Is the MT-20 rare?

[Blake]

I picked up this little keyboard to try out circuit bending for the first time, but I'm hesitating because I can't find any information about it online. It feels almost sacrilegious to tear into a Casio if it is scarce. Does anyone have any knowledge on this keyboard?

[Chas]

@Blake- From what I can find online, the MT-20 appears to be a model somewhere in-between the MT-30 and the M-10, probably released around 1981 or so. It's quite limited having only 8 x Tones with just a sustain and vibrato switch, and that's pretty much it. It will be able to produce some of those nice vintage Casiotone "Consonant Vowel" sounds that are common among early Casios/ Casiotones.

 

As far as circuit bending goes, bear in mind that most of the "bends" carried out on early Casio models are to the rhythm/ accompaniment sections, because those sections used discrete analogue circuitry. The voice part of most early Casios used a part digital part analogue synthesis called Consonant Vowel. Most of the tones are generated from digital waveforms from within an LSI CPU, then put through some fixed filters. You cannot modify the waveforms outside of building and incorporating separately built amp envelope and LFO circuits. There are no easy bends to drastically modify the Tones unless the model has Easter eggs, i.e. its LSI has features from bigger models than aren't activated. Sometimes these are extra tones or even an octave drop. You might be able to find a tuning circuit that can affect pitch, and sometimes you can change values of potentiometers to give greater pitch adjustment ranges, or a crystal oscillator that can be swapped for one of a different value to also change the CPU clock speed that can also affect pitch.

 

Same applies to the vibrato circuit. If it uses analogue circuitry then you might be able to change component values to affect the behaviour of the vibrato.

 

Search online and see if you can find a service manual for this model, as then you will be able to see how the circuits are constructed and what components would have the potential for bending.

 

Lastly, the MT-20 is not one of the sought after early Casios and thus neither valuable nor collectible. The MT-30 likely shares the same tone set and has a lot more additional tones, and the MT-40 "Sleng Teng" adds the famous accompaniment section famously used to generate the bass and rhythm for Wayne Smith's hit reggae single "Under mi Sleng Teng". Unlike the other early (1981 - 1983) MT models, the MT-40 is very sought after and valuable as a result of its use in a hit song. 

 

My advice would be to find the MT-20 service manual to see what, if any, potential bends might be available before attempting any bending. Then you can decide if it's worth spending time bending it, or whether it best to leave it standard as a nice little vintage Casiotone that can still produce some unique tones.

 

 

 

 

On 6/9/2021 at 1:55 AM, Blake said:

I picked up this little keyboard to try out circuit bending for the first time, but I'm hesitating because I can't find any information about it online. It feels almost sacrilegious to tear into a Casio if it is scarce. Does anyone have any knowledge on this keyboard?

[CYBERYOGI =CO=Windler]

The MT-20 sound engine is much simplified compared to actual Consonant-Vowel, so it lacks many filters and only layers 2 multipulses instead of stairwaves and (that's bad) volume envelopes use linear envelopes those decay too fast. So sounds are between consonant-vowel and VL-1 (rather the latter). The chip also has no interesting eastereggs.

 

The LSI "Hitachi HD44140" (54 pin SMD, pins count anticlockwise from below the left stub pin) was Casio's most simple polyphonic single chip main voice hardware of early 1980th keyboards. Compared with the very versatile first generation predecessor D77xG, it was cut down to have as few functions as barely needed for a short beginners keyboard. So despite 8 note polyphony the keyboard matrix supports only 37 keys and 8 preset sounds with 1 sustain and 1 vibrato switch. Unlike D77xG the chip produces no feelable heat, so the transistor count was likely minimized for long battery life (possibly created for the mythical solar cell powered mouth organ that became the PT-7). A bit strange is the integrated DAC that 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).

 

The sound generator is a very simplified variant of Consonant-Vowel synthesis. Each preset sound is made from 2 layered multipulse squarewaves (8 steps long?) crossfading by simple volume envelopes those do 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. (I.e. unlike D77xG there are neither ramp nor curve sections.) 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 (1.425 MHz?) prevents cold aliasing noise. The preset sounds 'clarinet' and 'piano' set the external fixed lowpass filter to 750Hz, else it stays 1.1kHz (seen in MT-11 service manual). Unlike D77xG there is no spread scale, i.e. holding the same notes in different octave causes no phasing (but phase changes among multiple key presses, thus there are no octave dividers involved).

