Roland keyboard

[woodchips]

Just repaired a Roland keyboard, can't remember the number, given it back!

Problem was one note wouldn't sound. This keyboard is fairly old but the keys are velocity sensitive, so hot harder, louder noise.

With the keys removed pressing the floppy rubber bit down would give a note, but only loud.

I peeled the rubber bits back and gently swabbed underneath with an optical cotton bud, and lo and behold it worked fine.

The question is, how do these things work? Attached under the floppy rubber bit were two black discs, each about 2.5mm diameter. When the key is pressed these go down and touch two pairs of tracks on the PCB. I am guessing that these are magnets and the tracks are actually Hall sensors? Obviously the tiniest piece of something was keeping the magnets (?) away from the track so not detected.

See it all ran on a 64180 with a large custom chip, but not enough pins to give four per key.

[ataradov]

Tracks are just tracks and the black disks are carbon impregnated rubber, which shorts the contacts on the board. This is a pretty standard design for the buttons.

And if the keyboard has velocity sensitivity, then there typically are a pair of such buttons, which are shorted at a slightly different time in the key travel. Time difference between the shorts is proportional to the velocity.

Wikipedia article about this type of buttons - https://en.wikipedia.org/wiki/Silicone_rubber_keypad

[floobydust]

Polyphonic after-touch measures the closed contact resistance, some Roland keyboards had the feature. I did not see it work so well, you have to really press down hard.
So some down-key switches were pressure sensitive, not strictly off/on.

[woodchips]

Thanks for the replies.

Certainly look like carbon impregnated rubber, couldn't see how the loudness was determined. It is polyphonic and the number of keys and wires didn't allow every key to have four wires. Perhaps it assumes you aren't going to play certain notes together to reduce the number, but then how does it know which note has been pressed?

Something so simple in a real piano isn't quite so simple in electronics!

The design of the rubber mount prevents any hard pressing down, the rubber simply won't allow it.

[ataradov]

The keys are probably connected in matrix, and that matrix is actively scanned. See if there is a bunch of diodes on the board.

If there are two sets of contacts for each key, then velocity (loudness)  is 100% determined by the timing of them shorting out.