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[Waaaaaark]

I just recently bought a used X3 from a less than reputable person and have since started going through it and fixing things. The biggest problem I've run into is that every C and F# in each octave doesn't trigger any sound. I got to the keybed and cleaned the contact and the rubber strip to no avail.

I would greatly appreciate any help. Thank you!

[T3owner]

Since the problem affects all C and F# keys in every octave, it would be quite a coincidence if the cause was a key contact issue. Rather, there's a problem with decoding of the key matrix.

See https://en.wikipedia.org/wiki/Keyboard_matrix_circuit

Although more serious problems can occur, the typical cause is a poor cable connection between the keybed and the matrix decoding circuit. All it takes is a single bad conductor.
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Korg: T3EX, 05R/W | Yamaha: Motif XF6 and XS6, A3000V2, A4000, YS200 | Fender Chroma Polaris | Roland U-220 | Etc.

[Waaaaaark]

Thanks a lot for your help. It makes a lot of sense what you said about the key matrix. I checked all the wires going from the keybed to the main board for continuity and they all seem fine. Where should I go from here? Taking it to a shop is the last resort, honestly.

Again, thank you.

[T3owner]

You're welcome.

I presume you checked the 11-pin and 12-pin cables. Did you also verify that connectors are making good contact, and that the connector pins are properly soldered?

Based on the schematic I have, the C and F# keys share two lines. They are connected via CN3 (12-pin connector), pins 6 and 12. I suggest you trace the paths as far as you can on the boards at each end, even beyond where the cables themselves connect.

If nothing seems amiss there, then there may be a problem with a pullup resistor for one of those lines, or the key scanning processor (IC20, M37451M4) may be bad.
_________________
Korg: T3EX, 05R/W | Yamaha: Motif XF6 and XS6, A3000V2, A4000, YS200 | Fender Chroma Polaris | Roland U-220 | Etc.

[Waaaaaark]

So I checked the connections on CN3 and found that on both ends everything is fine. I checked resistors R45-R79 that connect to CN3 and they all seemed to be fine, too. No damage that I could see to any components or traces anywhere. I'm just baffled at this point. I checked all the keybed diodes, too.

[T3owner]