Post #3656662
2026-07-07 19:56 UTC
The beautiful wooden keyboard of the Kawai MR25. There’s amazingly little dirt considering this piano was used on stage for decades… One of the five non working keys is fixed (simple adjustment, D#1), the others are next to each other (F1 to G#1) and are scanned using the same output pin of a 74LS138 3-to-8 decoder. So I guess either the output has failed or there’s a bad solder joint or cable. I need to bring a scope from the office tomorrow…
#Kawai #e-piano #electronics #music
Replies (1)
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@me_@sueden.social 2026-07-07 21:34
This is the excerpt from the keyboard schematic with the non working keys highlighted. It's a standard keyboard matrix implementation, the 74LS138 decoder pulls the output Y0...Y7 that corresponds to the binary number at the inputs A, B, C (pins 1-3, repeatedly counting from 0-7) to a logical 0 level. A pressed key connects the output connected to it to one of the inputs on the left (two per key for touch sensitivity), so the keyboard controller can figure out which keys are pressed.