Korg M50 intermittent keys need advice

[fredcapo]

I took in a Korg M50 synth/workstation that has intermittent operation of every fourth key, from the high end up to appx one fourth of the keyboard...that is appx 8 keys are intermittent.

I replaced the first 74hc138 and that seemed to fix it though short-lived though now it occasionally is intermittent on the same fourth keys though works when keyed second time...

It is not worn contacts since I swapped pads and chopsticking does not provoke a failure. Same problem in all voices.

I do not have a schematic and don't theory of operation so attacking it superficially as a problem local to the keyboard now. I am not sure if this is a mechanical problem or a logic or noise problem.

If someone can provide a schematic it would help.

Are replacement boards available from korg?

Thanks for any insight.

[Enzo]

You'd have to ask Korg, but what board are you talking about?

The every fourth key thing means a strobe line or a return line is failing. I assume the key contacts are not on one long pc board the length of the unit, I bet the key contacts are on two or three shorter boards. Are the affected keys all one one of those sub-sections? Generally there will be a couple cables from the main board to the key contact board(s). Then the subsections of key contact board have short ribbon cables connecting them. I'd be suspecting one of those short ribbon conectors or possible the connectors from the main board. I'd resolder all the pins on those just to be safe.


Sometimes the taking apart to replace some innocent IC jostles things enough to make it work, but the original problem returns. So like the replaced chip ws never really bad, just the act of replacing it made it work a while.

To troubleshoot it, you can use the lower end keys that work as a model. SCope the contact lines to see what it should look like, then scope the corresponding comparable lines in the affected area to see if they are different.

[fredcapo]

Enzo -thanks. From my pov the chip I changed was the only element common and unique to the affected keys... I have not tried to contact Korg yet...will try tomorrow. I need to understand the basic functional operation of the unit.

The unit had other problems that I fixed so it is playable. I returned it to the owner at minimum charge with instructions to use it while I research a possible fix or rebuild of the keyboard subsystem.