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This hardware class appears to be the last generation of Casio's famous Consonant-Vowel synthesis. It has the whole digital part of the hardware integrated into a single chip, including accompaniment, simple sequencer, ROM-Pack support and key lighting.
Unlike in modern such keyboards, not the keys itself but a row of tiny LEDs above the keys flash up to teach keyboard play. It has analogue percussion and nice sound, although the sound quality is lower than its (technically far more complex) ancestor Casio MT-800. In opposite to earlier ROM-Pack keyboards it has not only melody guide, but also a chord guide feature that teaches to play fingered chords.
An earlier version with stereo chorus came out in 1986 as Casio MT-820.
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Most preset sounds of MT-88 are bright or even a bit harsh, but not necessarily in a negative sense. Only the volume envelopes seem to be linear and thus end too soon. The 'piano' sounds quite realistic, the harpsichord a little grainy. The 'vibraphone' has chorus effect; its vibrato gets the faster the higher notes are played. (The same kind of vibraphone exists in Casio CT-6000). The 'pipe organ' has a dose of chorus and sounds a bit harsh and rather accordion- like. The 'jazz organ' is a sort of Hammond imitation which percussive attack phase rapidly turns duller and then sounds a little hollow in the bass range. The 'flute' sounds realistic and has vibrato, while 'strings' sound rather harsh and uses beside vibrato also a chorus effect. The 'synth sound' is a harsh and buzzy electronic organ timbre with fast attack phase. The 'clarinet' has some vibrato and sounds ok. The 'violin' has chorus and sounds rather harsh, thin and unrealistic (Casio VL-1 had a much better one). The 'reed' is another bright tone with chorus and fast attack rate. (Both resemble somewhat sounds on small Casio ToneBank keyboards.). The 'celesta' sounds a little harsh and resembles in the bass range a distorted harp, because in the release phase of short bass notes apparently the duller one of both subvoices fades silent sooner. All preset sounds contain a short sustain that prevents playing extremely short notes.
Unlike its crippled ancestor Casio MT-800, the fingered chord mode of the MT-88 fortunately accepts non-chord key combinations again. Its organ chord mode (without rhythm) uses always a grainy multipulse brass timbre. The accompaniments of the rhythms add a decay envelope to it and thus turn it into an e-bass. In opposite to Casio PT-100 it has (instead of its blip percussion channel) an additional warm bass tone.
The key lighting of the MT-88 behaves modern and thus lacks the flaws
and glitches of the MT-800 and has the same walking light like Casio
PT-80 etc. (but no start-up jingle). Also the 4 modes (with/ without
waiting, with/ without light) are present. New with this instrument is
that beside the normal melody guide key lighting (to teach monophonic
melody play) it also features chord guide. In this mode in the chord
section of the keyboard flash 3 LEDs to train fingered chord play. For
the correct timing a walking light moves from right to left in the melody
section and the player has to play the chord at the moment when this light
reaches an arrow mark. (The melody voice plays automatically.)
circuit bending detailsThe Casio MT-88 employs a more expensive version of the Casio PT-100 hardware class, thus for general hardware info see there. This description only explains the special features.While the main voice is basically the same, the MT-88 additionally introduces semi-analogue percussion and an additional SRAM "D446C-3", which is apparently used only for sequencer since all other functions also exist in PT-100 (accompaniment) and MT-28 (ROM-Pack, key lighting).
