Sunday, June 5, 2022

EHX 16 SECOND DELAY MOD




A friend of a friend contacted me in the summer of 2020 to have his EHX 16 Second Delay reissue modified. I had done some pedal mods for him in the past, so I was willing to give it a go, even though I was pretty bogged down with remodeling projects in my house. I was happy to be in my shop again and working with something other than drywall.. I didn't end up finishing the mod until the winter of 2021, but the owner didn't seem to mind the wait. Thanks Joe!
I can't remember what exactly he originally asked for, but I think he complained that the sound was too digital and he wanted to be able to switch the feedback on and off. Those were easy enough things to address, but I also came up with a bunch of other mods that I thought would compliment the pedal, and he agreed. So I spent some late hours of the nights that I wasn't too sleepy to mod this crazy beast.
The original plan was to create an analog signal path from the output of the pedal to the input with highpass and lowpass filters, but unfortunately there was just too much aliasing noise feeding back and being amplified, so I had to scrap that plan. I kept the filters, just created an alternative analog output to give the pedal a warmer sound. At least I think that's what I did... To be honest, at the time I am writing this, I am having trouble deciphering my notes and schematics, and my Kicad files are a mess from a catastrophic version change... I do not have a video to reference and the pedal got shipped home over a year ago, so I am just trying to recall from memory. I am pretty sure that the filters do NOT feed back into the delay though.
All of the controls on the pedal are 3.6 volt CV inputs so I added mod switches for most of them to be modulated by either a guitar audio triggered envelope generator or an external CV/pedal. The slope of each can be inverted independently. I drew up a pretty useful manual, so I will defer to that... 
I found a pretty cool local seller of high quality toggle switches. I love the stubby little ones!
Well, I guess I'll keep this short since I cant recall too much about building this. I will say though that this pedal is powerhouse of drone! If you see one, get it! It is always a treat to occasionally mod or repair rare things that I was never aware existed. I only wish i had recorded it before sending it back.






FUNKY GLITCH BUDDY-REVISED



While cleaning shop, I came across an old friend. I built this little keyboard monster back in 2013. The FUNKY GLITCH BUDDY was an interesting concept, but it suffered from some pretty challenging design flaws that I had always wanted to fix, but never had the time. For one, the keyboard would draw so much current due to the fact that the keys were all infrared proximity sensors. There were a lot of 7400 series chips and opto-isolators in the design too, so overall this thing was pretty inefficient and noisy. Two of the voices were 8-bit LFSR noise generators which didn't particularly sound like noise. The third oscillator was a basic two transistor oscillator that I ripped out of a cheap organ toy, and had died some time ago. At the time of this build, I had just started using my CNC to engrave and cut my own circuit boards. This was my first project using the CNC, and the design and orientation was pretty sloppy. Everything was jammed together, and I never even took the time to draw out my schematics, so finding and fixing problems years later was difficult to do. Instead I thought it would be quicker and more fun to just completely redesign the circuit.
The concept for the new version would pretty much stay the same, but there would be three LFSR noise generators instead of just two, and the pattern select keys would just cycle through the patterns rather than shifting through patterns depending on how far down the key was pressed. I replaced all of the 7400 series chips with their 4000 series counterparts, and got rid of any opto-isolators and unnecessary reference voltage trimmers. I used OTA VCA's. Unfortunately I hadn't yet discovered the Lunetta style diode VCA' at this point, which would have been perfect for this application... Oh well. Since the pattern select switches were sequential, I had three extra switches to make use of. The original design could only shift through 8 patterns with the variable-press keys, so I had "SHIFT" switches for each pattern to access the other 8 available patterns. Now that I didn't need the shift switches, I used them to set different taps for the LFSR noise generators. I have found that when it comes to LFSR noise, there are certain bit lengths that sound better than others, and they are not always the longest. Shifting the taps is a nice feature but can be problematic sometimes, because shifting the inputs can sometimes cause the LFSR to lose its "SEED"... Without going too far into LFSR's, I'll just say it was a little troublesome towards the end but I was able to create stable LFSR's with a little trial and error. I found that the more finicky tap combinations preferred CD4014 to CD4021 chips, which I don't fully understand why, but whatever, they work.. 
I had to get creative with some limitations to the interface, too, since there was no "freq" or "lfo" hardware for the voice that had been the toy organ oscillator. The pitch and LFO of the third(red) voice is shared with the second(blue), and the "amt" control is an attenu-verter, so the third voice can not be totally attenuated with the VCA. However, one of the 16 patterns is 0000000000000000, so it is possible to mute the voice that way.
The new circuitry was pretty easy to design and prototype on breadboard. I tend to use my CNC less and less these days since ordering PCB's from china is so affordable and easy. It seemed fitting that I design and cut all of my circuit boards with my CNC for this project since it was the first ever project I used the CNC for. Designing circuit boards this way is pretty challenging and frustrating sometimes, but it is gratifying when it comes out. 
The FUNKY GLITCH BUDDY is now finally up to my liking and it is pretty fun to play.











