I was recently sent back this keyboard that I circuit bent some nine years ago. I remember spending a considerable(relative) amount of time making it, and in the end I was not very satisfied with how it turned out, but it worked, and I ended up selling it. More details on the original build can be found HERE. The keyboard had stopped working, so the owner contacted me to ask if I would repair it. I agreed, and before long, it was back home on the bench.
When I first powered on the keyboard, the first thing I noticed, other than no power was that the supply was being shorted, so I opened it up to see if it could be remedied easily. I was greeted by a massive pile of rainbow ribbon cable, perfboard circuits and lots of hot glue. It really took me back seeing that mess, and how I used to have to build things. However nostalgic I was feeling, I was in no mood to try and dissect the circuit and all of those wires. Instead, I felt it would be easier to just completely gut the keyboard and rebuild it, and it was easier, even if it took a good week or two to finish.
I stripped the keyboard down to the single COB PCB, and plugged it into a breadboard. I then whipped up a VCF, two LFO's, an amplifier, an auto-power-off disable circuit, some summing and boosting amplifiers, a CMOS distortion circuit with PWM, and a bunch of CV modulation buffers for the pitch bend, cutoff, PWM, LFO's, and distortion. Everything was pretty simple an straight forward, so the circuit came together pretty quick.
The original faceplate was looking pretty rough, and most of the hardware was nasty, so I decided to cut a new faceplate, and use all new hardware. It was pretty straight forward, but space was limited, and when I was trying to lift up the keys, one key broke off.. The broken key set me back about a day, but I was able to find a replacement set that was only slightly smaller, but nice and new. The new keys also gave me a lot more room to work with since the original keys were and old set of used casio keys on top of clicky typing keyboard switches with lots of hot glue. They worked well enough, but took up a lot of space. I decided to add dust cloth to the back off the keyboard, since the PCB would be located pretty much under the keys, and debris could fall through the keys and compromise the circuit. Dust cloth is pretty useful. I also used some for the speaker cover after noticing the speaker magnet was sucking up all kinds of cut-off component leads, and making all kinds of buzzing.
|ROUGHING NEW KEYS|
|ROUGHING NEW KEYS|
Once the interface layout was decided, and the faceplate was all drawn up, I engraved and cut out the faceplate on my lasercutter. I got it right in two tries.
The PCB was straight forward enough, but it was a little difficult going back to single sided CNC engraved circuit boards after being spoiled with PCB software and board-house PCB's. I only needed one though, so the CNC would have to do. The main PCB ended up taking a couple of days to design, cut, and populate. In the end, there were very few errors, and they were easy to remedy since the traces are so big. That is one advantage to engraving PCB's on a CNC.
|WIRING IN PCB|
|PCB REVERSE SIDE|
The keyboard's new functions are essentially the same as they were before. Maybe a little bit more reliable though, and more effective. The keyboard chip on its own has several functions. Some are more redundant, like volume up/down, and demo songs, so I left them out. In fact, the auto-power-off disable circuit is just an oscillator the repeatedly triggers the volume up switch. When the keyboard reaches its maximum volume, the switch makes no sound, and resets the A-P-O circuit continuously.
the functions I did include are the eight drum beats, sixteen timbres, tempo up/down, and record/playback. All of the functions are selectable with the eight push-button switches. The modes of the push-buttons are selected with the respective right or left shift switch. The four buttons on the left can select rhythms 1-4, or playback, record, tempo up/down. The four buttons on the right can select rhythms 5-8, or the 16 timbres with a combination of of two button presses. The lower octave of keys can also be shifted to one octave above the higher octave. This was a feature on the original mod that I though should be included since there are only two octaves of keys.
The keyboard's main voice output is sent to an amplifier circuit to be scaled for the VCF input. the output of the amp is also sent to another amplifier with very high gain. That amplifier sends it's signal to a couple of NAND gates that have a sort of PWM filter. The effect is a heavily distorted signal that has a sort of high-pass filter, or "d-color" as I've called. The distortion signal is mixed back with the clean signal via "clean" and "distortion" volume pots before being sent to the VCF. The VCF is my go-to LM13700 12db resonant LPF. The VCF signal is then sent to an LM386 power amplifier to drive the output. There is also a line input that is summed to the main keyboard signal, so external signals can be sent through the distortion and filter circuits in the same way the keyboard is. The line in on the original mod did not work very well, from what I remember, but this is a huge improvement. There are two LFO's for modulation. Each has triangle and square wave outputs, and the LFO rates are controlled with a joystick. The LFO's have depth control pots, depth/frequency indicator LED's, and four-way select switches to modulate CV parameters. LFO1 can modulate "d-color", "pitch", or "cutoff", and LFO2 can modulate LFO1 rate, "pitch", or "cutoff". The joystick can also be set to modulate any combination of cutoff, pitch or d-color as well.
This keyboard is pretty sweet now. It is a whole new beast, and I am glad that I was able to finally get this keyboard to work in a way that I am actually satisfied with. Over the years I have taken on some pretty huge and long winded projects that this original mod doesn't even come close to, but at the time, this project was pretty defeating, and I think that stayed with me in a small way. It is nice to see this new thing connected to that time, and to see how far I have come since then. Now I must say goodbye again, and return it to its owner. See you again, maybe in another nine years :)