The illusive phantom crackle that plagued this early 60s Fender Bandmaster for years has finally been purged.
The problem—a sporadic fuzzy crackle that would sometimes fart out after playing a low note, and other times just sizzle in the background—would only reveal itself two hours after being powered on. Fortunately, a similar crackle was reproducible by tapping various components inside the chassis. After replacing a microphonic preamp tube and the phase inverter tube, I followed the problem underneath the chassis. A faulty filter capacitor (that tests perfectly within spec) was to blame.
It was by far the crunchiest sounding component when poked with the stick, and it was difficult to find. Poking the preamp side of the eyelet board or the socket of the phase inverter tube was initially causing a louder crackle than anything else. I assume these two points happen to jostle the eyelet board in just the right way to make the bad capacitor grumble.
The nature of the crackle had me suspecting a filter capacitor at first, but I saw that they had recently been replaced. I would have found this faster if I trusted my instincts.
I replaced the three 22 µF filter capacitors, but left the two 70 µF series caps, as they didn’t exhibit any crackling when prodded.
The amp was run through the gauntlet during a rehearsal last night, and while I’m happy to report that there is no crackling—phantom or otherwise, a new electro-zappy/lightsaber sound seems to have popped up.
My first plan of action will be to go through the power section again and make sure everything is in spec. Those 70 µF caps in series might not be long for this world.