PT100

The Noystoise PT100 is an experimental synthesizer designed and built around the HD61702xx sound chip. The chip can be found in many Casio keyboards from the mid-to-late eighties, including the Casio PT100, hence the name. Beyond the HD61702xx chip itself, the rest of the synthesizer design is completely original.

HD61702xx FEATURES:

The chip has 12 selectable "Vowel-Consonant-Synthesis" voices; Piano, Harpsichord, Pipe Organ, Flute, Strings, Violin, Vibraphone, Celesta, Jazz Organ, Clarinet, Reed, and Synth Sound. It has 6-note polyphony, and can support up to 49 keys(C1-C6). 37 keys(C1-C5) can be accessed by the keyboard.

Modification circuitry was added to;

 -Generate Gate and Trigger outputs from the key-scanning inputs,

-Add a variable clock rate to achieve pitch modulation

-Disable the Auto-Power-Off function 

-Derive a separate chaotic synth voice from an unfiltered melody DAC output. 

LEAD & DIST VOICE SECTION:

The main voice section of the PT100 is the center panel above the keybed. The LEAD voice consists of the original Casio VCS timbres sent through some light filtering and normalizing circuitry to prepare for the subsequent filtering and effects stages in the signal path. The timbre can be selected with the "TIMBRE" knob, and by modulating the "TIMBRE" parameter with the LFO, Sequencer, and Joystick.

The Pitch of the voices can be changed with the "PITCH FINE" and "PITCH COARSE" sliders, and by modulating the "PITCH" parameter with the LFOs, Sequencers, and Joystick. 

The DIST voice is a chaotic synth voice derived from the "LSB" test pin from the LEAD voice's DAC. The chaotic voice shares many fundamental frequencies and characteristics of the LEAD voice, but is dramatically stripped of its resolution and sonic detail. The result is a very harsh sounding modulated digital tone. 

The DIST voice is sent to a frequency multiplier circuit with 8 selectable super-frequencies; x3, x4, x5, x6, x7, x8, x9, x10. The voice can be transposed with "SUB OSC" knob, and by modulating the "SUB O" parameter with the LFO, Sequencer, and Joystick. 

The 3-position "DIST MODE" switch selects the output mode. In "DIST" mode, the signal is un-multiplied. In "XMOD" mode, the multiplied signal and the un-multiplied signal are sent through an XOR gate for a cross-modulating effect. In "SUB O" mode, only the multiplied signal is output. 

All three modes are sent then to a PWM circuit that can be modulated by the "PWM" knob, and by modulating the "PWM" parameter with the LFO, Sequencer, and Joystick. 

Both LEAD and DIST voices are then sent to resonant filters (L VCF/D VCF)that can be set to Bandpass or Lowpass mode. The filter cutoff can be changed with the respective "CUTOFF" knob, and by modulating "L(D) CUT" parameters with LFOs, Sequencers, and Joystick.

Both LEAD and DIST voices are then sent to voltage-controlled-amplifiers(L VCA/D VCA) that can be set with the respective "VOLUME" knob, and by modulating "L(D) VCA" parameters with LFOs, Sequencers, and Joystick.

The LEAD and DIST VCAs and VCFs have their own identical envelope generators. Each is equipped with Attack, Decay, Depth parameters, and a 3-way mode switch. In "MULTI" mode, the envelope will be re-triggered by subsequent key-presses in certain groups of keys on the keyboard. This creates a more dynamic pseudo-polyphonic effect to the filters, but is limited in that if two notes are exactly 6 keys apart, they will not re-trigger the envelope generator due to how the keys are scanned...

In "SINGLE" mode, the envelope is only triggered once until all keys are released. Only in this mode is it possible to re-trigger the envelope using the Sequence Pattern Clock(see Sequencer Section...).

