If you are experiencing problems with your bass pickups, such as hiss, hum, shielding, and other problems, you may wish to do some investigation and look at the text below.
First, of all, you need to determine, what type of pickups you have. There are basically three types: Passive, Active, and Active-Boosted. The latter is really a High impedance passive pickup with an active pre-amplifier and tone controls to modify the sound of the bass.
Passive Pickups (or rather–sensors) have no battery or external power to the sensor. They are essentially wire coils on a magnet structure. The common resistance of these sensors is 4000 ohms to 8000 ohms.
If the sensor is disconnected from the system, you can measure the resistance with a simple VOM (Volt-Ohm-Meter).
If you measure the resistance and appears to be infinite, you most likely have an open (a break in the magnet wire), and this is quite difficult to repair. Most sensors have a wax or epoxy coating on the entire wire coil, to prevent micro-noise from wire vibrating inside the coil.
Active Sensors are Passive Sensors with a built-in amplifier. The sensors are often low impedance types, but not always. Active Sensors require a battery to operate the amplifier and usually have three wires or four like the ones below.
Below, the red is the hot (positive) the black is the ground, and the white is the signal wire, which usually goes to the potentiometer or signals control board.
The photo above is just one example of many types of pickups for bass guitars. The ones above are an active Jazz Bass set made by EMG. Active sensors have a small built-in amplifier inside the pickup housing.
They also require a battery (usually 9 volts, or sometimes 18 volts) to operate. You cannot measure the sensor resistance with active-type pickups because the output side is really an operational amplifier. So all you can do is connect them up correctly, and try them out.
If it does not work, it may be the sensor (and amplifier inside it) or you may have some other problem like a bad control potentiometer.
If you have one sensor, like a soap bar type, and you have problems, your solutions will not be too complicated.
If you have two or more sensors, you have the added problem of correct phasing, which is corrected by changing the wiring of one of the sensors.
Another common problem is no signal, which usually results in just a hiss. This is usually the sign of an opening in the circuit, which could be a break in the magnet wire (not likely), or bad ground (most likely).
Poor soldering methods can lead to cold joints and poor electrical connection will result, leading effectively to an open circuit. If the solder joint does not flow onto the part or tab connection well but instead looks like balls of solder, lots of flux paste residue, and carbon residue, you likely have a bad connection that will need to be re-soldered.
Another issue is hum. This can be caused by electrically exposed wiring in the bass itself. (My 1974 Rickenbacher 4001 bass had no shielding at all!) The wiring compartment should be shielded with copper foil or CRT(Cathod-Ray-Tube paint–a resistive paint) paint, and then grounded to the system ground in the compartment.
If your hum only occurs when your hands are off of the strings, then you likely do not have a grounded bridge. Bridges must be grounded. Sometimes the hum comes from a poorly shielded control cavity.
Sometimes you can install a brass or copper plate under the sensor, and ground it into the system. Be careful to not short the sensor to the brass plate. You can use electrical tape between the sensor and the plate to prevent this from happening.
Avoid using aluminum or aluminum foil as it is hard to electrical bond to the material. Also, aluminum oxidizes and increases resistance, especially at joints.
Humbucking sensors should not hum, this is the whole point of making them. But if the wiring to them is open to electrical signals, you may be getting a noise from lights and nearby radio stations (which always gives a professional impression in most gigs).
If the wiring from these pickups is not shielded, the wiring should be twisted, which causes the hum and noise to cancel when it hits the wires. P-Bass split sensors are meant to be wired out of phase, so that the noise signals which fall onto the G & D coil, also fall out of phase onto the A & E coil, and thus cancel each other out.
Jazz bass sensors are meant to be fully humbucking, but only when the volume is equal on both pickups. This is the inherent downside of the Jazz Bass since most bass players like a little more of the bridge signal for clarity (e.g. Marcus Miller).
So if you turn up the volume on both pickups on your Jazz bass, you should not get any hum. Now, here is where you might want to consider installing split coil Jazz Bass sensors. DiMarzio and Bartolini are the only manufacturers that I am aware of that make sensor like these.
The advantage here is that each pickup is really two out-of-phase parts wired in series or parallel. So with just one sensor on, you can have humbucking enabled.
The above sensor on the right is a split coil design that can be wired to be hum-canceling. However, since I have it coupled with a single coil Jazz bridge sensor on the right, I have them wired as a single-coil type.
If you were the P-bass sensor, on a bass like this, as hum-canceling, in connection with a single coil Jazz bass sensor, the result will be that two of the four strings will be out of phase (two strings will sound like they have no bass).