Edited January 18, 2022 by CYBERYOGI =CO=Windler

[pianokeyjoe]

Funny to find THIS thread not in the CLASSIC CASIO section so good thing I found it! As Chas and Cyber Yogi explained, it is a simple keyboard but alas alas it is a rare Casio model! So no, do not bend it. Now, folks, here is a sample of what it sounds like!

It is just like the MT11 and MT45 but without the beats and accomp. So it was released with or actually AFTER the MT11,30,31,40,41,M10, and even the MT45/46.MT60/65/68/35/36, and CZ101! How do I know this you ask? The HARDWARE of the keys.. Those keys were released on newer Casio MT and err... PT models as of mid to late 1986. The MT20/21 used the MT11 sound internal hardware, but the same style strip keys released for the MT52/55/18/PT100,etc. Funny is that those keys were featured mostly on 49 note MT models from 86-89! So the shorties where indeed rare with those STRIP keys. The Casio keyboards made in the early 80s(80-85)were individual keys on both a metal keybed and molded into case keybeds. Casio recycled internal hardware in their models to keep using up the stock and obviously make money, but as you can hear from this video, the sounds used are MT11. The MT45 melody side if you want to go there too lol. So if you bend a MT45 or MT11, that is one thing, but the MT20/21 are more modern release of the same electronics as Cyber Yogi said. It is like the PT100. The PT100 has some recognizable tones from the MT205 but very sparse internal hardware of which you can not bend so easy cause the sounds where built into a more modern and reduced function LSI spider IC chip.. Aahh but fun to try!!

[Brad Saucier]

10 minutes ago, pianokeyjoe said:

Funny to find THIS thread not in the CLASSIC CASIO section so good thing I found it!

 

I hear you.  It's there now.   

[pianokeyjoe]

Lol! Thankyou my friend! I am actually on the hunt for classic Casio topics now through out the forum to respond to if I can. Now, here is another example of your MT20, but this time it is the MT21 which has 4 RHYTHMS.

As you can hear, it is the SAME Casio MT11/45 melody sounds, just now with beats added. Also note the sliders and buttons of your MT20/21. These were not in use until the late 80s(86-88).

[CYBERYOGI =CO=Windler]

The MT-21 uses for rhythm the MCS-48 microcontroller "Toshiba TMP8048P 1104". It is almost identical with "NEC D8048C-316" of MT-40. Even the bass accompaniment is fully functional (verified by inserting into Casio MT-40 and vice versa) despite no instrument is known to make use of it. Only rom dump behaviour differs (in Toshiba a section is scrambled/repeats).

 

The MT-205 is not related to PT-100 at all but a cheapened variant of MT-540.

 

Of Casio PT-100 there was a bigger version with genuine analogue percusion (IMO sounds boring) named MT-88. I figured out how to additionally enable the (much more interesting) PT-100 blip percussion in it. The CPU is "Hitachi HD61702 A02".

 

blip percussion, chord split point, octave

 

In MT-56 service manual there is a fixed diode shown at 93 KO11->47 KI8 to with the hint "model selection MT-56" and "(MT-56 selection)" The model selection 2 diode can be at 94 KO12->47 KI8. During power-on, the CPU scans presence of these 2 fixed matrix diodes to set the general operating mode of the instrument. The PT-100 contains none, the MT-88 both. Wire them through 2 switches to get interesting behaviour changes. Theoretically even push buttons are sufficient instead of locking switches, but you would need to hold them during every power-on to select the mode.

 

model select 2	model select 1	percussion type	chord spit point	octave
		blip	A#2	hi
	X	blip	A#2	lo
X		blip	C2	lo
X	X	analogue	F#2	lo

 

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.

 

The analogue percussion mode disables blip percussion and instead outputs an additional continuous bass voice in accompaniment and percussion trigger pulses on key matrix outputs KO1..KO7, those can be demultiplexed by AND comparison with CPU pin 98 FC4 to trigger external analogue percussion circuits (see MT-88). All blip percussion modes turn the "synchro start" button into a "synchro/ key select" button that first waits for pressing a white key to select a rhythm (and exits with wrong key press).

