Casio PT-7


 (instrument with tiny polyphonic touch sensor keypad & analogue rhythm)
This is clearly one of the most bizarre and innovative instruments created by Casio, because this tiny thing has a detachable keyboard with 29 soft touch foil keys, and this is not just a monophonic toy tablehooter but a real 8 note(!) polyphonic instrument with analogue rhythm and a small but high quality loudspeaker that makes a very respectable organ sound.



(This is an eBay picture, showing my specimen.)
This strange Casio invention permits special play techniques with very rapid glissandos and was one of the forgotten milestones in the struggle of overcoming the clumsy piano key relic on electronic consumer instruments.

main features:

  • 29 tiny soft touch foil keys on a detachable slim line keyboard with short cable
  • built-in astonishing reasonable sounding small speaker
  • 8 notes polyphony
  • separate knobs for main and rhythm volume
  • tempo knob
  • 6 preset rhythms {waltz, samba, swing, slow rock, pops, rock} selected by a slide switch
  • 8 OBS preset sounds {piano, elec. piano, organ, pipe organ harp, accordion, clarinet, violin} selected by a 4-step slider + switch
  • main voice CPU "HD44140, 3G 13" (56 pin SMD) with timbres based on multipulse squarewave tones with different digital envelopes, those are differently low pass filtered through capacitors. It makes very warm and pleasant timbres, but envelope release phase is truncated too soon, which makes piano- like tones less natural.
  • rhythm IC "NEC D8048C 316, 8322X7" (40 pin DIL, same like in Casio MT-40) that outputs trigger pulses for analogue drums
  • analogue percussion {base, snare, open cymbal, close cymbal, clave}; cymbals and snare use transistor noise.
  • jacks for AC adapter & headphone

eastereggs:

  • vibrato and sustain switch addable
  • up to 8 additional keys addable (makes no sense here)
  • holding down multiple sound select buttons during play makes wild cacophonic sounds (likely keyboard matrix mess by missing diodes).
  • possibly bass accompaniment (with additional keypad) addable
  • rhythm shitshot button addable

notes:

The main voice sound chip of this instrument is the same like in the Casio MT-45. It plays high quality analogue timbres, those although not always natural, reproduce a warm and pleasant sound; it does not sound typically C64- like thin, but resembles rather full- size home organs of that era. Like with Casio VL-1, the release phase of main voice envelopes seems to be linear and thus sounds unrealistic since it fades silent too soon with an audible end click. When sustain is switched off, all sounds stop almost immediately after releasing the key. When sustain is on, sounds with decay envelope (piano, elec. piano, harp) ignore the key press duration and sound always with a fixed duration instead. The "elec. piano" sounds like a banjo, and also the normal piano resembles more a picked string. The "harp" and "organ" sounds seem to add a bit of analogue distortion (or a mixed suboscillator with very short independent envelope??) during attack phase. All sounds include a mild vibrato. The smooth touch sensor keypad responds quite sensitive and permits special play techniques with rapid note clusters and polyphonic glissandos (but no portamento - this is not a theremin!).Attention: Never play with sharp, spiky or rough objects (like finger nails) on the sensor keyboard surface - the foil may get damaged easily.).
The percussion timbres resemble my Casio MT-70 and have the typical analogue home organ style. The rhythms are the same like in Casio MT-40and their tempo can be set ridiculously high but not extremely low.



circuit bending details

The main unit of this instrument is crowded with 3 stacked PCBs those contain the rhythm IC and a lot of analogue components. The PCBs are partly shielded by aluminiumized cardboard - likely to prevent interferences in the transistor noise generator of the analogue percussion. The detachable keyboard hangs on a very short, shielded 4 lead cable that apparently only conducts supply voltage, GND and the main voice audio signal. The keypad contains a small SMD PCB with the HD44140 CPU, various discrete components and a clock oscillator with 2 trimmers (for pitch tuning?). Attention: The sensor keyboard case is held closed by 3 tiny screws and 4 plastic tabs; to open it it is important to pull the plastic cover backwards (in the direction of the cable hole); do not bend it upward because this would crack off the plastic tabs. Be generally extremely careful with dismantling this delicate component.
The following I have concluded from my Casio MT-45 but not verified yet: the permanent vibrato seems to be hardwired by a chain of 2 diodes in series from CPU pin 55 to pin 56, that may be interruptable by an external switch. A sustain switch can likely be added by soldering it with 2 diodes in series from pin 54 to 56. The CPU has key matrix outputs from pin {2..7, 9..15, 17} and the corresponding inputs at pin 54..56, where theoretically 8 additional keys could be added, but in practise this makes not much sense because it is mainly the special sensor keypad that makes the special feel and the unique play techniques of this instrument possible and not just the sound.
The rhythm is generated by the accompaniment CPU NEC D8048C, which is the same IC like in my Casio MT-40 (also see there), where it additionally generates a monophonic bass accompaniment. Thus theoretically it may be possible to add also a bass button keypad like in MT-40 to the PT-7. But I expect that this would also need various external components for implementing the analogue bass decay envelope circuit (I did a similar mod for the arpeggio upgrade of my Casio CT-410V) and possibly even a new key matrix decoder, because in the MT-40 it is multiplexed by a "TC 4049 BP" IC (which existence I haven't checked in my PT-7). The bass keypad in the MT-40 is connected from pins {3, 5} of the 4049 to pins {12 .. 19} of the D8048C. The "fill-in" button (which also starts the accompaniment) is wired through a diode from pin 7 of the 4049 to pin 13 of the D8048C.
A lot of wild random crash sounds can be generated in my MT-40 by adding a shitshot button from pin 3 of the D8048C (for safety through a 1 kOhm resistor) against GND, which likely works also with the PT-7.
(I haven't upgraded my PT-7, not I have examined this hardware further yet.)
Unfortunately the Casio PT-7 with its special glissando keypad seems to be one of the rarest Casio keyboards at all (I yet saw no other specimen oneBay yet) and I also never heard of any other polyphonic instrument with this kind of controller. The same main voice sound like the PT-7 had also the Casio MT-45 and likely also Casio MT-11 and MT-21, but they all have ordinary midsize keys.



Casio PT-20

(photo from eBay, showing my specimen)

This tiny instrument seems to be just a downsized Casio PT-30 and sounds very similar. (I haven't examined the hardware yet.) Unlike the latter it is missing the LCD, the cartridge slot, some of the sequencer buttons, the transpose buttons and 2 high note keys (thus also the "arp. 6" rhythm). Instead of separate rhythm and chord volume sliders it has only a combined slider. Its small case resembles the Casio PT-1. The original German retail price in a German Conrad catalogue from 1986 was 199DM (about 100€).

The likely most advanced member of this hardware family was the ultra- rare keyboard boombox Casio KX-101 (37 mini keys, chord button pad, complex sequencer that saves data on audio cassettes), which was even 4 note polyphonic.