Saturday, June 4, 2022

WADDINGTONS COMPUTE-A-TUNE



Hello!?...
I have been clearing out my workshop and getting rid of things I don't want or have time for, in the hopes of someday getting back to circuit bending and blogging again, but just for my own amusement.
I am making a lot of progress, and my workshop is looking great, but I have run into some things that are too precious to throw out but not worthy of selling in the condition they are in. I had been holding onto this Waddingtons Compute-A-Tune for at least a decade, and I couldn't bring myself to throw it out. It had some major issues and was not worth even trying to sell for $20...
The Compute-A-Tune is a very old toy keyboard that i think was only sold in the UK in the early 80's. The synthesizer is based around the TMS1000, which if I am not mistaken-was the first microcontroller!
Unfortunately the original knobs that controlled the volume and "effect" had broken off, and the circuit-board inside was heavily corroded since there didn't appear to have ever been a solder mask. 
So rather than just throwing the whole thing away, I decided to give it a face lift. I started by reverse engineering the circuit, and found an additional tempo, and another chord mode. Now to come up with a some modern upgrades. I started with the power supply. The TMS1000 needs at least 15 volts to even turn on. The keyboard ran off two 9 volt batteries in series. This seemed wasteful to me so I built a circuit that would provide the TMS1000 17 volts from one 9 volt input using an ICL7660S charge pump chip. These chips can be noisy and don't deliver much current, but in this case there were no issues, since the keyboard is already pretty alias-y, and very digital sounding. The next step was to try and find a way to get the playback button to repeat after the recorded melody had played through. For this, I used a CD4098 dual monostabe multivibrator chip. I have been using these a lot lately. They are a very useful chip for lots of reasons, but in this case i used the chip as a re-settable pulse delay. Essentially, the gate output from the recorded melody re-triggers a a delayed pulse, and as soon as there is no gate to reset the delay, the pulse is able to re-trigger the "PLAY" switch, causing the recorded melody to  play continuously.
The next thing to do was create a more interesting mix of the square wave tone and its three available sub octaves. I had seen a schematic floating around for a diode VCA used in Lunetta circuits, but I had never actually tried it before. I was pleasantly surprised with how well it worked, so I built one for the top octave and another for the three subs. I used a CD4093 quad Nand gate as inverting buffers for the VCA's, and for the sub octaves, I added a PWM filter and a mixing switch to set different combinations of the sub octaves Nand-ed together. With the remaining two Nand gates, I made two separate trapezoid  LFO's; one to modulate the PWM, and another for vibrato.
Next, I needed envelope generators for the VCA's, so I crafted two looping attack-decay envelope generators from a CD4013 dual flip flop chip. I haven't seen a circuit like it, so if you're inclined, take a look at my schematic. It is simple but useful.
Once my circuits were all tested and working, I went to work redesigning the keyboard and the interface. I cut out circuit boards on my desktop CNC, which is always fun... The most challenging part being the circuit board with the thumb-wheel pots, because the circuit board had to line up perfectly with the existing mounting hole, and the pots had to stick out of the shell just right. Somehow it came out perfect the first try. Completely measuring the shell and creating a 3D model of it helped.
Assembling everything was slow and tedious. I really didn't want to screw it up, and there wasn't a lot of room to make mistakes. I was able to find a battery compartment that hold 6 AAA batteries from a strange flashlight repair parts site. It seemed like a strange site, but the battery compartment was only $4, so I gave it a shot and it arrived in a couple of days. The battery compartment needed to modified slightly, but it fits perfectly inside the original cavity. 
There were a few mistakes along the way, but for the most part, everything came out as planned. One thing that I only noticed once the mod were done, was that the "TUNES" function is pretty much useless... I had thought that you could use the four settings as places to store separate recorded melodies, but I was wrong, and they only work to set the demo melody when the power is reset. Had I known this, I wouldn't have included the setting in the mod. Oh well.
I took lots of pictures along the way. click on any of them to see larger views.