 

Interesting how other modes introduce various bizarre glitches and throw a wrench into key lighting modes (of MT-88). Without these diodes, the keyboard is assigned 1 octave higher, so in melody guide mode you have to play 1 octave lower(!) than what the leds indicate, and because soon you end up in the chord section, you can not reach some requested notes at all but only step further through the song by pressing "one key play". Also the highest keyboard octave in 'flute', 'strings' and 'celesta' makes mess; above C4 they play disharmonic low notes those form no regular tone scale and grunt lower than the normal C1. Maybe a lookup table ended too soon to save ROM space, but possibly this was a test mode or even a hidden message in the chip. The keys from C#4 to C5 play the notes {B#-1, B#-1, B#-1, B#-1, C#-1, C#-1, C#-1, B#0, B#0, G1, E#1, B-1}. At least the G1 and E#1 are detuned against the normal notes. The falling sequence length of equal notes hints to a part of a waveform or envelope curve, or did aliens compose this sequence to contact us?!? (Recording this in the MT-88 sequencer and replaying it on a normal preset sound plays the regular notes C#4 to C5, which proves that the glitch is in the sound engine itself and not keyboard decoding. Switching back to normal diodes keeps the high and faulty notes in the sequencer, which shows that the SRAM stores note numbers and not key numbers.) With "model select 2" diode the chord section is too short to play chords like displayed in chord guide mode. Instead you have to play those chord notes an octave lower, which is pretty confusing. Only with "model select 1" or both diodes the guide modes work ok.

Edited January 28, 2022 by CYBERYOGI =CO=Windler

[pianokeyjoe]

@CYBERYOGI =CO=WindlerWow! Ok, now that makes sense about the PT100. Sadly, I never had the MT88. Would the MT55 which I only played briefly as a child in a Pawn shop in Puerto Rico, be related in some way to the PT100? I always thought so. I think the MT88 you speak of would then be the 49 note version of both the PT100 and MT55? The sounds in both of those units sound the same, including the blip percussion you speak of. I actually like the blips myself. The SK1 has a more in your face version of those blips which I never heard on any other Casio model before or since. The MT56??? Is that a 49 note version of the MT55? The MT55 is 44 notes only like the MT52. So with the info you give here, IF I should come across a MT88 I can hack it and have BOTH the analogue percussion voices(MT65 type I guess), and blip voices(MT55 type I assume)? Is there any other model that you can affect the analogue percussion voices that way too? MT45/65? I have those! So many choices, so little money! That is my Casio collector's motto of the year lol!

[pianokeyjoe]

@CYBERYOGI =CO=WindlerCorrection on comparison of PT100 to cheapened version of MT540? Is it instead MT520? The MT540 was a new generation PCM midi based 210 tonebank keyboard with dual tone layering, and PCM sound effects pads. I have this model as well as the PT100. The MT205/MT520 however, do have some like digital pulse analogy tones to the PT100, namely SYNTH-SOUND, and VIOLIN(the PT100 has VIOLIN as a hidden easter egg when you set the tone select slider inbetween no sounds, and power on). The vibraphone sounds alike too but obviously the PT100 is mono speaker so it sounds chorusy and loud. Now back on point about the MT20/21. You say the MT21 in particular has the same Percussion as the MT40? I only owned a MT41 once. So basically, the MT21 is a cross breed of the MT11 with MT40 beats.. I thought it would have been more like the beats section of the MT35/36 but I have MT35/36, I never had MT21 so I can only go by this one rare youtube videos of both the MT20 and MT21. Again.. so many choices! So little money!

[CYBERYOGI =CO=Windler]

Yes, MT-205 is MT-520 based. The CPU is close relative of PT-100 and MT-88 but with different software, and because MT-520 has a sample based percussion IC "OKI M6294-02", it doesn't need the CPU blip percussion channel and so uses the surplus polyphony to layer 2 audio outputs for a thicker main voice in the manner of classic Consonant-Vowel synthesis and even can do a stereo chorus within the CPU.

 

Percussion often differs among Casio models with same CPU due to changed analogue filters and in analogue percussion instruments even omitted individual drums (output pin disconnected or wired to another drum). E.g. the famous rusty base drum of PT-100 sounds much tamer in other keyboards with this CPU. E.g. MT-40 had a terribly dull analogue snare that got fixed in later models.

 

Casio PT-100 is a cheapened single-chip version of consonant-vowel hardware with accompaniment (sort of dumbed down MT-65).

