Wednesday, March 18, 2009

CASIO PT-100


i found this keyboard at the bins. it cost about $4 and was covered in filth. despite how dirty it was, the keyboard was in perfect cosmetic condition. i had one of these a long time ago, but i harvested it for parts because it was in such bad condition. from what i had remembered, the pt-100 is pretty limited. there are not a lot of functions on this keyboard. it pretty much just has 8 rhythms, 8 voices, and a very limited accompaniment section. after looking it up on tablehooters , i quickly got work trying out some of his mods. i thought it would be kind of cool to select the rhythms and voices through the keyboard and replace the slide switches with cutoff pots for two separate VCF's. i also replaced the volume switch with a volume pot.

the two VCF pots had to be modified. the 100k slide pots i have are really slow to slide. this is because there is this thick grease in there that slowed them down and makes them smoother. i wanted these pots to be quick so i opened them up and wiped out all of the grease. the fast action makes for better filter modulation in my opinion. originally i had planned on adding an AR generator to modulate the filter, but i soon found that this keyboard is very stingy with its mA's. this was a real problem when it came to finding a good source on the board for the VCF's and the DCO for the pitch bend. eventually i had to resort to replacing the weird power switch with a normal power on/off switch. the pt-100 has a dpdt switch for the power, but it could be replaced with momentary on and off switches because power is still going to the board even when its off. i thought this was kind of pointless so i wired the on switches together and turned the slide switch in to a power on/off switch. this way the VCF and DCO are sure to not drain the batteries while the power is off. i had the DCO running from a 5v point at the amplifier section, but it was starving the CPU for some reason so i threw in a 7805 and a decoupling capacitor at the 7.5v source, and it fixed the problem. the pitch bend is just under two octaves.
anyone who has ever circuit bent a casio pt-100 knows that there are some great distortions to be found when shorting the R/C mixer section. for my mods i added 3 thumb wheel pots. the top one distorts and de-syncronizes the PCM of whatever voice you are on. with the combination of the variable amount of the bend, the pitch bend, 6 voice polyphony, the VCF, and 12 voices, this keyboard can make some pretty interesting waveforms. the sound kind of remind me of some of the better bends found on a CASIO SK-1. the middle thumb wheel pot controls the gain of the chord section. this one can get pretty nuts too. when turned all the way up, it makes a wall of distortion that can then be filtered by the VCF. the third thumb wheel controls the gain of the bass chord. this makes the bass chord sound much richer and analog with the VCF. the bass is mixed with half of the drum section, so it shares the same filter and distortion mods as the bass. the chord and bass also share the same VCF, but there is a 3 position switch to change between bass/bass&chord/chord. the other half of the drum section which consists of the bass drum and toms bypasses any filtering and goes straight to the amp unless you turn it off via the little black switch in the middle of the keyboard. when the bass drums are off, the rest of the accompaniment gets a little bit louder even when the drums aren't playing. this kind of doubles as a volume limiter for the accompaniment section say if you want the lead to be a bit louder than the chord. the other 3 position switches are for selecting rhythms and voices on the keyboard. in the center position the keys will act like normal keyboard keys, but in the up or down positions, you can select 12 rhythms and 12 voices. this is an upgrade from the previous 8 rhythms and 8 voices that were selectable through the slide switches. thanks again to tablehooters for the idea.
alright, what am i forgetting? the pt-100 also includes a chord on/off switch, a rhythm stat/stop switch, tempo up/down switches, a demo button, and an added 1/4" line out. during this project i developed a greater sense of respect for people who circuit bend vintage casios and yamahas. i almost forgot how frustrating it is to not know what the hell anything is without a service manual. it will probably be a long time before i circuit bend a vintage keyboard again. this one took over two weeks to debug. i've had to reopen and re close this thing more time than i can recall. during all the debugging, the paint job acquired a few scratches and scuffs too. oh well, i guess thats just part of it.

i am planning on selling this keyboard on eventually. if you are interested, just look up circuit bent on ebay in the coming week.