The LEAD and DIST voices are then summed together, and with a Reverb signal before being sent to the output via MASTER VOLUME slider. The LEAD and DIST voices can be sent to the Reverb channel independently with their respective "RVRB L" and "RVRB D" button switch. The Reverb channel has its own REVERB VOLUME slider located next to the MASTER VOULME slider. The Reverb bit-rate can be changed with the "REVERB BIT RATE" slider, and by modulating the "RVRB" parameter with the LFO, Sequencer, and Joystick. 

SEQUENCER SECTION:

The Sequencer section consists of two separate "KEY-HOLD" circuits, two separate clock sequence "PATTERN" banks, and a two channel 8-step CV Sequencer. 

The "KEY-HOLD" circuits have two modes. They can simply be turned on or off, or be gated on and off by the respective Pattern clock. All 49 keys are accessible, although some Timbres only support 37 notes. The note can be selected with the "KEY 1(2) HOLD" knobs, and by modulating the "KH1(2)" parameters with the LFOs, Sequencers, and Joystick.

The clock sequence "PATTERN" banks are 16 pre-programmed Patterns that are synced to the Master Clock source. The 16 Patterns can be changed with the respective "SEQ1(2) PATTERN" knobs. The clock Patterns can also be used to gate the  Envelope Generators using the corresponding "VCA" and "VCF" button switches, while the EGs are set to "SINGLE" mode.

The CV "SEQUENCER" has 8 tunable voltage source "STEPS". The Steps are accessed by two Step Sequencers. Each Sequencer has its own Pattern Clock that is synced to the Master Clock. The Master Clock determines the overall Tempo, while the Pattern Clock determines the Pattern in which the Sequence will advance. Each Sequencer has a 3-position "MODE" switch. In "UP" mode, the Sequence will advance through the CV Steps 1-to-8, and repeat. In "DOWN" mode, the Sequence will advance in reverse order from 8-to-1, then repeat. In "PROG" mode, the Sequencer's Pattern clock will be disabled. The output Step can be selected with the "SEQ1(2) STEP" knob, and by modulating the "STP1(2)" parameter with the LFOs and Joystick. Each Sequencer has a "DEPTH" knob that is set to Zero in the center position. The output can be variably set to positive or inverting by turning the knob to the Left or Right of Zero, respectively. The output of Sequencers 1 and 2 can select 1-of-7 modulation destinations, or be set to "OFF". Sequencer 1 can be sent to "L VCA, L CUT, TMBR, KH1, PITCH, RVRB, and LFO1". Sequencer 2 can be sent to "D VCA, D CUT, PWM, KH2, PITCH, SUB O, and LFO2".

LFO SECTION:

There are two identical LFOs with four different waveforms to choose from. LFO1 can be set to "OFF, L VCA, L CUT, PWM, KH1, PITCH, RVRB, or STP1". LFO2 can be set to "OFF, D VCA D CUT, TMBR, KH2, PITCH, SUB O, or STP2".

 The "LFO1(2) SHAPE" knobs select "RAMP, SQR(square), TRI(triangle), or S/H(sample/hold)". "LFO1(2) RATE" knobs can be used to change the rate of the LFOs. The "LFO1(2)" rate parameters can also be modulated by the Sequencers and Joystick. 

The "S/H RATIO" knobs are used to set the Sample/Hold gate time in "S/H" mode, and Duty Cycle of the Square Wave in "SQR" mode. The "RNDM/LFO1(2)" button switches can be used to switch the Sample/Hold input signal from the Pseudo-Random Signal Generator, to the output of the other LFO.

The LFOs are gated by key-presses, or Manually using the "MANUAL LFO GATE" pushbuttons on the Joystick section(see Joystick section). The "KEY/MANUAL" push switches select between key-press and manual gating. The LFO1(2) DELAY" knobs control how fast or slow the Depth of the LFOs reach their maximum each time they are gated. 

Each LFO has a "DEPTH" knob that is set to Zero in the center position. The output can be variably set to positive or inverting by turning the knob to the Left or Right of Zero, respectively.