 

The 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 DAC bits for each channel have separate outputs 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.
 

software number	hardware class	notes & features
A02	MT-88/PT-100	original version
A03	MT-520	Super Drums support, uses percussion IC M6294-02, melody=2 layered outputs
A04	SK-200, SK-100	combined with sampling CPU M6283-02 + percussion IC
A06	Hohner PK60 (SK-100 variant)	modified A04?, combined with sampling CPU M6283-06 + percussion IC

 

Edited January 29, 2022 by CYBERYOGI =CO=Windler

[pianokeyjoe]

@CYBERYOGI =CO=WindlerYou sir, are THE MAN!! Nice concise and detailed info! You are like the guru of Table Hooters!! Definitely the guru to go to when collecting ..well.. TABLE HOOTERS lol! Man oh man. So that is why I can circuit bend the MT205/520 so easy and get so many extra sounds that are musically pleasing with just that model set? The layering feature of that software revision? Cause wow, what a sound I get when I tinker inside those units! The PT100 would not let me do sound tinkering for NOTHING. Thankfully I did not fry the PT100s I had but still, would have been nice to get more tone variations. The MT11/45 and MT20/21 would seem also to allow sound tinkering as does the MT52/CT320 and MT65/68/CT405/etc but in different ways. Your research of the PT100 explains so much as to why it is so limited I can not even bend it, at least not so easily. Though I could not bend the SK1/SK5 either so easy and yet, the Casio circuit bending movement started with these models?? I may be wrong but seems you google Casio circuit bending or midi retrofit, and SK1 pops up first. Now because this is very interesting I may start a thread on the PT series. I can bend the PT1 and indeed that was my first ever Casio and as such my first ever experience into electronics and then into bending.. I am amazed no one here has a thread on PT series keyboards and there tech, bending possibilities and features,etc..

[CYBERYOGI =CO=Windler]

Casio PT-100 hardware has tons of eastereggs. I only absolutely hate this crappy fartsmone-ready $*@%! forum GUI that does not let me properly edit anything despite Firefox pastes HTML. So the bullets jam and spill dud jackets everywhere. I haven't tried much with MT-520 although I got the original service manual.

 

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. Also MT-28 and PT-180 are crippled variants. 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.)

 

keyboard matrix

 

83 KO1

84 KO2

85 KO3

86 KO4

87 KO5

88 KO6

89 KO7

90 KO8

91 KO9

92 KO10

93 KO11

94 KO12

pin

out 1

out 2

out 3

out 4

out 5

out 6

out 7

out 8

out 9

out 10

out 11

out 12

out/ in

demo

memory start/ stop

R. synchro/ key select

R. key select

R. tempo -

R. stop

one key play 2

sustain off

G. IV

model select 1 [diode]

model select 2 [diode]

in 8

47 KI8

memory record

R. synchro

R. fill-in

R. tempo +

R. start/ stop

one key play 1

sustain on

G. II

R. bank select

O. bank select

in 7

46 KI7

o F1

o B1

o F2

o B2

o F3

o B3

o F4

o B5

G. III

M. chord guide 2

R. samba

O. violin

in 6

45 KI6

o E1

o A#1

o E2

o A#2

o E3

o A#3

o E4

o A#4

G. I

M. chord guide

R. swing 4 beat

O. strings

in 5

44 KI5

o D#1

o A1

o D#2

o A2

o D#3

o A3

o D#4

o A4

C. casio chord

M. melody guide

R. swing 2 beat

O. flute

in 4

43 KI4

o D1

o G#1

o D2

o G#2

o D3

o G#3

o D4

o G#4

C. fingered chord

M. auto play

R. 16 beat

O. pipe organ

in 3

42 KI3

o C#1

o G1

o C#2

o G2

o C#3

o G3

o C#4

o G4

C. chord off

M play

R. disco

O. harpsichord

in 2

41 KI2

o C1

o F#1

o C2

o F#2

o C3

o F#3

o C4

o F#4

o C5

M.  power off

R. rock

O. piano

in 1

40 KI1

 

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.

legend:
"o"	= keyboard key
underlined	= function needs locking switch (i.e. stays active only so long the switch is closed)
R.	= rhythm
C.	= chord
O.	= orchestra (main voice sound)
M.	= 'mode' switch
G.	= 'melody guide' switch
orange background	= easteregg (unconnected feature)

 

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)"; this likely switches the CPU (key matrix behaviour?) into a different mode (octave low?).

