Casio CT-670
A budget home keyboard with an interesting digital engine, and a service manual that goes deep on cost-engineered DAC design.
Overview
The Casio CT-670 gets dismissed on sight and then surprises you on the bench. It is a home keyboard, plain and simple - the kind of thing sold alongside calculators and digital watches, not in the same room as a Prophet or a Jupiter. But pull the case and the engineering underneath is more considered than the plastic suggests. Casio's own service documentation for the CT-670 lays out a signal path built around a μPD937G CPU, an LC7881C DAC, and an LA4127 power amp, with an LA5668N power IC tying the whole thing together. That is not exotic silicon. Casio clearly understood it down to the pin level, though, and the manual treats it that way.
The sound generation is where things get interesting. The LC7881C is not a conventional single-architecture DAC. Per Casio's own documentation, it contains two each of an R-string DAC, a PWM DAC, and a level-shift DAC - one set per channel. The 16-bit serial audio stream arriving from the CPU is split three ways: the upper eight bits (D15 - D8) go to the R-string converter, the middle four bits (D7 - D4) go to the PWM stage, and the low four bits (D3 - D0) go to the level-shift stage. It is a segmented converter. It splits the word across three conversion methods to keep costs down while preserving resolution where it matters most. That is a real engineering decision, not a parts-bin accident.
The digital plumbing is just as specific. The CPU's MCLK terminal outputs a 2.7156 MHz pulse - one-eighth of the 21.7248 MHz master clock - and the manual flags it as the check point for the main clock generator. The same CPU drives the DAC over a serial bus: BCLK (bit clock) on pin 107, DATA (SWD1) on pin 109, WCK1 (word clock) on pin 111, and LRCK (left/right channel separation) on pin 112. The CT-670 also carries MIDI in and out on the CPU - MIN on pin 18, MOUT on pin 17 - clocked from a separate 4 MHz domain shared with the rhythm tempo generator. So a keyboard in this class was not strictly a standalone toy.
The CPU also has a pair of built-in ADCs. They are powered from the +5V rail, and per Casio's own pin table they are simply not used on this model. Plenty of manufacturers would have left the pins undocumented and let a technician guess.
The power section is where the CT-670 shows its age in the most practical sense. The LA5668N power IC handles the main rails, and the service manual's troubleshooting chart treats it as the first suspect for almost every failure mode: no power, no sound at all, distortion. The +5V rail on pin 6 and the +9V rail on pin 2 are the two voltages to check first. The reset IC, an S8053, should output a low-level pulse on pin 1 whenever the AC adapter is inserted or batteries are installed. If it doesn't, the CPU never initializes and the keyboard sits there dead. Auto power off is also handled in the CPU: leave the keyboard untouched for six minutes and the APO line drops low, shutting down the VC, AVDD, DVDD, and VDD rails through the LA5668N. That saves batteries, and it is also a predictable source of "my keyboard turns itself off" complaints from owners who never read the manual.
Key scanning runs through a matrix, with KO0 - KO3 driving keys and switches and LEDs, and KI1 - KI8 reading them back. The manual's fault chart is blunt about the most common failure: a single dead key is almost always dust on the contact, not a failed IC.
The audio output chain is short and conventional. The DAC's outputs (pins 1 and 20) feed an M5218PR filter stage, and from there go into the LA4127 power amp. The manual documents the expected waveforms at each stage - RESET 1 on CPU pin 5, key scan signals on KO4 and KO9, DAC outputs on the LC7881C, filter outputs on the M5218PR. That is more diagnostic detail than most budget keyboards ever got in print.
Based on the available documentation, the CT-670 is not a synthesizer in the programmable sense. Nothing in the service material describes patch memory, filter sections, envelope editing, or any of the architecture you would expect from a CZ-series instrument. It is a preset home keyboard with MIDI and a segmented DAC. Collectors who expect a CZ-101 in a cheaper case will be disappointed. Collectors who expect a well-documented, repairable, odd piece of Casio digital design will find exactly that.
Reception history for the CT-670 is thin in the surviving period press. The magazine chunks available for this model are mostly dealer clearance listings and mail-order catalog pages from the early 1990s rather than reviews. That kind of retail ephemera confirms the keyboard was in distribution and being discounted, but says little about how players received it. One such listing in HiFi-Stereo (October 1990, p.117) advertises a clearance sale of consumer electronics without singling out the CT-670 specifically, which is itself a small data point: the CT-670 was the sort of thing that moved through clearance channels rather than sitting on a featured shelf. The CT-670 was not a landmark instrument. It was a competent one, and the documentation Casio produced for it is better than the instrument's reputation.
For anyone repairing one today, the service manual is the whole game. The fault chart is direct: no power means check the LA5668N and the power jack. No sound at all means work through the LA5668N, the master clock at CPU pin 105, the CPU's MI and data lines, the S8053 reset IC, the LC7881C DAC outputs, the LA4127 power amp, and finally the ROM and RAM address/data buses. Distortion points at the LA5668N rail voltages, the DAC outputs, the LA4127 pin 11 voltage and pins 9 and 12 waveforms, and the CPU. A patient hobbyist can follow that sequence with a scope and a schematic.
Parts availability is the real constraint. The mechanical parts - battery springs, rubber feet, the battery cover, the dust cover - are catalogued in the manual with Casio part numbers, which helps, but only if you can find someone sitting on old stock. The key matrix is the most serviceable subsystem, and the manual's note about dust on the contacts covers the most likely fix for a working unit that has developed a dead note.
The CT-670 sits in an odd spot in Casio's catalog. It is not one of the CZ-phase-distortion instruments that earned the company a serious reputation among synth players, and it is not one of the tiny VL-1 or SK-1 curiosities that became collector icons. It is a preset home keyboard with real MIDI, a segmented DAC, and a service manual that treats the reader like an adult.
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