important: All PCB locations in this text are viewed from the solder side with keys facing down. Regard that all PCB writings at capacitors only tell their capacitance; they are not part numbers and thus ambiguous. The CPU contains 2 DAC halves for each of its audio output lines; those for bass, chord and obligato channel are mixed by voltage dividers inside the DAC hybrid "JS3918 IAM". The main voice channel additionally has the bit compensation trimmer VR7 to tweak DAC linearity. There are several methods of adjusting it; service manuals suggest using a scope at its wiper pin, select a sound with sustain and adjust for least jumpy decay curve. I found it more useful to select 'flute', play a high note and switch the analogue scope's triggering off until the sine turns into horizontal scanlines at the vertical height of each bit. Tweak the trimmer for equal distance. But the quickest and easiest method needs no scope at all; select 'flute', play a note cluster on several adjacent high note keys (sounds nasty) and tweak the trimmer for least crackling distortion. The Casio MT-820 has an additional stereo chorus. This is a phase shifting delay circuit with plenty of analogue stuff, using a 128 steps BBD IC MN3206 clocked by IC MN3202. keyboard matrix pinoutAll keyboard matrix pins are accessible on the 2 broad ribbon cables in the middle of the upper mainboard rim. The pins count from right to left.
semi-analogue percussionThe discrete analogue percussion hardware strongly resembles Casio CT-410V (see there). On the main PCB are 5 trimmers to tweak their decay time. The rightmost vertical trimmer row (top to bottom) adjusts base, low conga, high conga, claves; to their left is the snare trimmer.
The 'claves' circuit is not used by the rhythms at all but only as blip signal for the memory buttons. I first thought I broke it because no rhythms triggered its discrete circuit. (How stupid is that?! The CPU should use a cheap digital blip instead.) Unlike CT-410V, all analogue percussion trigger signals are multiplexed with the key matrix outputs KO1..KO7. 2 logic ICs "TC4081BP" decode them by AND comparison with CPU pin 98 FC4. How ever the digital blip percussion of the cheap PT-100 sounds much more interesting. You can enable it with 2 switches in the keyboard matrix. rhythm volume controlUnlike CT-805, the MT-88 has no separate rhythm volume control. The outputs of the individual analogue percussion circuits are mixed together in a horizontal row of 8 vertical resistors in the upper right PCB section. Their common output trace is routed from there to the left through a yellow 100nF capacitor, a 56k resistor and a wire bridge to the mixing point of the accompaniment. To separate the percussion, cut the bridge and wire the output of that 100nF cap to the right pin of your new 100k potentiometer. Connect its left pin to GND (of the accompaniment pot) and the wiper pin through a 10k resistor with the other end of the 1uF electrolytic cap at pin 2 of the near op-amp IC BA4558. Important is to use short shielded cables to avoid interferences of key matrix buzz; I even had to cover my plastic potentiometer with grounded aluminium foil to reduce hum because it was so sensitive. (The real CT-805 uses different wiring with a brown cable to an additional transistor in empty PCB holes - possibly to prevent this.)Regard that blip percussion volume can not be controlled by this, because it is embedded in bass and obligato audio. digital blip percussionAnalogue percussion can sound nice and certainly can be modified in many ways. But this one not even does accents and plays boring like plain whitebread, thus it is very recommended to also enable blip percussion to unleash plenty of crazy new digital whole-grain sounds. To activate it, you only need to disconnect 2 fixed matrix diodes (see here) below the left end of the 15 pin ribbon cable JA and wire switches in series.IMO it sounds even somehow more natural, so I don't understand why Casio in 1987 decided to install analogue percussion at all (other than saving polyphony). This is not the cheapish squarewave blip percussion of PT-82 anymore but uses very well tweaked rough timbres those in realism at least compete with the analogue stuff and are all different. bass & obligato timbre controls (blip percussion)In MT-88 hardware the bass and obligato channels are rather dull, which sounds nice but particularly will make trouble when you enable blip percussion, because it muffles their hiss waveforms way too much. The mufflers are 2 yellow capacitors 100nF ("C104", bass) and 47nF ("C473", obligato) against GND near the big round hole in the upper right PCB section. Desolder them and instead solder there 2 shielded cables; reconnect the original capacitor through each a potentiometer at the end. I used 25k for bass and 50k for obligato. Regard that they have to be wired "wrongways" because the capacitor is reducing volume and treble at lower resistance, so the "right" pin stays unused. I have cut the carbon track there to maximize volume/ brightness when fully cranked right. (Additionally I even swapped wiper for GND and left pin for signal to ground the metal lid of my plastic trimmers for better shielding.)Unfortunately the blip percussion still sounded weaker than in the real PT-100, which plays opener with more punch and less static noise. This happens because the drum signal is squeezed through a too small 4.7nF capacitor ("C472", looks like a resistor above transistor T12) which removes most of the bass contents from the obligato channel. To fix this, simply bridge it with a 1uF cap, but because this makes ROM-Pack music sound bad (obligato too loud), install at least a switch in series; I instead wired a 50k pot (plastic trimmer, from wiper to right end) with left end of the carbon track cut to dose the blip drums precisely. If you want to make also snare and cymbals stronger, bridge also the "R39" resistor below the plastic post hole (near unused solder holes) with an 1uF cap in series with a potentiometer (from wiper to right end - I used 100k plastic trimmer with left track end cut). With this hiss control cranked up and the 1st bass pot closed, it will distort cymbal hiss into a different timbre. Regard that everything needs shielded cables and depending on the pot type both used pins at it may need to be swapped to reduce hum sensitivity.