• main voice sounds ("tone")
• The main voice sounds are selected by 6 individual button inputs together with a bank switch input. Normally these are controlled by the 8 position "tone" slide switch which automatically activates the bank switch through a 2nd contact row while selecting the sounds {vibraphone, jazz organ, synth sound}. To get access to all 12 preset sounds, disconnect the "O. bank select" line from the slide switch and instead wire it through a diode to a (non- locking) button switch. According to comparison with other keyboards of the same hardware class, the names of the new preset sounds seem to include "violin", "clarinet" and "celesta". The last sound is a harsh electronic organ tone with fast attack phase and chorus, that resembles "brass ens." on small Casio ToneBank instruments.

• note: After the modification the "tone" switch positions {vibraphone, jazz organ, synth sound} will instead select {piano, pipe organ, violin} so far the 2nd bank is not selected.

• To select sounds will now become a little tricky, because to select any sound of the bank 2:
   
  1. hold down the bank button,
  2. then move the "tone" slider to the intended position,
  3. then move it to an intermediate switch position next to it,
  4. and finally release the bank button.

• If you release the button while the "tone" switch is still in a valid (not intermediate) position, it always switches back to bank 1. 

• It would be likely better to add a locking switch instead of the button, but I discovered that the button can be also well used as a realtime sound control to rapidly bounce back and forward between both banks during held notes (which also re- triggers their envelope); if you are in doubt, add both wired parallel. (The "tone" slider itself responds rather slowly and thus constitutes no good OBS realtime control.) 

• rhythms
• The rhythm bank select input "R. bank select" works very similarly like the sound one (see above), but fortunately it is not necessary to rewire here anything, because you can instead add the "R. key select" button that selects all 12 rhythms through the leftmost white piano keys. These rhythms are {rock, disco, 16 beat, swing 2 beat, swing 4 beat, samba, bossa nova, beguine, tango, march, slow rock, waltz} and exactly correspond to the rhythm set of small ROM-Pack keyboards (like Casio PT-82).

• fingered chord

• Originally the instrument has the "casio chord" 2 step slide switch to switch chord/ accompaniment either "off" or to single finger chord mode. But additionally also a fingered chord mode exists, which permits much more versatile accompaniment play and makes great organ basses with rhythm off. Thus I disconnected the "casio chord" switch and added instead 3 buttons to the inputs "C. chord off", "C. fingered chord" and "C. casio chord". Theoretically also a 3 step switch can be added here, but the only disadvantage of separate buttons is that the instrument always comes up in single finger mode after switching it on.

• sustain

• Either a locking switch or 2 buttons can be added to switch sustain for the main voice on and off. I rewired the disabled "casio chord" slide switch for this purpose. The pedal sustain of CT-805 is no additional easteregg but only activates normal sustain (does nothing when already on).

• fill-in

• A fill-in button can be installed at KO4->KI7.
   
• blip percussion, chord split point, octave

• During power-on, the CPU scans presence of these 2 fixed matrix diodes to set the general operating mode of the instrument. The PT-100 contains none, the MT-88 both. Wire them through 2 switches to get interesting behaviour changes. Theoretically even push buttons are sufficient instead of locking switches, but you would need to hold them during every power-on to select the mode.

•  

  model select 2	model select 1	percussion type	chord spit point	octave
  		blip	A#2	hi
  	X	blip	A#2	lo
  X		blip	C2	lo
  X	X	analogue	F#2	lo

  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.

• The analogue percussion mode disables blip percussion and instead outputs an additional continuous bass voice in accompaniment and percussion trigger pulses on key matrix outputs KO1..KO7, those can be demultiplexed by AND comparison with CPU pin 98 FC4 to trigger external analogue percussion circuits (see MT-88). All blip percussion modes turn the "synchro start" button into a "synchro/ key select" button that first waits for pressing a white key to select a rhythm (and exits with wrong key press).

