Casio SA-65 Part III - fixed it and learned a lot

In my two earlier posts, I described a Casio SA-65 electronic keyboard that appeared to be working (after removing lots of battery compartment corrosion on the negative terminals) except for the annoyance of failing to produce notes correctly on specific pairs of adjacent keys on the piano.
These pairs of keys, spread over each octave of the piano's 37 leys, would sound both pair's notes (e.g. D and D#) when only one of the keys - either one - was pressed.

Opening up the keyboard to investigate, I've spent some hours reviewing the schematics (see Part I for a link to the Casio SA-65 service manual, or just Google for it). My experience of electronics is still very basic, but while I am pleased to state here that the keyboard now works well, I must admit that I've only really overcome the symptoms of the problems and not truly resolved the deeper issues with this keyboard. It can now be used as intended, while inside lurk some glitchy CPU processes!

What bad diode?

My trials and errors in finding a solution to make the Casio work again, led me into a few alley ways and perhaps some crazy conclusions. This is all part of my learning process. One example was my confident assertion that there was a bad diode, D104, on the PCB.

This is flatly untrue! 

Measuring all diodes in circuit, I measured the expected 0.6V forward bias voltage drops and 'OL' reverse bias levels. All except in one place, D104. My inexperience and enthusiasm got the better of me and I wrongly deduced this D104, measuring 0.1V forward and reverse and 100Ohm resistance, was defective. I desoldered it and replaced it with a known good diode: but the original diode measured out of circuit was fine and the good new diode measured in circuit was bad!

I'd misinterpreted and ignored the schematic details (above) and forgot that measuring voltage across D104 would involve all components in parallel with its anode and cathode - like that 100Ohm resistor (see above).

So there were no bad diodes, and my assumed cause of 'ground bounce' was not the source.

There IS ground bounce in the circuit, as seen from a CPU output pin (KO0) and the noise in sync on a quiescent input pini (KI0) that was not affected by the phantom signals:

blue - KO0 output, yellow KI5 input (good signal), purple KI0 input (phantom signal)

Soooo, I know there is some 'bad' effect from the quiescent pin ground noise, but have no idea what is causing it or how to resolve it.

But that does not mean I cannot resolve the issues that make the piano unplayable: the two-tone keys. Looking at the above screenshot as representative of the idea that follows, we can see that the phantom signal is somewhat lower in peak voltage compared to the desired signal.

Using this observation, I added a potentiometer (470KOhm) in series with KI5. The idea is to create a voltage drop across the resistor which will attenuate the signals on pin KI5 - that includes the wanted and unwanted square wave pulses. By finding just the right resistive level (hence the adjustable pot), the wanted signal is above the CPU threshold for a 'high' input, while the unwanted phantom signal is below.

I did this successfully on the affected pins KI5 and KI6 (different resulting resistances were needed) and soldered these new resistances in series with the circuit towards the CPU pins.

The two-tone sounds went away after that, so closed up the keyboard and happily sent this keyboard onwards to new owners.

Thanks for reading.