With all 4 pots installed you can get close to PT-100 percussion and make them knock even stronger. Only the chord voice stays slightly brighter (likely by a smaller lowpass filter cap against GND). With the pots fully turned left, you get back to the MT-88 default timbre. For general hardware info see PT-100. |
My Casio MT-88 came with the default ROM-Pack RO-554 "Family Songs". This cartridge has only 6 (partly short) songs, which is hard to understand since my MT-800 from 1983 was shipped with a 15 full- length song cartridge and the cost of ROM memory did not rise but fall over time.
A stereo version of the MT-88 was released as Casio MT-820 (additional stereo chorus circuit, cinch output jacks and "stereo chorus" switch next to the sustain switch | verified by sales prospect). A fullsize keyboard without ROM-Pack (49 keys, mono) based on this hardware was the Casio CT-350. Many short midsize variants exist; one with more interesting percussion is the Casio PT-100.
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Due to similarity I only list here the differences to Casio MT-88.
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Most preset sounds of the Casio PT-100 are bright or even harsh, but not necessarily in a negative sense. The "piano" sounds quite realistic, the harpsichord a little grainy. The vibraphone is made from plain squarewave with chorus effect; its vibrato gets the faster the higher notes are played (although this is no sample). The "pipe organ" has a dose of chorus and sounds a bit harsh and rather accordion- like. The "jazz organ" is a sort of Hammond imitation which percussive attack phase rapidly turns duller and then sounds a little hollow in the bass range. The "flute" sounds realistic and has vibrato, while "strings" sound rather harsh and uses beside vibrato also a chorus effect. The "synth sound" is a harsh and buzzy electronic organ timbre with fast attack phase. When modified, you get 4 additional sounds: The "clarinet" has some vibrato and sounds ok. The "violin" has chorus and sounds rather harsh, thin and unrealistic (Casio VL-1 had a much better one). The "brass ensemble"(?) is another bright tone with chorus and fast attack rate. (Both resemble those sounds on small Casio ToneBank keyboards.). The "celesta" sounds a little harsh and resembles in the bass range a distorted harp, because in the release phase of short bass notes apparently the duller one of both subvoices fades silent sooner. All preset sounds contain a short sustain that prevents to play extremely short notes.
The single finger chord uses (without rhythm) a slightly rough and buzzy multipulse organ tones (those resemble accordion basses). With rhythm, the accompaniment uses chords with piano envelope instead. Both timbres don't change among rhythms. When modified, you can get a well working fingered chord mode that also accepts all non- chord key combinations and without rhythm it makes even a fantastic, massive droning, buzzy organ bass, that even does not repeat within one octave and thus behaves almost like a real key split. This bass slightly resembles a wooden accordion or reed organ, but sounds more massive. But compared with MT-88 it lacks one warm bass channel (here re-used for blip percussion) and the chords sound a bit duller. Unfortunately the accompaniment is a bit too loud and has no separate volume control, but this can be likely upgraded also. Also the remaining main voice section of the keyboard is a little short in this mode, which would be complex to upgrade. (But other variants of this hardware class have a longer keyboard.) Compared with MT-88 the PT-100 sounds opener, cleaner and less noisy - likely because there are fewer components in the analogue audio signal path.
