MKS-70 Distortion on Bottom (B) board


Working through a strange issue on an MKS-70 where the bottom voice board had aliasing like crackling distortion when the resonance is on. Distortion is on all 6 voices (which makes bad filter chips low probability). I replaced the mixer op amp, no joy. Chorus has no effect (on/off). I’m thinking the last might be the VCA but the M5241L’s are hard to come by. Anyone have and ideas? -before I hunt the VCA down and replace it.

Hi I’m not an electrical guru but own a 70 and I can say that it’s surprisingly easy to overdrive the VCA. Are you 100% certain it’'s a chronic thing and not a patch specific thing? Like if you assign the same tone to each board, will the lower always distort before the upper with balance set to 50/50?

No, I can see the noise on the scope before the VCA. It first appears at the output of U41/P1 but not on U41 P7 (SYN2 or Xmod enabled) so I don’t think it’s the quad switches. The one possibility I saw was some hash on the balance and resonance S&H outputs (U33/P7 and U34/P1) at the same apparent frequency as the “hairs” on the output of U41/P1 that is not on the Upper board’s outputs. The same hash is on the CD4051 outputs U25/P2,4. Tried replacing the little caps at C40 to no avail. It’s only on the 4051 U25/P2,4 and no other pins. I’ve replaced the 4051 twice. Again, this does not appear on the Upper board.

The U25/P2,4 hash puzzles me as it’s on no other output, not apparently on the supply and the same on 3 different 4051’s.

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I’ll be curious what you find. Please update when you know more. Have you already gone through this tech tip from Guy’s site?