• Interesting how other modes introduce various bizarre glitches and throw a wrench into key lighting modes (of MT-88). Without these diodes, the keyboard is assigned 1 octave higher, so in melody guide mode you have to play 1 octave lower(!) than what the leds indicate, and because soon you end up in the chord section, you can not reach some requested notes at all but only step further through the song by pressing "one key play". Also the highest keyboard octave in 'flute', 'strings' and 'celesta' makes mess; above C4 they play disharmonic low notes those form no regular tone scale and grunt lower than the normal C1. Maybe a lookup table ended too soon to save ROM space, but possibly this was a test mode or even a hidden message in the chip. The keys from C#4 to C5 play the notes {B#-1, B#-1, B#-1, B#-1, C#-1, C#-1, C#-1, B#0, B#0, G1, E#1, B-1}. At least the G1 and E#1 are detuned against the normal notes. The falling sequence length of equal notes hints to a part of a waveform or envelope curve, or did aliens compose this sequence to contact us?!? (Recording this in the MT-88 sequencer and replaying it on a normal preset sound plays the regular notes C#4 to C5, which proves that the glitch is in the sound engine itself and not keyboard decoding. Switching back to normal diodes keeps the high and faulty notes in the sequencer, which shows that the SRAM stores note numbers and not key numbers.) With "model select 2" diode the chord section is too short to play chords like displayed in chord guide mode. Instead you have to play those chord notes an octave lower, which is pretty confusing. Only with "model select 1" or both diodes the guide modes work ok.
• 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.
• rom pack modes

• The additional  'melody guide' and 'mode' switch settings need a rom pack port. The 'chord guide 2' mode not even exists in MT-88; it combines chord guide for the left hand with free play (key leds off, no automatic melody) for the right hand. Installing a ROM-Pack port to the CPU would be electrically simple but mechanically complicated. Adding key led circuitry is not really worth the effort.

separate volume controls

The CPU outputs many sound channels on individual pins, thus separate volume controls can be added here.

timbre filter switches

Like 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.)
 

preset sound:	FC5 (coarse)	FC6 (fine)
piano	H
harpsichord		H
pipe organ		H
flute	H
strings		H
violin
vibraphone	H
celesta		H
jazz organ	H
clarinet	H
reed
synth sound		H

 

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

• The sequencer function 'memory start/stop' is at KO2->KI8 needs an optional sequencer SRAM "NEC D446C-3"; without it will crash (lockup?).

Edited January 30, 2022 by CYBERYOGI =CO=Windler

[HauntedHarmonics]

Sorry to revive this dead thread, but google led me here when researching…

 

Does anyone happen to have a service manual for the PT-100 or any of its close variants? MT-56 or MT-105 perhaps?

 

I’ve been looking for ages and have turned up nothing :/

 

By the way, it’s an honor to post in the same thread as you, Windler. I’ve been using your site for years, it’s an incredible resource for circuit bending & vintage keyboards, i’ve referenced it during my builds too many times to count

[Chas]

@HauntedHarmonics  -  I did a search online but couldn't find a Service Manual. I did find this site that states a Service Manual may not even exist for this model. It does, however, have some useful information that might help:

https://circuitbending.miraheze.org/wiki/Casio_PT-100

 

[CYBERYOGI =CO=Windler]

I have no service manual of the PT-100. Some discrete percussion filter parts likely differ from other models, but the rest can be concluded without. Here is some more CPU info, if anybody needs it.

pinout HD61702

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).
 

pin name purpose 1 FC6 melody filter switch out (fine) | in MT-520: chord filter switch 2 FC7 melody /mute switch out 3 MNOISE metallic noise out (for analogue percussion) 4 WNOISE white noise out (for analogue percussion) 5 VDD1 ground 0V 6 VDD2 rom pack pin 4 | in MT-520: percussion ic ground 7 GC (+2.3V in MT-88) 8 GND0 supply voltage +5V 9 GND2A DAC O2A supply voltage +5V 10 O2A chord audio out (upper bits) | in MT-520: melody beta audio 11 O2B chord audio out (lower bits) | in MT-520: melody beta audio 12 GND12B DAC O1B, O2B supply voltage +5V 13 O1B melody audio out (lower bits) | in MT-520: melody alpha audio 14 O1A melody audio out (upper bits) | in MT-520: melody alpha audio 15 GND1A DAC O1A supply voltage +5V 16 GND4A DAC O4A supply voltage +5V 17 O4A obligato+base+hi conga+lo conga audio out (upper bits) | in MT-520: bass audio 18 O4B obligato+base+hi conga+lo conga audio out (lower bits) | in MT-520: bass audio 19 GND34B DAC O3B, O4B supply voltage +5V 20 O3B bass+cymbal+hihat+snare audio out (lower bits) | in MT-520: chord audio 21 O3A bass+cymbal+hihat+snare audio out (upper bits) | in MT-520: chord audio 22 GND3A DAC O3A supply voltage +5V 23 VDAC1 DAC ground 0V 24 VDAC2 DAC ground 0V 25 /O11 (stays hi, not used) 26 O11 (stays lo, not used) 27 O12 melody DAC /LSB (not used) 28 O13 melody DAC LSB (not used) 29 O14 melody DAC LSB (not used) 30 O15 melody DAC LSB (not used) 31 O16 melody DAC2 LSB (not used) 32 O21 chord DAC /LSB (not used) 33 O31 bass+cymbal+hihat+snare DAC /LSB (when blip percussion, else hi | not used) 34 O41 obligato+base+hi conga+lo conga DAC /LSB (not used) 35 GND5 supply voltage +5V 36 GND2 supply voltage +5V 37 GND1 supply voltage +5V 38 OSI clock in (7.808MHz) 39 OSO clock out 40 KI1 key matrix in | MT-520: snare pad 41 KI2 key matrix in | MT-520: base pad 42 KI3 key matrix in | MT-520: clap pad 43 KI4 key matrix in | MT-520: rimshot pad 44 KI5 key matrix in | MT-520: lo bongo pad 45 KI6 key matrix in | MT-520: hi bongo pad 46 KI7 key matrix in | MT-520: cymbal pad 47 KI8 key matrix in | MT-520: hihat pad 48 MI power switch on 49 RESET reset 50 DTE (wired to ground)

