Guitar electronics help needed

[ilan]

Took my '65 Höfner Senator bass to band rehearsal today. All treble and no bass... When I turn down the tone knob, it kills all volume. Yesterday she played just fine. Is it the pickup? knobs? Anything else I'm not aware of? (All parts are original, the pickup was rewound last year).

[Psycho Bass Guy]

It sounds like the tone capacitor may have shorted internally or the pot itself may have problems; you may have some kind of crud in there. Whatever the problem is sounds almost certainly like it is in the tone knob/cap, but there is always the possibility of something else. Could metal shavings or heavy dust might have made it into the electronics or the bass have been exposed to phantom power, even awhile ago? Capacitors block DC, but the voltage value drops as electrolytic caps age and the "floating 48vDC" of phantom power could have started breaking down the cap long ago. That's weird. A bad pickup will also do that.

[ilan]

Well, I really wished it was the cap but it was the pickup. Bummer. Luckily I had a spare '65 staple-top pickup. I'll have the original one rewound.

[Psycho Bass Guy]

Did the coil wrap wire get broken somehow? Pickups don't just die for no reason and it by some freak of nature, the magnet had lost its magnetism, it would have gradually weakened in output over time, not just died out of the blue overnight. Something had to have happened to it. Any ideas?

[ilan]

I have installed the spare pickup, and will have the dead one checked. I'll inform when the results are in.

[Psycho Bass Guy]

Thanks. I'm very curious about what could have happened.

[Rob]

Same here if it is the pick up I'd suspect a cold solder joint at the connector between the wrap and the lead.
Bit please advise.

[amptech]

Thanks. I'm very curious about what could have happened.

Same here.

I've repaired pickups that has been 'mechanically unstable' , mostly japanese 60's pickups with thin wire rubbing against bare metal and eventually shorting, but again, it's usually the connection between the magnet wire and the wiring to the controls.

I have only once had a pickup to repair which was low in volume or different sounding in the owners ears.
Again, connection problems. He had installed the pickup  himself, probably using a too big iron. Fortunately, there was enough inner wire not destroyed to fix it, wrapped off one turn of the outer wire and soldered it back properly - advising him to cut/unsolder pickups at the controls, not the pickup itself. Usually they work, or they don't!

[ThunderBucker]

This sounds like the one I repaired for you, it was a bad connection to the coil under some 50 year old dead tape.  Very brittle, hard to repair under the microscope (I don't see very well anymore....).  It might have failed again, or it might be the connection on the *other* coil....sorry!

[ThunderBucker]

Oh, and a simple test for this. Select the pickup in question (if there are 2). turn the volume all the way up, plug a guitar cord into the bass, and read the resistance from sleeve to tip on the guitar cord.  If the pickup is good (not opened up) you will read the resistance of the pickup (maybe 8k for single coils, maybe 10k for a humbucker, maybe 20k for a mudbucker, but a resistance in the 10s of k ohms).  if the pickup is open, you will read the resistance of the volume pot (250k for a fender, 500k for a gibson).

Most pickups fail by going open. There are miles of Leetle Tiny Wires in there, and many of them are very old and tired.  Fender single coils usually fail because corrosion on the Alnico poles eats into the wires wound directly on them. Oddly, an open pickup will make some sound (capacitive coupling for you budding engineers out there ) but it will be very tinny and weak.

This is a simple test that doesn't require opening up the bass or unsoldering anything.

[Happy Face]

Great thread. Much info! Thanks!!