upon doing some random research for circuit ideas, i decided it might be time to finally find out what PLL's do. when i started circuit bending some 10 years ago, i remember reading about Phase Locked Loops in a textbook. i think i got about two sentences in before my eyes crossed in boredom. since then i have increased my capacity for understanding electronic applications. i couldn't believe how simple it was to understand. basically, in the case of the CD4046, there is a voltage controlled oscillator that synchronize its frequency to the frequency of whatever signal is inputted to the CD4046. i guess i never understood why you would need to do that. why not just use the input frequency? well, apparently if you divide the VCO output frequency before sending it back in to the CD4046's phase-comparator, the frequency of the VCO will be increased until the divided output frequency matches that of the input frequency. so if the phase-comparator input was set to 1/2 the VCO frequency, the VCO frequency would be raised X2 to match the input frequency to the CD4046. BRILLIANT!!! i remember how exited i was when i learned to divide frequencies, and now i can multiply them!! and not just by 2, 4, 8, and so on. with the PLL you can multiply by whatever number you want. there is one drawback though, at lower frequencies the PLL takes longer to stabilize. it kind of sounds cool though because the frequency stutters a bit before it locks in to sync. hearing something fall in to sync is so vindicating.
after coming up with a decent PLL circuit, i devised a simple bidirectional sequencer that would drive an LTC1799 chip to pitch the YM2413 voice chip of the PSS170. i wanted to add an envelope generator to the keyboard that would be triggered by note-on messages, but unfortunately there was no way to obtain a decent note-on gate from the PSS170 other than to use the audio signal as a gate. and since there are so many different voices in the PSS170, there would be no way to get the envelope generator to respond to all of the voice presets. instead i just synced the envelope generator to the sequencer clock. i also threw in a divider so that the envelope generator could be triggered by 1/1, 1/2, 1/4, or 1/8 the clock frequency.
the envelope generator would be set up to modulate the cut-off frequency of the melody filter before being sent to an echo circuit. while i was experimenting with creating a note-on gate from the audio signal, i realized that the voices coming from the YM2413 sounded really crunchy when you filter them just right and sent them through a frequency divider, so i ran those sub-frequencies through a joystick mixer and mixed them back with the original voice before they get sent to the filter stage. this is when i realized something else... although there are separate outputs for the melody and rhythm voices coming from the YM2413 voice chip, there is some bleed through from the rhythm output to the melody output, even when the rhythm output is fully attenuated. i suddenly remembered this same problem i had with other yamaha PSS models, namely the PSS-30. i think it is due to the fact that the melody and rhythm sections share some similar functions when it comes to the auto-accompaniment. the accompaniment melody comes through the melody output, but the accompaniment volume switch affects both the accompaniment melody volume, and the rhythm volume regardless of the analog volume outputs of the two sections from the YM2413. and of course, the accompaniment volume has no effect on the melody output volume. with all of this in mind, i safely concluded that there was no solution to the bleed through. the only real problem it gave me was that when the rhythm volume was set all the way down, and the melody divider was engaged, the bleed through would get picked up by the melody divider and get amplified quite a bit. the divider seemed to be picking up some aliasing noise too, so to solve the problem, i used a potentiometer with a switch at one end that would disconnect the divider circuit when its volume was set to its minimum. in addition to the independent volume controls for the melody and rhythm sections, i thought it would be kind of cool to have independent echo circuits for the two too.
after about a week of reverse engineering the keyboard and coming up with an overall circuit, i came up with a design plan. the PSS170 has a lot of room inside, but i would need a lot more if i wanted to fit all of the circuits i had built. the first thing i did was replace the huge 6xC battery compartment with a much smaller 6xAA battery compartment. then i cut as much of the front panel off as i could without compromising the structural integrity of the keyboard case. the next step was to clean the hell out of what was left. this keyboard was absolutely filthy. once it was all clean, i took some measurements and started drawing up the new face-plates. after playing around with the orientation of the interface for a few days, i settled on a design, and got to work drawing up a circuit board for the whole thing. rather than using multiple small circuit boards, i decided to be lazy and just put it all on one big circuit board. the circuit board took most of a day just to cut and drill. CNC engraving a circuit board of that size can be slow going. i had to keep going back over it a little deeper each time around the corners to get all of the traces it missed the first time around.
populating the circuit board was pretty fast. the only thing that bothered me was seeing the huge number of parts that this circuit consumed. it's not that i hate ordering more parts, it's when i need parts that i don't have anymore. oh well, i can always pull parts from scrap circuit boards if i get desperate.
once the circuit board was built and ready to go i cut out some prototype face-plates on my laser cutter using cardboard from a cereal box. i have found this to be the best way to connect the hardware to the main-board without having to worry about over abusing the face-plates. then once everything is wired up and working properly, i just swap the disposable face-plate for the real thing. i had originally planned to paint the three face-plates all different colors, kind of going for a crazy candy color scheme, but i kind of liked the way the cereal box cardboard looked with the bright orange buttons. i had a can of krylon that was basically the same color, but i've had a lot of trouble in the past using solvent based paints with acrylic face-plates. i read somewhere that if you reheat the acrylic and let it cool, it will reduce the risk of cracking the acrylic when the paint dries. once the panels were engraved, cut, and i had colored the graphics, i hit the the panels with a MAPP torch for a few seconds on each side until the acrylic started to bow out. i kept heating them up until they wouldn't warp any more, and then i let them cool. once they cool, they return to their original size, and all of the stress in the plastic is relieved. then they are ready to paint. there was a little blistering from the torch, but it is barely noticeable.
i would have to say the most rewarding part of a project that takes so much planning, is when it's time to assemble everything, and it all comes together smoothly. however, being the most satisfying phase of the project can in turn create a perfectly devastating situation. that's usually what happens, and this time was no exception. once all of the new keyboard was assembled and working, and i was putting in the very last screw that would finish the piece using a very powerful screw-gun, i went too deep and cracked a face-plate... i cut out a replacement, painted it, and was able to swap it in by the end of the day. lesson learned.
once the keyboard was closed up i notice a couple of things that i wasn't entirely satisfied with. for one, the envelope generator was biased too low, so it would make this irritating popping noise every time it swept down. not only that but the envelope generator didn't sweep deep enough because the envelope was buffered by an op-amp, so it couldn't reach more than 4 volts. i opened it back up and swapped out the envelope buffer with a couple of transistors to fix both issues. i seem to remember having the same problem with IVAN. why didn't i think of it when i was designing the circuit? oh well, fixed now. while i had the keyboard open, i threw in a little input jack to the CD4046. now the sequencer can accept incoming clock signals and/or the rhythm tempo. i thought about adding a tempo sync output too, but i'll leave it out for now.