I decided to try something new this week. I have a couple old guitar pedals laying around my shop and I wanted to open them up and see what I could do as far as circuit bending them. The first I bent was a Danelectro T-Bone Distortion mini pedal. I chose this as my first because it is small, relatively uncomplicated and most importantly cheap. I have a second Danelectro mini pedal (A BLT Slap Echo) which I will be bending next but before I do I wanted to get some practice with pedal bending. Also this distortion pedal will give me a better understanding of how these devices are put together as the two pedals are constructed very similarly.
After opening the T-Bone Distortion pedal up my suspicions about it’s simplicity were confirmed. The pedal is made up of two separate boards connected by a short ribbon cable. The lower board (brown board in this image) appears to primarily handle the functionality of the switch and the inputs and as such I will be largely ignoring it. A quick look in my BLT pedal confirmed that this lower board is shared between all pedals in this series and is therefore not involved in the effects production. I will be focusing my efforts on the upper (green) board.
The effects board on the T-Bone Distortion pedal appears to be built around an LM324 quad op-amp chip. This chip is essentially four op-amps (LM741s) arranged on a single chip with shared power and ground. Another helpful thing I found with this board is that there are numerous unused solder points throughout the circuit which made experimentation and modification extremely easy. Likely this board was developed to serve multiple purposes or be used in multiple pedals depending on which components or points were used.
By feeling around on these unused solder points I was able to find a number of areas which created additional effects though the majority of these effects were very similar to each other. In the end I decided to keep things simple for this project and settled on three soldering points. When these points are used in combination they allowed me to create two new effects which I found interesting and fairly unique. By connecting the blue wire shown above to either of the green wires (through a potentiometer) you can create these effects.
For the first (using the green wire on the left side of the board) I used a 200K ohm potentiometer. This creates a high gain bass boost effect. As you get closer to 0 ohms of resistance this boost devolves into a crunchy noisy mess and the melody for your guitar (or other inputs) is all but lost. That being said if that is the effect you are after it is quite pleasing. As you raise the resistance up close to 200K though you can get a very nice (if still a bit crunchy) bass boost added onto the melody you are playing.
The second bend is a bit harder to describe. For it I used a 10K pot and added a 27K resistor in series. With this bend the pedal will die if you lower the resistance below about 25-26K so the extra resistor stops this from happening. with no inputs going to the pedal this bend will create a smooth square wave oscillation, the pitch of which can be adjusted with the potentiometer. Once this is connected with inputs going to the pedal though things get a bit strange. The oscillation and the audio playing through the pedal begin to modulate each other and create some very fun and interesting effects.
Since there is very little free space in these pedals I opted to run the wires out to a small container I had on hand. I drilled a small hole in the side of the pedal and ran the wires out through there prior to attaching the switches and pots. This is not a very permanent solution and I expect I will be re boxing this pedal down the road. That being said it works for the time being. To protect the wires I loosely wrapped them with electric tape (heat shrink tubing will give a cleaner effect if you have some on hand) and created plugs for the holes on the pedal and box using hot glue. This will prevent the wires from being tugged which could disconnect the solder points. With that my Danelectro T-Bone Distortion was ready to play!
That’s it for today, I hope you guys enjoyed this project. Once I have finished the BLT echo pedal I will upload a video showing off the effects. Until next time, happy soldering!