The digital percussion uses very unusual grainy and impulsive electronic
timbres containing much zipper noise. The cymbals are made from a hissy
metallic waveform similar like in Casio MT-36
or Casio SK-1, but unlike SK-1 the
drums are no blips at all but still have a vaguely realistic timbre. Particularly
the base drum is fantastic; its timbre resembles rather a sort of wooden
jungle drum with much zipper noise. This drum sounds incredible grainy
and buzzy, but at the same time confusingly natural - a bit like
a snare but much more tonal; this thing goes "bormm bormm bormm" - perhaps
like drumming on a rusty, half- full trash can or the like. The patterns
even seem to use several dynamic steps for accents. My oscilloscope hints
that shift register feedback noise is mixed with something that may be
ultra-low resolution samples or digital waveforms with coarse decay envelope.
Only low and high conga are typical squarewave blips.
circuit bending detailsThe Casio PT-100 contains only a single digital IC (on the solder side of the PCB); although there are still many discrete components (muffling capacitors for the timbres etc.), it is far less complex than early polyphonic Casio keyboards. The slide switches slide directly on carbon traces on the back of the main PCB. For my modifications I have cut various traces at the slide switches and connected them by wire wrapping to new inputs.
The CPU "Hitachi HD61702A02" is also used in the more complex Casio MT-88 (there connected with an additional 2KB SRAM "NEC D446C-3" for sequencer), so this text also describes the common aspects of both hardware architectures. The CPU contains 2 DAC halves for each of its audio output lines, those are mixed by external voltage dividers. In PT-100 there are empty solder holes instead of the bit compensation trimmer for tweaking the main voice DAC (see VR7 in MT-88), but its shorted left and right pin traces would need to be split to make it work. keyboard matrix
All unknown function names and in/ out numbers in this chart were chosen
by me. The input lines are active- high, i.e. react on +Vs. Any functions
can be triggered by a non- locking switch in series to a diode from one
"out" to one "in" pin. In MT-88 the key leds are wired through 12 transistors
to CPU pins 66..71 and 73..78, multiplexed though pin 72 and 79 those switch
2 ICs 74HC174P.
Fist I had made very incomplete schematics of PT-100. Later I studied the MT-56 and MT-105 service manuals and analyzed the MT-88 to complete it. In PT-100 I had measured a 'rhythm stop' doublet at KO3->KI7, but it was likely a mistake because at least in MT-88 it does not exist. Also 'C. chord off' and 'C. fingered' were swapped. In MT-56 service manual there is a fixed diode shown at KO11->KI8 with the hint "model selection MT-56" and "(MT-56 selection)"; I found out this switches the CPU into a different mode to set octave low (else the longer keyboard would glitch).
While upgrading blip percussion instruments with analogue percussion is not worth the effort (plenty of additional discrete circuitry), the opposite is very easy and even automatically disables analogue percussion (CPU pin 98 FC4) when a blip percussion mode is selected. Great is that the chord section key split point printed on the case allows to identify which type of percussion is inside; only melody section starting on F#2 means analogue.
When I first saw that Casio MT-88 used the same CPU like PT-100, it
remained a great riddle to me because they sounded and behaved so different.
