[sdiy] 1980s Casio keyboard modification instructions in a PDF

Robin Whittle rw at firstpr.com.au
Tue Feb 3 03:33:25 CET 2009

Hi Peter,

Thanks for your reply!

You wrote:

> I wonder if there is a difference in the schematics between you MT-Series Casios and the CT-Series.

MT could mean MT-30 or MT-40 which involves a single uPd77x chip, or
MT-65 which involves a CPU driving a 42 pin uPd931 chip.

I guess there are other MT models which work on different principles.

The uPd77x chips and the uPd931 both use similar digital techniques
to construct their waveforms, with a sampling rate of ~500kHz.  They
both have a 14 bit DAC output (I recall - my schematics are not
handy) and both raise similar challenges making a DAC which is linear
at least near the mid-point, while also having little in the way of
capacitive glitches.

My page:


has some written notes on this DAC challenges.  I don't at present
have better documentation of this, but I have solved the problems
with my modified CT-202.

> I had a look at the CT-320 and CT-620 Service Manuals I bought years ago and I think
> they have similar circuits. And yes they also use the uPD931c melody circuit.
> And they also have the ladder network DAC but with an "bit compensator" circuit and a 
> sample&hold circuit after the DAC circuit. The bit compensator circuit was casios way of getting 
> rid of some of the "time lag noise" as they call it caused by the accuracy of the resistor ladder 
> network and the response speed of the buffers.

Accuracy of the resistor network and the DC impedance of the buffers
which drive it is not what I would call a "time lag" problem, unless
they use this to refer to the very messy problems caused by different
slew rates up and down, as discussed on my page.

> The circuit allow you to add the positive or negative phase of the MSB.

That is adjusting the weight of bit 13.

> Did you have a look at the MT-series schematics?

No - but maybe they are similar to the MT-65's.  Do you know of any
of these on the Net?

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