CASIO PT-100,
Casio MT-28		 small keyboard with accompaniment & great tekkno percussion,
small keyboard with ROM-Pack & key lighting

Casio PT-100

This midsize Casio PT series keyboard from 1987 (embossed case date) has polyphonic sound, nice rough accompaniment and extremely unusual sounding grainy digital percussion that is great for tekkno. All sounds seem to be made from 2 layered squarewave timbres with different envelopes. When modified, there are plenty of great eastereggs to explore.
 
A black key was missing yet.

main features:

eastereggs:

modifications:

notes:

This keyboard seems to belong to Casio's last and most advanced multipulse squarewave based instrument series, because unlike earlier models it contains a single chip CPU without external digital ICs, and its timbres are well tweaked and contain partly delayed vibrato or sustain and some even use their 2nd subvoice for a chorus effect although they still seem to be  based on the same old "Consonant- Vowel" synthesis technology that was employed in the great CT-410V and many earlier Casios since Casiotone 202. But I am not entirely sure if it the PT-100 uses beside multipulses already some rounder waveforms, because e.g. the "jazz organ" sounds a bit smooth for a squarewave timbre, although this may be also just a result of different analogue filters. The accompaniment section occupies 15 of the 32 keys, which shows that this hardware was mainly designed for larger instruments. Embossed dot stamps in the case indicate that my specimen was apparently built between 1987-04 and 1988-06.

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 squarewave organ tones (those resemble accordion basses). With rhythm, the accompaniment uses squarewave piano chords 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. 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.)

The percussion uses very unusual grainy and impulsive electronic timbres those contain much zipper noise. The cymbals are made from a hissy metallic waveform similar like on Casio MT-36 or Casio SK-1., but unlike SK-1, the drums are no blips at all but have still a vaguely realistic timbre. Particularly the base drum is fantastic; its timbre resembles rather a sort of African wood drum and is likely also made from 2 layered multipulse squarewaves (or ultra- low- res waveform samples?, or one of them a low shift register noise??) with much zipper noise. This drum sounds incredible grainy and buzzy - a bit like a snare but much more tonal; this thing goes "bomm bomm bomm" - perhaps like drumming on a rusty, half- full trash can ot the like.
 

circuit bending details

This instrument 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.), this one 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 "HD61702A02" was apparently also used as accompaniment CPU in Casio MT-88.

keyboard matrix

83
84
85
86
87
88
89
90
91
92
93
94
 
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
[crash]
R.
synchro/
key select
R.
key select
R.
tempo -
R.
stop
 
sustain off
 
 
 
 
in 1
47
 
R.
synchro
R.
stop
R.
R.
tempo +
R.
start/ stop
 
sustain on
 
 
R.
bank select
O.
bank select
in 2
46
o
o
o
o
o
o
o
o
 
 
R.
aRhythm
O.
violin
in 3
45
o
o
o
o
o
o
o
o
 
 
R.
aRhythm
O.
strings
in 4
44
o
o
o
o
o
o
o
o
C.
casio chord
 
R.
aRhythm
O.
flute
in 5
43
o
o
o
o
o
o
o
o
C.
chord off
 
R.
aRhythm
O.
harpsichord
in 6
42
o
o
o
o
o
o
o
o
 C.
fingered chord
 power
on
R.
aRhythm
O.
pipe organ
in 7
41
o
o
o
o
o
 o
o
o
o
 power
off
R.
aRhythm
O.
piano
in 8
40

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.

note: My documentation is a bit messy, thus this chart may contain some errors and is possibly incomplete. Especially the order of preset sounds may be wrong.
 

legend:
"o"
 = keyboard key
R.
 = rhythm
C.
 = chord
O.
 = orchestra (main voice sound)
orange
background 		 = easteregg (unconnected feature)

  • 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.) 
     
    These are the additional controls I have added. Note at the "tone" slide switch the red marks for the sounds those need to activate bank 2, and the white dots those indicate the intermediate switch positions.

  • 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 could 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.

separate volume controls

The CPU outputs near its lower right corner many sound channels on individual pins, thus likely separate volume controls can be added here.

others

    During measurement the CPU sometimes crashed into a mode where any keys produced only a blip noise. Likely the CPU intended to select melodies from a non- existing ROM- Pack cartridge here, but this might even be part of an "oriental keyboard" tone scale programming mode, because the "arabic" mode of my modified Casio SK-8 makes similar blips during tone scale selection.

    Connecting in the keyboard matrix lines "out 2" with "in 1" causes a crash (lockup?). Possibly optional additional ICs were intended to use this for communication.

Attention: I have only very incomplete draft schematics of my modification, thus this description might be partly inaccurate or even wrong since it is based on them. 

Casio MT-28

This is a Casio PT-100 variant with ROM-Pack, key lighting and each 12 preset sounds and rhythms. Unfortunately it completely lacks the accompaniment and manual chord of the latter, and also the great electronic percussion sounds here thinner and way less spectacular.
 
 

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.

different main features:

modifications:

notes:

Its really a pity that Casio omitted the manual chord & accompaniment of the PT-100, but they can likely easily added as matrix eastereggs. Unfortunately the wonderful rusty booming base drum of the PT-100 sounds here rather thin and boring (likely by changed discrete components), and generally rhythm and main voice sound brighter. The case of the MT-28 is made from the same dark blue plastic like with Casio MT-36. The preset sounds are selected by 6 locking buttons and a locking select button; with my specimen that select button didn't lock anymore due to a bent spring that engages into a tiny groove of the small plastic slider under the button. After many attempts I finally found out that the spring had to be squeezed shorter because it was pressing into the groove with too much force and thus prevented the slider from sliding fully up again after unlocking.

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".
 

An MT-28 variant without ROM-Pack was released as Casio MT-25; instead of the "guide select" slide switch it apparently had a 2 step "casio chord" switch for single finger accompaniment like Casio 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) was released as Casio MT-55 (all seen on eBay). A more expensive variant of this hardware was used in the ROM-Pack keyboard Casio MT-88.
 

 removal of these screws voids warranty...    
WarrantyVoid
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