MT-70 Analysis and (hopeful!) repair

[ZorakIsStained]

Bringing this topic back again, I just got an MT-70 that was sold as for parts-or-repair. Turns out most of the problem was just a bad battery compartment, but now that I have the thing powered on I'm noticing that notes F and B for each octave don't sound right, it's like they play F+F# and B+C when they're pressed, respectively. All other notes are fine, and all other functions (accompaniment, bassline, etc) seem to work correctly. Is this likely a DAC issue as above?

[Ra226]

Does F (the bad one you mention) sound any different when you press down the F# next to it (the note you say sounds like it's being added)? Or same question but for B/C?  And what happens if you hold down the key for a problem note then try to add 7 more notes that are not any of your problem notes?  This keyboard should have 8-note polyphony, so if you can only add 6 notes to the F you're playing and nothing happens on the 7th, then chances are you really are playing 2 notes with one key (as opposed to some weird, internal problem in the chip or whatever).

 

Assuming a second note is being triggered with one key, I'd bet your issue is somewhere with the keyboard matrix.  Maybe a short somewhere, so that two lines are being connected rather than just one.  You'd have to study the different lines, but the idea of the keyboard matrix is you have, for instance, 7 "in" lines and 7 "out" lines, for 14 lines total.  Different notes are played depending on which IN lines are connected to which OUT lines (this allows you to control, for example, 49 notes with a lot less than 49 lines).  You should be able to find more info on the web if you lookup "keyboard matrix circuit" to see how they work.

 

Oh, another thing that might help narrow it down: when playing a barcode song (if you can load one), when the song has an F or B, do those notes sound fine?  That would prove the keyboard is capable of playing those notes fine and just isn't when you press those keys.

 

Edited November 22, 2024 by Ra226

[ZorakIsStained]

Wow, I really appreciate the quick response on a 2 year old topic! I'll try checking polyphony once I get the keyboard back in front of me, though I must say my ears have a tough time picking out more than 5 notes at once, but if I play F+B+some 4 voice chord I should be able to hear if the F/F# drops out. The only reason my mind didn't immediately jump to the keyboard matrix was because this was happening on every F and B. This topic jumped out at me because of the "every N notes" nature of it. If each F and B are on the same row or column, then maybe that's all the issue is. A service manual would really help here.

[Ra226]

Ah--yeah, the "every N notes" was when played successively, not simultaneously.  Play a note, it sounds fine, stop playing it.  Play the same note again, it sounds fine, stop playing it.  Do that over and over, and you get a pattern of 4 notes in a row are fine, then 3 notes in a row are bad, then the pattern repeats (regardless of what notes you do).  It happens because the two synth chips take turns on which one actually plays the notes and one of my chips was bad.  Very likely unrelated to your issue.  I'd have to check, but I don't think the first notes drop out when you try to play more than 8--I think instead the 9th note simply doesn't play.