CYBERYOGI =CO=Windler

The GZ-500 looks like a castrated version of Casio CTK-650 or CTK-680 (lacks accompaniment section, likely by different ROM) sold as a more PC oriented GM keyboard/sound source. Check CTK-650/680 manuals if it operates similar (e.g. midi details/sysex etc.).

You may also make a mould e.g. of silicone from a keycap of identical shape to copy it e.g. in acrylic or epoxy resin. Particularly with liquid/uncured epoxy be very careful not to get it on your skin; it is severely toxic and causes cancer.

HD43720 can be found in MT-36, MT-200, MT-90, CT-102. It was apparently Casio's cheapest polyphonic sound IC with percussion generator. Yamaha made a somewhat similar thing named DSG (digital sound generator, YM2163 and YM2142 (GE8)) which however employed simple pulsewave and sawtooth. But common is that both only have simple linear volume envelopes those make them sound artificial. Although it can sound similar, the real Consonant-Vowel synthesis is not multipulse-/squarewave at all. The waveforms look like pixelated symmetric skyscrapers morphing into each others. The volume envelope switches through a a sequence of multiple linear sections turning gradually shallower to approximate logarithmic decay, and its clock rate is driven by the waveform clock, which makes (like playing a sample at different speeds) low notes decay slower, which mimics the natural behaviour of most plucked string, piano and mallet instruments. The sinewave instruments (1000p, MT-70 etc.) sum 5 sines like a drawbar organ. Beside somewhat low resolution these were actual sines. The sound IC is very detailedly explained in the Casiotone 1000P patent (US patent 4538495) and further details in US patent 4453440, which describes digital envelope control by adding phase shifted copies of the same sine wave to avoid multiplication, and how overtones are produced by a kind of phase distortion predecessor. In this patent Casio also describes how to mix the outputs of multiple sound ICs digitally, which was never done in any other 1980th Casio keyboards I am aware of. I think the main reason why Casio had so much secrecy about their sound generators was that to be first on the market, they often sold keyboards with a new sound source inside before the patenting process was finished, and so had to try hard to prevent competitors from figuring out what they did. But beside FM variants (phase distortion, using no exact sines to avoid the definition of "multiplication") they didn't copy much from Yamaha. To learn more about the inner working, it is more useful to websearch for Casio patents with "priority date" close to the release date of the instrument you are looking for. Unfortunately patent names are often very generic or even grossly misleading (e.g. "System for generating sample tones on an electronic musical instrument" is about playing a demo note during preset sound selection, which has absolutely nothing to do with sound sampling) and the interesting parts are often not even that what is patented, but technical details about other parts of a described instrument. The disclosed reference implementation often also differs from the actual finished product, which can make it hard to identify the actual instrument and to figure out how it functions. But patent texts are the only officially published technical documents about Casio hardware those can be found online in large quantities, which makes them worth to look into even though they can be hard to understand. Because one purpose of patent texts is legally documenting the state of art of already known inventions (to prevent patenting twice), unlike websites they are not prone to suddenly disappear. So not every hobbyist website or forum talking about them is in the need of keeping backups online (telling a patent number is better than an URL). Patent texts can be hard to understand, but often they are the only traces that orphan hardware left in the noosphere. It is on us now to find out and index what they are about. There is are a huge online archive of unidentified technical information out there, that is useful far beyond the realms of companies and lawyers.

A recti-fire? In Germany these things are named Gleichrichter and had the nickname "gleich riecht er" ("soon it will smell") because they tended to stink apart (early ones with not too healthy selenium fumes). In my parent's house a Saba Ultracolor TV set once blew the house's main fuse, because a diode in the degaussing coil circuit had shorted and so vomited the current from a big charged electrolytic capacitor back into mains.

Even if it works without, do not keep it permanently running with a capacitor removed. Resulting HF dirt on DC supply voltages can slowly damage irreplaceable ICs by internal capacitive currents resulting in local overheat.

Do you mean extracting data through a TEMPEST attack? I doubt that anybody takes the effort to decode that (unless AI in mobile radio networks spies out every offline device in the background). Or is there a hidden bluetooth or wifi transmitter? At least older keyboards definitely do not communicate with pulsed microwaves, and it would waste battery to keep such a transmitter always on. (How ever it may be designed to respond to a certain wakeup signal, like the "roving bug" spy more in mobile phones that is legally required by US mobile radio standards.) Or did you find a radio-controlled clock (RCC) feature inside newer Casios to set correct file dates? (They only receive, not send.) Every digital device produces some radio waves. In my childhood I placed my Casio VL-Tone 1 onto the radio and switched the latter to AM or SW, which makes tons of funny distorted synth sounds receivable in the radio (depending on the radio's tuned frequency). I have collected on youtube much info about EMF shielding and radiations. https://www.youtube.com/playlist?list=PLCMrVbN84CCYrhCbgKXzknfx0PjWrBZLA

