Making A Guitar Pickup
This article describles the making of a guitar pickuo for an 8-string lap steel guitar.
Here is a picture of the 'flatwork' before being cut and drilled. It is made from fibreglass printed circuit material with the copper etched for so as to provide electrostatic shielding, notched so that it does not form a shorted turn. Termination points fo rthe winding wire and outgoing connections are also provided.
Drilling the flatwork is not as simple as you may think. Any inaccuracy caused by the wandering of the drill results in uneven spacing of the magnets which is readily visible. I made a drill guide from 10mm square steel bar. It was clamped in my lathe tool post and drilled at 3/8 in centres for the 8 magnets and for the mounting holes. I clamped the guide to the flatwork pieces for drilling.
The main difficulty here in in aligning the guide with the etching. This I did by aligning the mounting hole guides while viewing against a light.
The holes for the magnets were drilled slightly under-size and then opened up with a round file to give the desired fit for the magnets. They were tight and had to be hammered (very lightly) into place. The magnet position was set by 16 gauge tin-plate and 10mm wooden spacers. It turned out to quite difficult to align and hammer the magnets at the same time. So much so that I intend the make a jig for next time, if there ever is a next time.
Below is the assembled pickup prior to winding. I am applying a little super-glue just to "make sure".
Winding The Coil
Now comes the Interesting bit, putting on the copper wire. Here is a photograph of my pickup winder. Made from the "junk-box" and not very pretty, it works quite well. It has an electric motor which does the winding, a stepper motor which controls the position of the wire and a cam operated wire guide (the rod emerging though a slot on the right). The wire goes around this guide and as the pickup rotates, it moves back and forth to give a constant-velocity feed for the wire. The winder has for its brain, a pic micro-controller. A PIC16F84 if I remember correctly!
Seen here, it has a pickup spool mounted on its faceplate and you can just about see the wire. The wire unwinds from the end of the reel, over a tensioner (above and unseen), down to the oscillating guide, through the horizontal and vertical guides of the wire positioner and to the pickup spool.
The micro-controller contains a number of "recipes" given wind speed, wire thickness and advance per turn. A recipe is selected by number, the positions of the two flatwork cheeks are input and the winder --, well --, winds. It stops when it has finished.
Here is a video of it in action
The winding of 7000 turns is complete and the winding is pretty even. The accuracy of the pickup spool and the setting of the travel of the wire feeder are all important to achieve this level of even winding. It is not easy. Indeed the other pickup of the pair is not this good. I have enhancements to the winder in mind to help with setting the travel. Probably won't ever make them.
Note that the connection is made with a short length of insulated, flexible wire. I find it easier to solder these fine wires to a thicker wire than to a PCB. Also it allows some flexure without straining the winding wire. The other end is terminated similarly, although with a much shorter flex.
I wrap the winding in insulating tape followed by copper tape, which is connected to the ground planes, then with linen thread. The whole thing is then immersed in hot candle fat for an hour then allowed to cool.
Once off the winder. I measured the coil resistance. It read 15.3kΩ. Far too high. Both pickups are the same so it does not appear to be a connection problem. Checking the winding wire, I see that I have used the wrong wire. 48 gauge instead of 42 gauge. I'll see how they sound before I rewind them. Though I expect they will lack treble bite.