Repairing a Casio SA-65 electronic keyboard
Purchased at a car boot sale, this keyboard was in need of some TLC. The battery compartment was badly corroded with the guts of long dead AA batteries. And the supplied wall wart power supply was incorrect for the required 7.5v DC supply.
I removed the battery spring and soaked it in a small glass filled with vinegar and 1 tbsp salt for 6 hours, then soaked in a similar glass of water and 1 tbps of sodium bicarbonate (neutralises the acetic acid). Using a wire brush, the corrosion fell away and the resulting spring was shiny (if somewhat pitted).
Reinstalled, I discovered the keyboard had a fault that recurred every octave: a pair of adjacent keys emitted
a two-tone note of both keys rather than a single note for the key pressed.
This video describes this behaviour and how I fixed it with a simple resistor to attenuate the phantom signal below the CPU threshold for a high input, while leaving the intended signal above this same CPU threshold. Hence, the CPU sees the desired input (above the threshold of a high input) and ignores the phantom one.
The repeated issue of pairs of keys, one pair in each octave across the keyboard, is caused by just 2 CPU input pins due to the arrangement of the key switch matrix. So, adding just 2 resistors to attenuate the signals on these 2 pins is enough to stop the two-tone issue across all pairs of keys in the keyboard. A neat solution!
I found the Casio SA-65 service manual at http://elektrotanya.com/casio_sa65.pd... . The manual shows the schematics, key switch matrix, parts list, etc. I could not have made any progress in solving this without this service manual.
This does not resolve WHY the CPU has this fault, nor other issues such as the bad jitter of the CPU output pulses to the keyboard switch matrix. The fix makes this keyboard playable again and I enjoyed the electronic challenge to trouble-shoot, play with my oscilloscope and resolve the issues to make it work once more.
Shame I don't know how to play the piano!
Thanks for watching.