Acoustic guitar

Build Your Own Guitar DCR Test Tool

Welcome to Mod Garage. After receiving many requests to show more DIY tools for guitarists, today we’re going to explore one of my favorites. For years I’ve used this one daily in the store and I’m sure you’ll like it. It’s cheap and easy to build, but very effective at analyzing the circuits of electric guitars and basses without opening the electronics compartment or lifting the pickguard. It is a kind of adapter or extension to measure the DC resistance (DCR) of a pickup from outside the guitar. After building one, we’ll see how to interpret the measurements.

A mic’s DCR is by far the most common parameter you can read when reading mic descriptions and is often used as an indicator of the output. The reason for this is that it’s easy to measure, but, unfortunately, it tells us nothing about a pickup’s output or its tone. To quote pickup truck designer Bill Lawrence: “DC resistance tells you as much about a pickup’s tone and output as shoe size tells you about a person’s intelligence.

DCR is not a primary parameter in pickup design. It’s simply the result of the pickup wire type and gauge, number of turns, and other parameters like winding pattern, etc. But it’s not completely useless, and we can use it as a good benchmark for analyzing pickups. inside and outside a guitar or bass circuit. All you need for this is a digital multimeter (DMM). You don’t need an expensive calibrated precision digital multimeter – any entry-level digital multimeter will work. You can get a simple digital DMM for $10, but if you want to invest in a better device, that can’t hurt.

The easiest way to analyze a mic is outside of a circuit. Simply set your DMM to ohm and connect both sense leads to your DMM. If your DMM does not have an autorange function, set it to 20k ohm. You will now get the DCR reading for your pickup. You can compare it to your pickup’s factory specs and it should be close. If your digital multimeter reads “infinity” or “overload”, you know the sense lead is broken. Let’s say your mic should read 7k ohm, but yours reads around 2-3k ohm. Your pickup probably has a short somewhere in the coil. Used this way, DCR is still good for quickly checking whether a mic is active or not.

To quickly analyze a guitar or bass circuit with one or more pickups, you first need to build the DIY matching tool discussed in this article. There are two different versions, and you don’t need much for it:

  • Version 1: This is the quick and dirty version. You need a standard 6.3mm straight mono plug (the same type on all your guitar cables), wire of your choice (preferably two different colors), and two insulated alligator clips.
  • Version 2: A sleeker version that you can also use with a scope if you have one. You need the same parts as version #1, but instead of two alligator clips, you need two 4mm banana plugs, and both wires need to be longer than you would use for version #1 .

So fire up your soldering iron and let’s get to building version 1.

  1. Solder a piece of wire to the HOT terminal of the mono jack and another to the GROUND terminal. I prefer a red wire for the HOT and a black wire for the GROUND terminal (Picture 1).
  2. Solder an insulated alligator clip to each end of the two wires, preferably one black to the black wire and one red to the red wire. Ready!

Version 2 is constructed the same way, but, instead of alligator clips, you solder a 4mm banana plug to each end of the two wires, if possible also black and red. Both wires should be long enough to place your DMM and/or scope some distance from the guitar. In Picture 2you can see version #1 at the top and version 2 on the background.

The difference between the two versions is that with version 1 you put the plug in the output socket of the guitar, connecting the two probes of your DMM to the crocodile clips: the black probe of the DMM goes to ground (black wire) and the red probe goes to hot (red wire) as we see it in Picture 3. With version 2you need to remove the two probes from your DMM, plug the two banana plugs directly into your DMM or oscilloscope, also seen in Picture 3.

Both versions work equally well. Version 2 is just easier to use when you also want to use the adapter for a scope.

For a quick check you can also touch the hot and ground terminals directly with your DMM’s probes, but you need both hands or a second person for this if you want to fiddle with the controls or the pickup selector .

Now we can easily check four things with this tool, assuming everything is connected as it should be and your DMM is set to ohm and auto range or 20k ohm scale if your DMM doesn’t have auto range mode autorange:

  1. Are you getting a reading on your connected DMM? If not, check if the volume pot is fully open. Are you getting a reading now? If so, close the volume pot completely and see if you still get a reading. Nope? Great, you just proved the volume pot is alive and well.
  2. With a volume pot fully open and a reading on your DMM, slowly lower the volume and watch the reading on the DMM. If you’re getting a crazy reading, chances are there’s a treble bleed network on your volume pot. If the reading slowly drops to zero, you know there is no treble bleed network on the volume pot, and you can check if it is an audio or linear volume pot ( plus the cone it has, if it is an audio pot). Let’s say we have a volume pot of 500k. When you close the volume pot halfway and receive a reading of around 250k, you know it’s a linear pot. An audio pot, depending on its taper, will result in a much higher reading on the first 50 percent of the volume pot. If you read 500k until the volume pot is almost fully closed, that means the pot has an audio taper of 90:10, exactly the type of volume pot you don’t want to have. If you read something around 300k in the middle of the volume pot, you know it’s a 60:40 audio cone.
  3. If the volume pot is fully open and you’re not getting a reading from your DMM, chances are your output jack is broken, not connected, or improperly connected. Please make sure there is no kill switch activated in the circuit which can also cause this “problem”.
  4. Turning the tone knob(s) will make no difference to the reading you receive. If you receive a slightly higher reading with a fully open pot compared to when it is closed, you know it is a no load pot.

There’s a lot to discover from the outside of any guitar or bass. Now let’s see what we can measure from outside the instrument starting with a Telecaster with a 4 position selector. The readings in all the examples are the readings I received with guitars I had in the store, but they may be different in your instruments:

  • Bridge pickup only: 5.85k ohm
  • Neck pickup only: 6.76k ohm
  • Both mics together: 3.18 k ohms
  • Pickup selector in position #4: 12.30 k ohm

Readings for both pickups are within factory spec and within a typical range for a set of vintage-flavored Telecaster pickups. With a reading of 3.18k ohm for both mics together, you know both mics are in parallel. With the 12.30k ohm reading, you know the two pickups are in series with each other.

Here are the simplified calculations behind these readings:

  • Serial connection: Micro DCR #1 + Micro DCR #2
    • In our example, it’s 5.85k + 6.76k = 12.61k ohm, which is very close to the 12.30k reading we received. The missing 0.31k ohm is swallowed up by the resistance of the potentiometers and the tolerance of your DMM. For this test, I chose the cheapest DMM I could find in stores. A high quality calibrated DMM will have much less tolerance.
  • Parallel connection: (micro DCR #1 + micro DCR #2) divided by four
    • In our example, that’s 5.85k + 6.76k = 12.61k ohm divided by four = 3.15k ohm, which is very close to the 3.18k ohm reading we received.

Now let’s repeat this with a standard Stratocaster:

  • Bridge pickup only: 7.07k ohm
  • Middle pickup only: 5.88 k ohms
  • Neck pickup only: 5.70k ohm
  • Bridge + Middle pickups together: 3.26k ohm
  • Neck + middle pickups together: 2.94k ohm

All three pickups are factory specs for this Strat. We have a slightly warmer bridge and two vintage-flavored pickups. The two intermediate positions are in parallel.

Finally, let’s try a vintage Les Paul loaded with PAF:

  • Bridge pickup only: 7.77k ohm
  • Neck pickup only: 7.09k ohm
  • Both pickups together: 3.74k ohm

Both PAFs have the typical vintage DCR and are parallel in the middle position.

That’s it. Next month we’ll be looking at changing the wires on the pickups in more detail, which I’ve been asked a lot about, so stay tuned!

Until then…keep modding!

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