Casiotone 401

This was Casio's first keyboard with accompaniment and (so far I remember well) also the world first keyboard with automatic single finger accompaniment.
The Casiotone 401 has 16 unusual, partly incredible dry semi- analogue preset sounds and 16 analogue rhythms; also the bass sounds nicely warm and seems to be based on a triangular wave. Unusual is the "hold" switch, much like the manual fingered chord mode (with chord memory) on newer keyboards, here also the melody voice notes hold until any other chord or key combination is played. The noble designed case is unusually heavy, because it is made of plastic coated steel instead of genuine wood.

main features:

  • 49 fullsize keys
  • built-in big speaker (with reasonable bass)
  • main voice polyphony 8 notes
  • chord polyphony 12 notes
  • volume knob
  • separate accompaniment/ melody volume balance knob
  • tempo knob
  • 16 semi- OBS preset rhythms {rock, slow rock1, swing, bossanova, march1, waltz, rhumba, habanera | rock'n'roll, slow waltz, shuffle, samba, march2, rock waltz, beguine, mambo} (selected through 8 locking button switches + "variation" bank switch)
  • 14 OBS preset sounds {organ, flute, oboe, clarinet, trumpet, violin, cello, piano, harpsichord, celesta, accordion, electric piano, funny, frog} (buttons with each a red status LED)
  • "casio auto chord" switch {off, fingered, on} (manual chord mode with rhythm off)
  • "chord" switch {rhythmic, continuous}
  • chord memory switch (chord stays held after releasing key when on)
  • "octave down" switch (transposes main voice 1 octave down, works only in chord mode).
  • rhythm "fill-in" and synchro buttons
  • switches for vibrato, vibrato delay, sustain, hold
  • main voice timbres based on 2 mixed multipulse squarewave tones with different digital envelopes, those are differently low pass filtered through capacitors. Vibrato modulates CPU clock oscillator.
  • analogue percussion {base, snare, low tom, high tom, open cymbal, close cymbal, woodblock} which uses transistor noise for cymbal and snare
  • complex multi- chip hardware:
    • main CPU= "NEC D773G, K05266, Japan" (64 pin zigzag DIL)
    • accompaniment CPU(?)= "NEC D8049C, M0Z076-049, 084, Japan" (40 pin DIL)
    • IC "NEC D8243C, E01039-005, Japan" (24 pin DIL)
    • ROM(?) "Texas Instruments TMS3615NS, MBS [triangle] 8048" (28 pin DIL, socketed)
    • IC "AMD(?) AM 6012PC, 8050FM" (20 pin DIL)
  • tuning knob
  • mains operated
  • jacks for headphone, line out, aux in, volume pedal, sustain pedal, foot switch
Casio Auto ChordSERIAL NO. 181707, CT-401


Although on the case back this instrument is called CT-401, but its official name on the front panel was Casiotone 401. All preset sounds are selected through non- locking OBS push buttons with individual small red indicator LEDs; apparently Casio wanted to visually demonstrate here their use of advanced digital technology, since the similar looking OBS rhythm buttons employ still old fashioned locking switches. But all buttons and switches have still the classic 1970th home organ appeal with even different button colours per preset sound group; they all respond easily and feel quite noble. (The controls e.g. strongly resemble those on older Yamaha Electone organs of the A-, B-, C- and D- series.) The speaker grill has still the same style like with the Casiotone 201 (the world first Casio keyboard), although here it is dark brown instead of black. Impressive are also the massive count of 6 jacks on the back. Unfortunately there are no individual volume controls for rhythm and accompaniment (although these can be certainly added). My specimen came with the volume pedal Casio VP-1 and a stylish chromed original Casio keyboard stand.The Casiotone 401 is quite heavy (about 10kg?) because despite its case looks almost perfectly like wood, it is mainly made of plastic coated sheet steel. (Casiotone 201 and 202 were of genuine wood.) In spite of this it is not as robust as it appears, because at the corners of the side pieces (made of pressboard?) the plastic woodgrain tends to peel off. I had to hotglue mine back into place, but this sheet plastic stuff also cracks off easily when accidentally folded to hard. I bought my specimen in very dirty state; the whole case was severely sticky especially at the bottom. I am not sure whether it was fatty kitchen dirt or if anybody had tried to apply furniture wax on the case despite it is not of wood. I removed most of that gunk with a paper towel and vegetable oil (stay away from rubber parts) followed by water with dish washing soap.
The dirty speaker grill had to be removed also for cleaning, because someone spilled cacao or similar on it.
Be careful with the 5 metal lashes with screw holes those hold the rear part of the control panel from inside. With case opened, these lashes are sharp as razor blades and easily damage the panel and PCB traces; with mine a preset sound LED failed by an accidentally cut trace after working on the opened keyboard. After soldering the trace I glued ribbon adhesive tape over the sheet metal lashes to make them less dangerous. After opening, a lot of broken small black rubber rings fell out of the case - likely they belonged under the piano keys somewhere for damping (keys feel a little loose), but turned brittle by oily room air or ozone. The quite complex analogue hardware of this instrument has the size of an old style PC mainboard and contains many discrete components for the analogue percussion. The percussion have individual trimmers for their decay time. The main IC numbers and sounds have some similarities with the smaller Casio MT-40. The power amplifier is a quite big hybrid module. Unusual is that the fingered chord is up to 12 note polyphonic with additional monophonic bass, while the main voice has a polyphony of only 8 notes. (I haven't analyzed the hardware further yet).

