This project was supposed to be a quick quick one, but it ended up taking nearly the entire month of October to complete. it was't terribly difficult to build, but this month has been so full of distraction. initially all i had intended on doing was modding up a toy keyboard with some simple mods that could be built quickly, and would yield some interesting sounds. i started with a Rhapsody programmable electronic organ. i forgot to take a picture of mine before i totally dismantled it, but i did find a pretty good picture of it on google images.
The keyboard comes equipped with a built-in sequence recorder and rhythm generator, so i thought it would make a great base for a novelty synthesizer. the rhythm generator was actually just two separate pulse generators that were filtered slightly before being sent to the amplifier. my first thought was to add a step sequencer that could be counted up by one pulse-generator, then counted down by the other. it seemed to work, but for whatever reason, i decided not to include the pulse generators or the internal sequence recorder in the end. i think the reason was that the pulse generators made a popping sound whether they were connected or not. i only wanted to use them as a clock source, and not hear them at all. they could also be heard when the recorded sequence was playing back, so i just got rid of both features and moved on.
Keyboard case cutting before and after.
I came up with a rough idea of what i wanted the keyboard to do, and breadboarded the circuit. the keyboard pitch would be sent through a bi-directional 8-step sequencer. the sequencer would also control the pitch of a little drum voice chip i found in the basement. it was an HT82105 from an old drum toy from the 80s. the chip has four drum sounds that would be triggered by the sequencer clock. since the pitch of the drums would be sequence by the same hardware that pitches the keyboard, i thought it would be a good idea to have the two voices be pitched in opposite directions. so basically, pitching the keyboard voice up will effectively pitch the drum voice down. the drum chip is kind of unique in the fact that when the voice is triggered, the drum sound will repeat as long as the gate is high. at higher pitches, the voice is able to repeat more than at lower pitches. to get a little more versatility from this effect, i added a duty-cycle control to the sequencer clock to be able to adjust the gate time to the drum voice.
The melody voice would be sent to two CD4046 PLL circuits. one would output 1/2 and 1/4 the frequency of the keyboard voice, and the other would output X2, X4, X8, and X16 the frequency of the keyboard voice. each PLL circuit would have individual filter controls, to modulate the tracking speed of the circuit. one of the great things you can do with PLL chips is drastically reduce the tracking speed and efficiency by replacing the filter resistors and capacitors with ones that are not nominal. the result is squirly, phasey, rough and beating tones reminiscent of circuit bent sounds, but much more stable and not overdriven. the voices have a mind of their own, and that is what i want!
Anyway, then the two PLL voices would be mixed together with the original keyboard voice via switches, then sent to a 12db lowpass filter before being sent to the power amplifier. the filter would be slightly resonant, and the cut-off frequency could be controlled either manually, or with an envelope generated by the sequencer clock. there is not really any decent way to shape the modulating envelope to the filter without effecting the sequencer speed and the drum trigger gate time, but because there is a duty cycle control, the envelope can be changed. everything being connected in some way is kind of a recurring them in this synth.
Schematics and PCB layout.
Once all of my ideas were on a breadboard and working, i drew up some circuit boards and faceplates. surprisingly the circuit boards came out with no problems at all. i think i'm getting good at this. the one thing i forgot to include in the faceplate design though, was the Key Hold switches. unfortunately there was no way to hold notes on this keyboard without making a physical connection to the note switches. i wanted to be able to hold any of the 25 notes, so i would have to use multi-position switches. the keyboard chip has 8 data lines, and 4 common busses, but the 4th is only responsible for one note and all of the other unused functions, so i left that one out, so the high F can't be held. also, because the PLL circuits need a gate signal to disable their VCO's while a note is not playing, i used the gate signal coming from the 4th common buss. unfortunately though, the PLLs cannot be gated when the high F is played, only the original keyboard voice will sound.
Anyway, to hold the 24 notes, i used a combination of a single pole four position switch with a single pole eight position switch. the 8-position switch will select the note address, and the 4-position switch will select the one of three common busses, or off. unfortunately the 8-position switch i used is a "make before break" type, which means that the note cannot be changed with the 8-positions switch alone, the 4-positions switch will need to be turned to activate a changed note. sadly, i didn't realize this until it was already built. good to know for next time though.
Sequencer board on the right.
Fitting everything in the keyboard enclosure was kind of a challenge. there was not really a lot of room to work with, and it all got filled. the finished keyboard is actually pretty heavy for its size. i had thought about painting the keyboard enclosure a different color just because of how old and sun-stained the white plastic had become, but then i started to like the stains, and started to think that painting the whole thing might just make it look too "arts-and-crafty". it's not like it wouldn't be obvious that i painted it. also, it's been super wet here in Portland lately, and that has caused me more than a few paint-job heartaches in the past. best to let it be itself as much as possible. i am glad i did, because i don't think i could be happier with the way it turned out.