Signal Injector for Pickups

General

This device can inject a signal into the face of a guitar pickup without strings installed.

There have been many times I have wired a guitar from scratch, made a modification, or performed a repair. After putting the guitar back together, I find that I have overlooked something or made a mistake. A Telecaster has a control cavity so most problems can be resolved without removing the pickguard or strings. I really hate it when I work on a Strat, get all the strings installed, and find out I have a wiring problem and need to take it back apart. One method I used for years is to take a screwdriver and tap on the pickup you are testing to see if the switch and volume controls work. This works OK, but the "thud" created by tapping the pickup with a screwdriver is a very low frequency signal. This sound is about the same with the tone control at any position, so it's not the best source for testing the tone control operation.

I have seen some innovation in this area. In one case I saw a fixture that holds a guitar string under tension that can be placed over the pickup and plucked. I decided to make an electromagnetic signal injector that can be placed over the top of the pickup and inject an audio tone. It is nothing more than an 1kHz oscillator that is connected to a guitar pickup.

The Transducer

I chose a very cheap Telecaster neck pickup for the transducer. I removed the cover and pushed out the magnetic pole pieces. It would not be a good thing to wave a magnetic pickup over the top of the pickup under test. It could potentially alter the field of the magnet (I presume). Magnets are not required since induction of the signal is accomplished by the magnets already on the pickups in the guitar. You could use any old pickup for this, but I would recommend you chose one that has easily removable magnets. Here is the Tele neck pickup with the cover and pole pieces removed. This is little over a mile of 40AWG wire wrapped around a plastic structure.

The Oscillator

I wanted to make the simplest oscillator possible to drive the coil, and I wanted square wave. A sine wave is much harder to generate, and square wave contains many harmonics. The harmonics will be at high frequencies so the effect of adjusting the tone control while testing will be quite apparent.

One very versatile device is the LM555 Timer made by National Semiconductor. It can do many things, but in this application, it is an Astable Multivibrator (fancy word for oscillator). The typical LM555 astable application is configured below:

It cannot produce a 50% duty cycle the (time the signal is high is the same as the time it is low). There is a little known configuration that can result in a near 50% duty cycle. It can be accomplished with only (1) external resistor and (1) external capacitor shown below.

NOTE: Most images below this point are small thumbnails. Click on the image to get a pop-up image in larger resolution.

Parts List

Here are the required components. The guitar pickup, resistor capacitor, momentary contact pushbutton switch, project box, battery, battery clip, and a project circuit board. The parts list has active links to the suppliers.

Assembly

This project begins by cutting a Radio Shack project Printed Circuit Board (PCB) to size. Measure and mark the board 0.8" from the mounting hole center. The holes are on tenth inch centers.

One method of cutting the PCB is with large shears as shown. A hacksaw will suffice as well.

Here is the board layout.

Use this layout to stuff the components into the board. There are only 3 parts, but they must be placed in the same hole relationship as shown in the layout.

Bend the leads slightly on the backside so they don't fall out of the board.

Insert the board in a vise to hold it while soldering.

Solder all the leads to the round copper pads. Then use bare and/or insulated 30AWG wire to connect the circuit per the layout diagram. (Make sure to click on image for higher resolution)

Here is an image of the layout mirrored that you can print as a guide that is representative of the back side of the board. Notice how it matches the connections above.

Now connect the components external to the board. There is a ground (GND) for the battery clip and a ground (GND) for the pickup. The red wire of the batter clip is cut in half and the switch is wired in series with the battery. The white wire of the pickup goes to (PU SIG). It would be a good idea to test the unit now before going further. Hold down the switch and wave the pickup over the pickup of a plugged in guitar. You should get a 1kHz tone with rich harmonics.

Drill the mounting holes for the pickup and switch. Countersink the holes for the pickup so the bottom of the box will be smooth. The screws required for a Tele neck pickup are #4-40. Here is the finished assembly. The PCB can be fastened to a boss in the floor using a single short #4-40 pan head screw.

To secure the battery, cut a piece of foam (polyethylene in this case) and spray one side with 3M Super 90 (or equivalent) and lay it on the battery with the adhesive side up.

Place the cover on the box so the foam will stick to the cover.

Here is the assembled unit.

Testing

To test a guitar, simply hold the box with the pickup side next to the plugged in guitar under test while pressing the switch. A 1kHz square wave signal can be heard through the amp. Work all switches and controls to see if they work as planned. Here is a video example. I apologize for the poor quality. This was uploaded to YouTube directly from my Blackberry.

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©2009 Terry Downs Music™