Again, this sound source is named "consonant vowel" synthesis, NOT Vowel Consonant. 😡 The smaller Casios (PT-series, VL-1) were not walsh. They have only one multipulse squarewave with linear volume envelope and LFO. It may be that the idea of synthesizing timbres this way originated from Walsh (there was that Allen Organ patent lawsuit), but multipulse squarewave is not identical with Walsh. Layering multiple Walsh drawbars would still sum them with different volume levels, while within a multipulse (16 bit steps those each can be hi or lo) the steps always keep the same height (i.e. 1-bit signal of repeating 16 steps). I remember I read in a magazine(?) long ago an interview that Casio had invented multipulse squarewave coincidentally when they experimented with LCD control voltages for calculators (consisting of blocky 4 step (2-bit) waveforms) and noticed that they sounded like organ tones. I also doubt that consonant-vowel or SD internally layers actual Walsh drawbars (which are a series of certain mathematically well defined special multipulses). It simply crossfades/morphs between 2 blocky waveforms (and than layers 2 or 4 of them having different preset analogue filters) to synthesize timbres. The internal basic waveforms were likely tweaked by ear and not planned to implement Walsh. Walsh makes only sense when you want to be capable to systematically approximate all repeating waveforms by summing a high number of multipulses in the same manner like conventional drawbars sum sines (implementing the Fourier series). The HD44140 (in PT-7 etc.) is a specialized and badly crippled single-chip variant of the sound IC HD43720 (found in MT-36, MT-200, MT-90, CT-102). Its sound engine is quite unique but nothing great. sound generator of HD43720 This sound hardware was apparently Casio's first cheap polyphonic sound ic with integrated DAC, chord and percussion generator. The main voice generator is obviously a polyphonic successor of the great multipulse squarewave sound engine of Casio VL-1, but unlike there, the blocky waveforms are bipolar, i.e. made from 3 levels {-1, 0, +1} (which IMO did not improve timbre quality). So the sound IC has 2 DAC outputs for a main and a sub waveform, those are mixed through resistors in a ratio of 1:64. With held monophonic notes the main waveform looks like a normal multipulse, while the sub waveform has 3 levels {-1, 0, +1} and looks like the tone. During falling volume envelope, the equidistant "0" sections of the main waveform rapidly move many times linearly up and down while the sub waveform amplitude slowly shrinks. I am not sure if these observations (seen on analogue oscilloscope) are only artifacts of sending upper and lower waveform bits to a separate DAC, or if main and sub waveform indeed get computed this way by 2 independent generators to combine into the envelope. Likely the main waveform only scales the envelope; in most preset sounds it is plain squarewave, but in 'elec. piano' and 'clarinet' it is a multipulse, so it may be that this is a very crude kind of phase distortion (FM) mechanism involving analogue multiplication of both components (main waveform has 2 diodes in series to GND, which may intend nonlinear behaviour), although it does not vary modulation depth to change timbre. (It would be interesting to build a more programmable version of this, to examine what else it can do.) The unused 11-bit DAC output port makes me conclude that for 8 note polyphony (3 bits) each polyphony channel including envelope has 8 bit. The sub waveform of one note is up to 5 steps high (seen on analogue scope), hence the sub waveform is 3-bit while the main waveform has likely 63 steps and thus 5-bit. In preset sounds 'piano' and 'vibraphone' the sound ic pulls pin 11 lo to enable a discrete external filter that makes them duller and adds a little resonance. preset sound sub waveform main waveform envelope piano 11111100 11111100 decay 1s elec. piano 1--01-00 011X01XX decay 0.5s organ 1---1-00 11111100 hold + slow attack oboe 1-----10 11111110 hold + slow attack clarinet 11-100-0 11110010 hold vibraphone 1---0--- X111X111 decay 0.5s + always sustain strings 100----- 10011111 hold + slow attack elec. organ 1-101--0 11101110 hold bass & chord 11110010 (capacitor sustain) The complete waveforms on analogue oscilloscope look very distorted by capacitors, thus the bipolar multipulse patterns were hard to identify, but they seem to be indeed identical with their own "sub" waveform, which shape is better visible. The "-" indicates the intermediate level (center line) between 0 and 1. The "main" waveforms here are only an approximation. Their "X" sections move rapidly. The "0" section amplitude moves with envelope. But these don't strongly change the complete waveform anyway but mainly affect envelope. The volume envelopes are linear with very few internal parameters. Unfortunately they decay too fast. Sustain decays 0.5s after key release. pinout HD43720 The Music LSI "Hitachi HD43720" (54 pin SMD, pins count anticlockwise from below the left stub pin) is the squarewave sound IC of various early Casio beginners keyboards. It contains 8 polyphony channels for melody voice (4 of these can be re-routed as chord & bass voice) with linear envelope. Additionally there is a percussion generator with waveform outs for {base, snare, cymbal+hihat} and corresponding trigger pins for external analogue envelope circuits. Also the 4 separate chord & bass outs have each an envelope trigger pin. As a real sound IC it needs to be controlled by an external CPU through a data bus. The integrated DAC the integrated DAC outputs its lower bits (at increased level) only on a separate pin; also the highest bit is separately output in normal and inverted. All these have to be combined through an external voltage divider (small resistor network with ratio 1:64), which was likely done to reduce noise. The only 11 DAC bits also have separate output pins (not used in any instruments I know). Each waveform is made from bipolar multipulse squarewave (8 steps long) and a simple volume envelope that does not change with note pitch. Each step can have only the height {-1, 0, +1}, i.e. protrude fully up, fully down or be zero. The linear envelope and low bit resolution make it sound rather artificial, but the unusual sonorous waveforms are unique and the high internal clock rate (1.65 MHz) prevents cold aliasing noise. Unlike the more advanced D77xG there is no spread scale, i.e. holding the same notes in different octave (except the highest note) causes no phasing (but phase changes among trilled decaying notes, thus there are no octave dividers involved). It is unknown if synthesis parameters are send by the host CPU to the sound IC (like in D931C), or if it only can select 8 preset sounds from an internal rom (like done in early Yamaha keyboards). The fact that all known instruments with HD43720 contain the same preset sounds suggests the latter. See attachment for tech details. Casio_MT-36.html