JOYSTICK SECTION:

The X and Y Axis of the Joystick can be set to modulate "PITCH, D CUT, TMBR, KH1, LFO1, RVRB, STP1, D VCA, L CUT, PWM, KH2, LFO2 SUB O, and STP2" parameters, or be set to "OFF". 

Each Axis has a "DEPTH" knob that is set to Zero in the center position. The output can be variably set to positive or inverting by turning the knob to the Left or Right of Zero, respectively.

The two pushbutton switches at the bottom of the panel are the "LFO1(2) MANUAL LFO GATE" switches. They are used to manually engage the LFOs when set to Manual mode.

INPUTS/OUTPUTS:

-STANDARD 9VDC TIP-NEGATIVE JACK (1000mA)

-1/4 INCH LINE OUT JACK

-1/4 INCH CLOCK SYNC OUT JACK (5V, positive edge triggered)

-1/4 INCH CLOCK SYNC IN JACK (5V, positive edge triggered)

BUILD PROCESS:

The Casio PT100 Super-Mod started out as a potential commission in early 2024, and was finished as a personal project in 10-11 months time. The goal was to build a synth that had some glitchy-industrial characteristics. I had always felt that the PT100 had some very unique sounds when circuit bent. Being that the project started out as a commission, the design and signal path all came together pretty efficiently, and I was very excited for what I had come up with. However, I didn't feel that the customer would be satisfied with it, so I recommended postponing the deal until it was finished. Unfortunately, not having any kind of deadline or expectation on me made it too easy to put off, and it ended up taking a lot longer than it should have to finish. It was all for the better, though. I was able to spend more time playing with new ideas, and getting rid of some things that I didn't think were useful. I also had more time to create cleaner project files so that I can potentially build more of them in the future.

There was not really anything new in the design and build process that I haven't used already. I am still using my 15 year old K40 laser cutter to cut out the wood pieces, and cut/engrave the acrylic faceplates. I also made use of my 15 year old mini CNC to make small circuit repairs when needed. I am still using CorelDraw X5 from 2010 to design my faceplates and other vector jobs. I am still using  a Google Sketchup Demo from 2016 to draw my 3D models. I am still using Arduino 1.8.5 and KiCad 5.1.9, but wait, I did upgrade to Windows10 last year! HAHA

I have been buying my knobs and some hardware from AliExpress. So far, they seem to have good quality options there and some unique styles. I had to use long shaft 9mm snap-in potentiometers so that there would be enough clearance for the switches I used. Unfortunately the added shaft length causes the knobs to wiggle. It's not terrible, but it is just one of those subtle things I would like to avoid in future builds. 

I used some super old blue Tolex to wrap the enclosure. It was at least 15 years old, and had been folded up in storage for all that time. It is really good quality Tolex, but I kind of botched putting it on. I thought that if I lightly Ironed the Tolex flat under an old towel, it would get rid of the creases faster. It seemed to work great until the final inspection after I had already stretched and glued it to the box. The creased areas that I had flattened out were smoother than the rest of the textured surface, so there are these smooth stripes here and there... It is really only noticeable on the back, so it was acceptable. I won't be making that mistake again, though. Another minor issue I had with the Tolex was actually with the plywood I used to make enclosure. I used cheap 1/4" sand-ply because it is light and easy to cut. It is not very Ideal for making straight parts, though. It took a lot of effort to get it as true as possible. I even used wood-filler and sanded every surface flat, or so I thought. The Tolex I used is pretty thick and robust, but when it was stretched and glued down, every imperfection in the surface of the plywood pressed through... I was able to fix most of it, but some little cracks and bumps are still visible if you know where to look. Thankfully Tolex is great at hiding imperfections. In the future I will use some quality particle board, or anything other than cheapo sand-ply.

Unfortunately I didn't take very many photos of the build process. Or if I did, I can not find them anywhere. This is all I could find on my phone. I had posted a few teasers on my Instagram. I think they got about six views. LOL please follow me...