15 comments:

  1. I knew some fucking hipster with 185 dollars to blow was going to snatch this up. looked at his other purchases, all ironic bullshit. fuck them, I was gonna buy this thing and LEARN ALL THE SECRETS, but no my grandparents didn't die and leave me a hunk of money to jack off hookers with.

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  2. sorry friend, better luck next time. in the meantime you should relax and make use of what you've got. life's too short for namecalling, y'know?

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  3. woah. well uh good for noys toise.

    nt where did you learn, or get the schematics for filters and oscillators? is there like a sweet electronics book that could help me with that? what's a vcf?

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  4. VCF stands for voltage controlled filter. it pretty much implies that the filter can be modulated by envelope generators or LFO's(low frequency oscillator), or anything with a CV(control voltage)out. there are many different VCF's you could build, but each will most likely need to be adapted to the instrument you are modding. as far as LFO's, i'd recommend downloading the ne555 datasheet(the biggest file you can find). it will have examples of application circuits in it. i use the 555 and the 556 a lot, but you could use op-amps too. just start googling, you'll find lots.

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  5. Hey, remember how I said I was looking for PT-100 bends? Well I just finished bending mine today, I'm really pleased with the way it came out. I took a page out of your book when I made the schematic, splicing the pictures together. I think I found some original bends too! Check it out here:

    http://arcturusbtw.com/instruments/casiopt100/casiopt100.html

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  6. Hi,
    I was wondering if you could help me. I have this exact same keyboard (Casio PT-100). I recently bought it at a second hand store. It won't power up and I'm dying to unlock it's nifty 80's sound. My power source is 7.5 DC, and I opened it up to check the solder points, but I just don't know what's wrong. Any help would be greatly appreciated.

    Thanks,
    Mike Pettella
    mpettella@cox.net

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  7. Hey I guess you've probably moved on from this project, but I was wondering if you could tell me how you did the pitch bend mentioned in this modification? I've tried a power starve on my own PT100, which does drop the pitch, but not smoothly, so any advice would be really useful on making a more theremin like pitch bend control. Love all your other projects,

    Aleks

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  8. Ah right, I might not put one on mine, but I will bear it in mind for any future projects, I'm quite happy with what I've got so far. Thanks anyway, keep up the awesome projects!

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  9. Hi, this is very sweet job you did on this. The sum total of your mods are more extensive and more useful than nearly all the other "circuit bent" PT-100s I've seen on youtube. I've spent a while messing with my own PT-100 and have found and made most of the same mods you did, and a couple others. One thing I failed to discover though is the where the Tone Select pin combo is located. I found the Rhythm Select pin as described on tablehooters, and I have the 12 rhythms keyboard selectable with a simple push button. Any chance for one tiny hint here on the location of that one pin coordinate?

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  10. it has been a long time since i built this. i cant tell you what pin it is exactly but it should be obvious. if i remember correctly, the timbre slider switch has a common pin. each octave on the keyboard has a common pin as well. replace the common pin on the keyboard with the common pin from the timbre switch and there you go.

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  11. Hello, I have on of these Casio PT-100's but whenever I turn on the power switch, nothing seems to work. I've tried using various batteries, AC/DC adapters, plugs, and tried opening it to see if anything is loose in the circuit but nothing is visibly damaged. If you know what might be the problem please let me know!!! Thanks.

    reecewolff@rocketmail.com

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  12. What type of Vcf did you use? And where did you hook them into the main pcb?

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    1. hello! i think it was just a very basic filter, like the one on the music from outer space weird sound generator. i probably put it somewhere between the sound ic and the power amplifier. it has been a long time since i made this, so that is just a guess..

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  13. Hello! I bought pt 100 keyboard, but it is missing transistor T1 near dc jack. Can you please tell me which transistor it was?

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    1. sorry, i don't know. i do not have a pt100 on hand.

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