I expected complex tricks like an external ROM and that the same digital
percussion output was gated and routed through a variety of external filters
to change their timbres. When I found out that the only other digital IC
was SRAM and not ROM and no digital percussion waveform was coming from
the CPU, I got even more puzzled until I finally saw in the MT-56 service
manual a fixed matrix diode KO11->KI8 "MT-56 Selection" (also in MT-105
schematics). So I analyzed my MT-88 and found even 2 those change behaviour
when absent.
separate volume controlsThe CPU outputs many sound channels on individual pins, thus separate volume controls can be added here. timbre filter switchesLike in CT-410V, the main voice is routed through fixed timbre filters, those in MT-88 are controlled by 2 CPU outputs pins 100 FC5 (coarse) and 1 FC6 (fine) for up to 4 timbre settings. In cheaper hardware variants like PT-100 the filter uses only FC6 for 2 timbres (seen in MT-56 & MT-105 service manuals). Pin 2 FC7 is no filter control but mutes the main voice during "mode" or "chord" switch operation to avoid popping. To make the filter switchable, cut the trace at pin 100 and 1 (I did it near the CPU but you may use a safer spot, e.g. at the "4066" IC of the filter). Install a switch with at least 3 positions into each of them to input either the original CPU pin, hi (+5V) or lo (GND) to the filter. A 2 position slide switch with open (intermediate) center position will work as well when you solder a pulldown resistor (I used 22k) against GND to the filter control input and make the switch only select between CPU output and +5V. Of course you may also install potentiometers (like I did in CT-410V) to distort the timbre. In keyboards without 4066 this may even permit gradual changes instead. (In my wiring I installed a pluggable connector at the switches to improve serviceability.)
Pin FC5=hi makes the timbre duller, while FC6=hi makes it slightly brighter. The combination of both hi is originally not used, but produces an intermediate sound. others
pinout HD61702The LSI "Hitachi HD61702xxx" (100 pin SMD, pins count anticlockwise, xxx = software number of internal ROM) is the CPU of Casio's last generation of non-PCM home keyboards. It was their most sophisticated squarewave and stairwave hardware with functionality resembling a D930G and D931C pair with DAC integrated into a single-chip. Unfortunately compared with D930G the functionality is very restricted. So the keyboard matrix supports only 12 preset sounds and rhythms, and beside a simple sustain switch there are no sound or accompaniment variations. The HD61702 has separate analogue outputs for polyphonic melody, chord, bass and obligato channel; the 8 note polyphony is shared among these. Also the blip percussion (can be disabled through 2 matrix diodes) is part of bass and obligato. Like the early HD44140, the upper and lower bits for each channel have separate DAC outputs (each 6 bit) those need to be combined through an external voltage divider (small resistor network) to reduce noise; the mixing ratio is 1:69 (1:110 at both ends of bit compensation trimmer) for main voice and 1:100 for others. All DAC outputs need 390 Ohm pulldown resistors against DAC ground 0V. External trimmers can be used for bit compensation to adjust smooth transition. The internal DACs also have plenty of individual supply voltage and ground pins to permit signal tweaking, and even their LSB is accessible on pins 25..34. There are 3 switch outputs for fixed main voice filters; one is only used as a pop blocker to mute during switch operations ('mode', 'chord', 'memory stop', but not preset sound changes). The CPU supports a ROM-Pack port and key lighting LEDs. An optional SRAM can be connected as sequencer memory.The sound generator seems quite similar like D931C. The preset sound waveforms are 5 bit high (up to 30 steps during attack, 18 steps for continuous tones) and apparently consist of 2 stairwaves with independent volume envelope, those can be detuned against each others for chorus and phasing effects. Most waveforms are symmetrical and seem to be made from up to 4 quaterwaves those are each 8 steps long and can be flipped and mirrored. Like with all stairwave instruments there is no interpolation, so the wave shape and height does not change with note pitch and all blockiness also remains in bass range (unless smoothed by external fixed filters). Interesting is how the waveforms are composed. E.g. the 'flute' resembles a blocky sine with small square "nipples" on their peaks those vibrate irregularly for wind noise. The 'piano' uses a strange geometrical trapeze shape with varying sunken center to change the harmonic contents. It was really an art to construct good sounding waveforms at that time, and yes it can sound nice. However unlike D931C the envelopes seem to be linear and end too soon, but this also may be result of too low DAC resolution. As far my analogue scope can tell, it may have only 8 bit (5 bit waveforms + 3 bits for 8 polyphony channels), but possibly the envelope resolution is a little finer. The blip percussion even seems to use only one DAC halve; at least their decay envelope doesn't look linear, and lissajous patterns on my scope hint that rough timbres are not samples but partly contain shift register feedback noise with varying bit patterns. Possibly the synthesis uses noise modulation (US patent 4656428) that adds random numbers to the readout address of a very low resolution sample to make hiss timbres more vivid. The versions of "Hitachi HD61702xxx" ("xxx" = software number of internal
ROM) differ in accompaniments, preset sounds and additional features.