Never use or try out wrong polarity. It will destroy your Casio keyboard (unless it contains a protection diode, which might already exist in later generations like CTK-650).

Don't fall for eBay price scams. They often put a keyboard specimen for 5x the realistic price to auction (possibly even with several staged fake sales back to the same vendor). Unknown is if they try to sale for that on eBay, or to sell another specimen e.g. in their local thrift shop for slightly less by showing the moonpriced eBay auction as "example" what the prices would be. ("Look, I have this MT-100 for only 550€ for you. That's a really great deal... On eBay they sell these rarities now for 899...")

A zener diode normally is no high resistance part. A small one can handle about 100mA, and if overloaded (pulled to far above its breakthrough voltage) it would run boiling hot. Which voltage is on the lower left 100 Ohm 1W resistor (coming from mains trafo?). Does that voltage collapse too when connecting the stereo chorus circuit?

Check the power consumption of the stereo chorus by inserting a mA meter into its -15V line. If it pulls it up to -2V then either a semiconductor went bad (do any of the chips run hot?) or the PSU outputs way too low current on its -15V line. The sample & hold circuit in 1980th Casios functions basically like the distributor of an old car engine. Like the distributor assigns the ignition voltage to different spark plugs depending on the rotation angle, the sample & hold demultiplexes the signal behind the DAC among multiple audio lines by timing signals to produce multiple analogue sound channels (e.g. stereo or to permit a separate chord volume potentiometer) from a single DAC. Bontempi did similar things in stereo keyboards, while cheaper mono variants just used a resistor and capacitor to sum the output without demux..

Casio MT-65 is quite similar but more versatile than MT-60 (not the same chips). Its preset "funny" resembles "frog" and with the envelope switches it may be adjusted even closer.

Casio should make a small keyboard remake of the classic SA-series sound engine, but with fully editable synth parameters (perhaps velocity sensitive, similar like Yamaha PSS-A50). Cruicial is that it has to be self-contained (possibly USB midi for sysex and data storage) and not infested with app crap nor bluetooth. (I do not buy any mobile radio garbage.)

I did not doubt that Casio keyboards contain fully custom chips. It's only the usage of the term "LSI" that (if that is really Casio's definition) is rather camouflage than any standard meaning. This is like naming a solid state video playback flash memory chip in a block schematic "DVD" and explaining it with "Downloaded Video Deliverer". Nobody would in context of video playback ever expect that meaning when a system mentions a digital video source named "DVD". And in the age of Casiotone 201 their "LSI" (although custom made) for sure simply meant "Large Scale Integration". Also Casios true single-chip softsynths (SA-series etc.) do have a fixed polyphony channel count, because everything runs in a very tight loop of fixed timeslots processing each channel (according to patent even code stuffed with NOP to keep all tasks the same step count). This coding style may be only compared with the way Atari VCS2600 outputs video by bitstuffing scanlines with pixel data and handling tasks for each sprite of the game. Adding more voices in those softsynths would require higher clock rate (usually by a multiple of 2) or reduced DAC output frequency and modification of all frequency tables accordingly (that's why some preset sounds in MA-130 differ from their SA-series counterparts), and may also need deeper register files (somewhat similar like PIC or Transputer(?) - those may be even implemented as clock rate stepped shift registers for rapid task switching, like the early all-hardware Casiotones do to implement polyphony).

This may be a bad electrolytic capacitor, that causes an op-amp to oscillate when burdened with the sound signal. A similar bug is the infamous squeak in Casio Digital Horn saxophones (DH-100 and such). Someone needs to solder a new capacitor in. (But of course also a shaky contact may do such things.)