Casio CT-405 (MX-41)
A Casiotone that hides a genuine synthesizer voice chip and a real filter stage under its home-keyboard plastic.
Overview
The CT-405 is one of those Casios that rewards a look under the hood. Service documentation for the model - filed by Casio as the MX-41 - shows a 42-pin N-MOS LSI doing the tone generation. It is an eight-note polyphonic voice chip, and it runs under the control of a separate accompaniment CPU rather than handling its own housekeeping. Casio's own service notes describe the part as having "new features of digital reverberation" and delayed vibrato, which is a more interesting feature list than the typical preset organ box of the period. The chip takes its orders from the accompaniment LSI - data lines on pins 1 through 4, timing pulses on pins 6 and 7 - and puts out digital sound on pins 25 through 41. Two chips, then, which is why the voice architecture is a bit more ambitious than the plastic case suggests.
The filter is the reason to open one up. The service manual documents a filter cutoff frequency change-over signal. The major waveform plots trace both a DAC output and a filter output, captured at the F12 chip with an organ tone on C3. There is a real filter stage in the signal path, not just a static tone switch, and the documentation shows the cutoff being switched under digital control. For a home keyboard from this era, that is not nothing.
The percussion section is done the old-fashioned way. According to the service manual, the bass drum, snare drum, claves, and both congas come from CR oscillator circuits. These are capacitor-and-resistor tanks where the oscillation frequency is set by the relationship between capacitance and resistance, with the trigger pulse supplied by the accompaniment LSI. The manual even gives the formula: one over two pi times the square root of six times C times R. Each drum voice has its own trim pot and its own target decay length: bass drum 15 ms, low conga 27 ms, high conga 16 ms, claves 3.9 ms, snare 7 ms, all with a plus-or-minus ten percent tolerance. The cymbal, hi-hat, and snare also draw on white noise generated from two random pulse streams out of the accompaniment chip. That is a different synthesis method for the noise-based voices, and the service documentation lays it out in plain language.
The accompaniment LSI handles the chord, bass, and arpeggio duties. Pin 4 outputs only on seventh chords, so the chord recognition logic is doing real work rather than just triggering fixed patterns. Chord envelope signals occupy pins 7 through 10, bass envelope is pin 11, arpeggio envelope is pin 12, and the trigger pulses for bass drum, snare, claves, hi-hat, and cymbal come out on pins 13, 14, 18, 19, and 20 respectively. A Schmitt trigger pulse on pin 28 initializes the internal circuits when the power switch is thrown. The clock runs at 4.9 MHz into the tone LSI, with a divided 1.6 MHz output on pin 10.
Casio's own service notes for the wider Casiotone family say the MT-68 is the same as the MT-65 but with a different color case. Casio clearly liked spinning multiple model numbers out of one chassis. The CT-405 shares its service manual with the MT-65 (MX-40) and MT-68, which suggests a close family relationship, even though the CT-405 carries its own MX-41 designation and its own PCB part numbers. The parts list references multiple board revisions - M441-MA1M, M441-AS1M, and M441-CN1M all appear in the documentation - and at least one ladder network in the signal path was changed mid-production with non-interchangeable pin configurations. That matters when you are trying to source a replacement board.
One hard caution for buyers: the graphic equalizer IC, the LA3600, is documented in the related MT-100 service notes as a five-band part whose resonance frequency is set by external capacitors. The surrounding technical bulletin warns that a ladder network in the first production version was swapped for a different part in the second version, with different pinouts. If you are repairing a CT-405 and the board does not match your schematic, you may be looking at the other revision. Check the PCB silkscreen against the manual before you start desoldering.
HiFi-Stereo (April 1983, p.96) ran the CT-405's contemporaries in its mail-order specials pages through the spring and summer of 1983, which places the model's commercial window firmly in the early-1980s home-keyboard boom. The same magazine's June, July, August, and September 1983 issues carried the model in dealer listings alongside the rest of the Casiotone range. That is not a review — it is a paper trail — but it is a useful anchor for dating the instrument when the factory documentation does not give a clean production run.
The CT-405 is not a professional instrument, and the documentation makes that clear. It is a preset home keyboard with an accompaniment section, a handful of drum voices, and a filter that does more than most of its peers. The eight-note polyphony is generous for the class. The two-chip architecture with a dedicated accompaniment CPU is more sophisticated than the price point would suggest. But the percussion circuits are trim-pot-tuned analog tanks, the equalizer is a single IC, and the whole thing lives in a plastic case with a parts list full of commodity components.
For a collector, the CT-405 sits in an awkward spot: too complex to be a throwaway toy, not rare enough to command serious money, and documented well enough that a patient repair is possible. The service manual is out there, the schematics are legible, and the failure points are known. If one powers up, the filter alone is worth the price. If it does not power up, the Schmitt trigger initialization circuit on pin 28 is the first place to look.
Similar items on eBay
No exact Casio CT-405 listings are live right now — these searches show current Casio and similar vintage synth gear on eBay:
As an eBay Partner, we earn from qualifying purchases. This helps support our independent vintage technology research.