Electronics help - measuring resistance

fjon

I'm doing some troubleshooting on a pre-amp I have installed on bass and have been asked to measure the resistance from the pickups. Specifically, this is what I need to do:

"Disconnect the pickup from the preamp.
Measure the resistance from either pickup lead back to ground.
Measure the resistance from the pickup hot to pickup common."

I have a multi-meter, but am really struggling with understanding how to use this.
I don't suppose anyone on here would be able to give me some pointers on how to do this?

Quillo

You sellect the Ohm range on the Multimeter (indicated by the greek letter Omega) to measures the dc resistance.

Single-coil pups are usually less than 10kOhm and humbuckers less than 20kOhm so select an appropriate resistance range when measuring (say, 20k for singles coil and 50k for humbuckers).

fjon

Quillo wrote: »

You sellect the Ohm range on the Multimeter (indicated by the greek letter Omega) to measures the dc resistance.

Single-coil pups are usually less than 10kOhm and humbuckers less than 20kOhm so select an appropriate resistance range when measuring (say, 20k for singles coil and 50k for humbuckers).

Great - thanks! They're single coil, so I'll choose the 20k setting.
I've never used one of these things before, so not sure which of the two wires (probes?) I need to use, and where on the pickup wires I need to measure.

Quillo

A pickup is basically a long piece of insulated wire wrapped around some kind of magnet. In measuring the dc resistance you are measuring the resistance of this piece of wire. Some pickups have just the two wire ends coming out of them and so measuring is easy. Other have a coaxial arrangement where one wire is in the centre and the other wraps around it. Other have the two wire ends wrapped in an outer shield (and so on). Basically you need to find what are typically a black and a white wire and measure the resistance between the two ends.

On the metre, if its just two leads, use those. If it's two leads, but three sockets, then use the common socket and the voltage/resistance socket and avoid the socket used for current measurement.

If you Google "single coil pickup wiring diagram" you see lots of wring examples that may help.

fjon

Thanks, good information!
I set up the meter as you describe, and touched the red probe to the "hot" wire, and the black probe to the "common" wire (I ignored the ground wire).
The reading seems to fluctuate quite a bit, but it seems to be around 7-8, which seems about normal.

The one thing that I'm not sure about is

"Measure the resistance from either pickup lead back to ground."

I've tried touching one probe to the ground from the pickup, and the other to both the hot and the common wire but I'm not getting a reading for either.
Any idea how I measure this?

Quillo

Pup resistance sounds fine. A little fluctuation is normal.

With the pickup out of circuit the should be an open circuit (ie, infinite resistance) between each wire and "ground" - in circuit, the black wire is typically connected to ground and so you should get zero resistance from black to ground and 7-8kOhm from the white wire to ground whether in a passive circuit like the one below or in a active circuit (the input resistance of the preamp in an active circuit is typically much higher than pup resistance and so the multimeter will see the pup resistance to ground largely unaffected by the presence of the preamp).

See the wiring diagram for a passive Seymour Duncan single coil J pup setup at:

http://www.seymourduncan.com/images/products/basslines/501030-105.pdf

Really all you can check here with a multimeter is that the pups are not shorted out and that there is 9v (or 18v or whatever) going to the preamp and the things that are supposed to be grounded are grounded. Beyond that, you're talking tracking signal path with an oscilloscope kinda stuff.

fjon

Thanks!
Try as I might I can't really get my head around the second bit though!
The wiring for my bass looks is similar to the one in the picture, except I have a 3rd (ground) wire coming from each pickup (and also a ground wire coming from the bridge).
I'm touching one probe to the PU ground wire, and the other to either the white or black PU wire, but I can't get a reading with either (display says 1).
The only time I get a reading is when measuring the white and black wires.
Any help on measuring the ground resistance?
Sorry for the questions - this is all brand new to me.

Quillo

The third ground wire is probably a shielding wire. In circuit it would be connected to ground and to the black wire.

Out of circuit, if the black and white wires are disconnected there will be no resistance reading to ground from either of them. You will on get a resistance reading to ground when they are connected.

Can't really offer any more advice than that - suggest you take the guitar to someone with a bit more experience for a lookee see.