This pinout was based on the service manuals of Casio MT-56, MT-105, MT-820, CT-450 (fullsize MT-520), SK-2100, SK-100 and photos of my keyboard hardware. The Hohner PK60 is an SK-100 with different preset sounds and rhythms, but is technically still based on SK-200 hardware class with same pinouts. caution: The service manuals indicate that this CPU uses "negative
logic", i.e. technically +5V is its GND while 0V is its -5V supply voltage.
So the voltages are not was the pin names suggest. I use the positive
voltage naming convention (from 0V to +5V, not -5V to 0V).
The analogue percussion trigger signals are multiplexed with the key matrix outputs KO1..KO7; they can be decoded by AND comparison with pin 98 FC3. Depending on 2 fixed key matrix diodes, the CPU output either analogue percussion triggers or blip percussion, but not both, so there is no easy way to mix them. The 'claves' is used only as a signal (memory button click, ROM-Pack error) but not in rhythm. In blip percussion mode FC3 outputs only a shorter tempo led pulse. The hiss waveform outputs 3 MNOISE and 4 WNOISE seem to be shift register feedback noise; they apparently need to be initialized, because they only start working after first use of rhythm. Pin 33 O31 works only with enabled blip percussion; with analogue percussion it stays hi. Because the MT-520 hardware has a percussion IC and so doesn't need the CPU percussion channel, it uses the surplus polyphony to layer 2 audio outputs (named "melody alpha", "melody beta") for a thicker main voice in the manner of classic Consonant-Vowel synthesis, and it does a stereo chorus in software; for this it delays the alpha or beta melody channel by FM and externally mixes them with opposite phase into stereo channels. Because drumpad hits can be faster than the keyboard matrix scan cycle, for buffering the 8 drumpad inputs each set a flipflop. To read their contents, the CPU sets LO12 high, which makes the flipflop bank appear on the KI lines of the keyboard matrix. Afterwards it sets LO13 high to clear the flipflops to get ready for sensing new drumpad hits. The early percussion IC of Casio CT-6000 has the number HD61701, which suggests that its grainy sound engine was possibly based on the same blip percussion hardware like HD61702 or at least was developed at the same time. |
The melody guide key lighting has 4 modes (not the keys itself light up but a row of small LEDs above them). Due to extreme similarity I only list here the differences to the PT-100.
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Like with Casio PT-82 accompaniment exists only as part of ROM-Pack musics and the melody guide key lighting has 4 training levels {1= with light, waits for correct key, 2= with light, no waiting, 3= without light, waits for correct key, 4= without light, no waiting} (but no rating). The "accomp." slider controls rhythm, obligato and accompaniment volume together; unfortunately also here the obligato voice plays too quiet in ratio to rhythm and accompaniment.
My MT-28 came with the default ROM-Pack RO-554 "Family Songs".
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An MT-28 variant without ROM-Pack was released as Casio
MT-25;
instead of the "guide select" slide switch it apparently has a 2 step "casio
chord" switch for single finger accompaniment like PT-100. A longer 44
midsize keys version of the PT-100 was released as Casio MT-105
(aka PT-200? | 2 speakers, stereo?) and a mono version as MT-56;
an MT-56 variant with sequencer and each 12 sounds and rhythms (buttons
like MT-28) came out as MT-55 (all seen on eBay). They all
have blip percussion. (With this hardware class the chord section split
point on the case shows the percussion type; only melody section at F#2
means analogue.)
| removal of these screws voids warranty... | ||
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