Like with the Casio CT-410V, the main voice sounds of the Casiotone 401 consist of 2 mixed multipulse squarewave suboscillators with different pulse patterns and different digital volume envelopes, those are (depending on the preset) muffled by different filter capacitors. Thus both instruments have the same kind of sounds, although unlike the rather dull CT-410V,  many sounds of the 401 have a reasonably bright and partly incredible dry timbre, and when a key is trilled with sustain, each new note here occupies a new sound channel, which produces a great phasing sound and volume increase effect although this eats up polyphony. In the bass range many sounds turn into a more or less buzzy, sonorous purring drone, which is a characteristic style element of squarewave based instruments. These basses can resemble some of the famous POKEY sound effects on Atari XL homecomputers and are very different from the gradually duller and duller growing sine wave bass behaviour of average Yamaha FM keyboard sounds. (For further technical details about this hardware family also see here.)
By the use of simple low pass filters, many sounds play high notes too quite. The "organ" is a slightly dull metal pipe organ timbre, which seems to be a muffled variant of the that classic multipulse sound. The bass range reminds to a wooden accordion. The "flute" is dull and resembles a sine wave tone. As expected, "oboe" is harsher; the bass range resembles a saxophone. The "clarinet" turns thinner during its attack phase. The "trumpet" sounds quite thin and barely recognizable (like with many squarewave based instruments). The "violin" sounds ok (especially with vibrato), while the "cello" is dull and could be also a dull tuba or the like. The "piano" has a too dull bass range; the "electric piano" is the same in slightly brighter. The "harpsichord" sounds ok. The "celesta" is a musicbox sound that sounds unrealistic by a too slow attack phase; the timbre resembles a flute. The "accordion" has a fluttering attack phase (quickly quieter and louder again, more like a violin) and sounds too dull and plastic- like. The "funny" preset is a sort of rough digital slap bass (creaky picked e-bass) which sounds like "ennng!" (possibly it was initially intended to be called "funky", but sounded to artificial for this?). An even harsher variant of this is called "frog" (although it doesn't sound at all like one), which makes a wonderful rough and incredible dry bass noise. When sustain is switched off, all sounds stop almost immediately after releasing the key, and the sound presets itself also contain neither vibrato nor tremolo. The sustain duration depends on the preset sound. The vibrato is quite fast (6Hz) and its intensity depends on the preset sound. When "vibrato delay" is additionally pressed, the vibrato waits 1s before it starts (vibratos affect also sustain); with vibrato off this button does nothing. Very unusual is the "hold" switch, which makes the melody section of the keyboard behave like the manual fingered chord mode on most newer keyboards; the notes of any pressed key or key combination (e.g. chord) are held after key release, and after all keys are released, any newly pressed keys stops the old held notes and instead plays and holds the new ones. Apparently the algorithm of the manual fingered chord with chord memory was re-used for this. Despite there are nicely big semi- OBS preset sound buttons, they are no good realtime sound control since they mute held notes while pressed and cause a delay of about 0.3s after button release before held or new notes will sound again.
The percussion are typical soft analogue home organ drums; cymbals and snare are likely made from transistor noise. The patterns also include beside the usual stuff also a few nice latin rhythms and the "fill-in" button inserts a fill-in pattern (including accompaniment), which depends on the selected rhythm. Unusual is that during rhythm not only the "start/ stop" LED lights up during the first step of a bar, but both the red "start/ stop" and the (at that time expensive?) green "synchro" LED dimly flash during every step. Despite the locking button switches, the semi- OBS preset rhythm buttons switch a playing rhythm only at the end of the bar, which limits their use for realtime play tricks.
Unlike later Casio instruments, the accompaniment here was not yet called "Casio Chord", but "Casio Auto Chord" (abbreviated "CAC"), and it also did not feature yet an arpeggio. But the accompaniment of this historical instrument already accepts any disharmonic note combinations and not just those few ones that establishment has defined to be "chords". Apparently there was already more software behind than with the archaic Antonelli Star 2379, because the bass pattern here only plays its note sequence when it recognizes chord- like key combinations, while it plays only one note during wild clusters or a single key press. Unusual with the 401 is the "chord" switch; when set to "rhythmic", you get the usual automatic bass and chord accompaniment, but when set to "continuous" only the bass plays automatic the note of the lowest pressed chord section key, while all keys of the chord section otherwise are in manual organ chord note like with rhythm off. The accompaniment patterns are made from a monophonic bass line (some with walking bass) and simple staccatos of the current chord (or note combination). The chord voice is a plain squarewave organ tone, while the bass is a dull and warm organ bass tone with sustain that reminds to a dull Roland TB-303 bass and may be made from a triangular wave. With rhythm off there is a manual chord mode (12 note polyphonic!) with bass on the lowest played key. Unfortunately also here the notes always stay within 1 octave, thus it is no real key split. Regard that with this ancient Casio the chord section will not play any sound when the "chord" switch is set to "rhythm" while rhythm is off. With the "memory" switch the current chord is held after key release (with or without rhythm). Moving the "chord" lever switch slowly during rhythm varies the chord volume, which can be also used musically.
A strange Casiotone 401 variant without accompaniment section was released as Casiotone 301 (seen on eBay - likely extremely rare now since I never saw another one). This CT-301 had an identical case beside that it lacks the 4 white switches, the letter row above leftmost keys, the fill-in button and has only 4 jacks. Also the control panel sections are in different order (power switch, 16 preset sound buttons, vibrato switches, 8 rhythm buttons, rhythm start/ stop & synchro switches, 3 knobs) and employ the 2 black toggle switches here for rhythm start/ stop and synchro instead of the missing sustain and hold functions. Also the orange rhythm select switch apparently does something else, thus there are only 8 OBS rhythms.

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.