pin name purpose 51 TEST (wired to ground) 52 /WE ram write enable 53 /CE1 chip enable (ram, rom, percussion ic) 54 CE2 rom pack pin 2 55 C1 rom pack pin 5 56 C2 rom pack pin 6 57 OP rom pack pin 7 58 D0 data bus (ram, rom, percussion ic) 59 D1 data bus (ram, rom, percussion ic) 60 D2 data bus (ram, rom, percussion ic) 61 D3 data bus (ram, rom, percussion ic) 62 D4 data bus (ram, rom, percussion ic) | rom pack pin 8 63 D5 data bus (ram, rom, percussion ic) | rom pack pin 9 64 D6 data bus (ram, rom, percussion ic) | rom pack pin 10 65 D7 data bus (ram, rom, percussion ic) | rom pack pin 11 66 LO1 key led out | in MT-520: key matrix out, address bus A12 out 67 LO2 key led out | in MT-520: key matrix out 68 LO3 key led out 69 LO4 key led out  70 LO5 key led out | in MT-520: rom /CS 71 LO6 key led out | in MT-520: ram /CS 72 LO7 key led mux out | in MT-520: percussion ic /CS 73 LO8 key led out | in MT-520: tempo led out 74 LO9 key led out | in MT-520: record led out 75 LO10 key led out | in MT-520: rhythmSel2 /rhythmSel1 led out 76 LO11 key led out | in MT-520: toneSel2 /toneSel1 led out 77 LO12 key led out | in MT-520: drumpad flipflops read out 78 LO13 key led out | in MT-520: drumpad flipflops clear out 79 LO14 key led mux out | in MT-520: /APO auto-power-off out 80 VDD3 ground 0V 81 GND3 supply voltage +5V 82 CKM2 clock/2 out 83 KO1 key matrix out, address bus A0 out, base trigger 84 KO2 key matrix out, address bus A1 out, snare trigger 85 KO3 key matrix out, address bus A2 out, claves trigger 86 KO4 key matrix out, address bus A3 out, lo conga trigger 87 KO5 key matrix out, address bus A4 out, hi conga trigger 88 KO6 key matrix out, address bus A5 out, hihat trigger 89 KO7 key matrix out, address bus A6 out, cymbal trigger 90 KO8 key matrix out, address bus A7 out 91 KO9 key matrix out, address bus A8 out 92 KO10 key matrix out, address bus A9 out 93 KO11 key matrix out, address bus A10 out 94 KO12 key matrix out, address bus A11 out 95 VCON (stays hi)  96 FC1 tempo led out | in MT-520: melody gain & filter switch out 97 FC2 memory led out | in MT-520: melody gain & filter switch out 98 FC3 percussion trigger mux out | in MT-520: melody gain switch out 99 FC4 /APO auto-power-off out | in MT-520: melody gain switch out 100 FC5 melody filter switch out (coarse) | in MT-520: bass filter switch out

 

The analogue percussion trigger signals (used in MT-88) 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.