferrite bead, and would like to understand what possible scenarios could cause the ferrite bead to burn out.