Casio MT-65
A small Casiotone that hides an auto power-off circuit, a double-XX keyboard revision, and a parts list shared with half the line.
Overview
The MT-65 is one of Casio's Casiotone electronic keyboards. If you repair or collect vintage Casio gear, the useful thing about it is how much of the line's service documentation routes back through it. The service manual for the MT-100 says its own manual covers only the graphic equalizer and waveform circuits, and that "for the other circuits, refer to the MT-65 service manual." So the MT-65 was treated as a reference design, the baseline for documenting its siblings.
The MT-65 shares its core with the MT-68. The service literature spells it out - the two are "electronically the same," with case color the only difference. Casio repeated this in more than one place, noting that MT-68 is the same as MT-65 but has a different color case. An MT-68 on a shelf is an MT-65 in a different outfit, and parts interchange accordingly.
Casio revised the MT-65 mid-production, and the revision is documented in unusual detail. There are two distinct keyboard assemblies and upper cases. The old style can be identified by a single "X" pattern molded on the underside of a key. The new style carries a double "XX" pattern. The change wasn't cosmetic. The new style replaced two separate keyboard PCBs (KY1 and KY2) with a single board, and the key contact rubbers were redesigned alongside it. The parts are not interchangeable between revisions. Casio flagged this in the technical bulletin, along with a warning that the ladder network change on the related MT-100's main PCB involved parts that are likewise not interchangeable because the pin configurations differ.
The auto power-off circuit is documented as an MT-65-only feature in the service manual's circuit descriptions. It cuts VCC when no keys are struck for roughly six minutes. After approximately that interval, signal O-25 from the accompaniment LSI drops to logic low, transistors T2 and T3 turn off, and the supply is cut. A dead-looking MT-65 on a bench might simply be asleep rather than broken, and battery drain on a forgotten unit is less punishing than it could be.
The service manual's LSI description says the sound generation is built around a 42-pin DIP N-MOS LSI that is 8-note polyphonic and controlled by an outer CPU. It does not contain its own control circuits. It receives data from the accompaniment LSI and generates digital melody sound signals, with a 4.9 MHz clock input and a 1.6 MHz output derived as one-third of that. The documentation also notes the chip carries "new features of digital reverberation, delayed vibrato." The manual's waveform section characterizes the DAC and filter outputs using an organ tone at key C3, which shows how Casio expected technicians to verify a working unit.
The MT-65 also sits at the center of a small cluster of related models in the same service materials. The CT-405 (also designated MX-41) appears alongside the MT-65 (MX-40) on the cover of the shared service manual and parts list. The MT-100's manual defers to the MT-65 for everything outside its equalizer and waveform sections. The MT-68 is the same instrument in a different case. The MT-65's documentation is, in effect, the documentation for several of its siblings.
Period advertising from the early 1980s put Casio's small keyboards in the mail-order pages of the hi-fi press. HiFI-Stereo (April 1983, p.96) and HiFI-Stereo (May 1983, p.81) both ran full-page dealer spreads with toll-free order lines and free catalogs. These are dealer ads rather than independent reviews, so they say more about how the instruments were sold than how they performed.
For collectors, the MT-65's value is in its documentation. The service manual is unusually complete, with parts lists, technical bulletins, waveform checkpoints, and circuit descriptions. Because so many sibling manuals point back to it, it also covers repair knowledge for the MT-68 and portions of the MT-100. The mid-production keyboard revision is the thing to watch for when buying. A unit with single-X keys and a unit with double-XX keys are not parts-compatible, so identify which revision you have before sourcing a keyboard PCB or contact rubber.
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