Roland MKS-70 chorus issue

Synth Repair
1

I picked up a used MKS-70 a little while ago, and I am noticing an audio artifact that seems to be coming from one channel on one Tone board somewhere near the chorus circuitry.

I’ve been hearing this rhythmic ‘thud’ (like a heartbeat) type of sound. It happens about once every 2 seconds. I notice it mostly in the left channel, and only on certain patches. It disappears when I turn the volume down. It is audible, regardless of whether a key is depressed or not. After a bit of checking out different patches, and trying to identify the differences, I think I’ve isolated it. I hear this only when Tone A (Upper) has it’s chorus enabled (to either ‘expansive’ or ‘rich’ value). The tempo of the ‘thud’ seems to be increased when the chorus setting is on ‘rich’.

Does anyone know what could possibly fail in the chorus circuitry to cause this (i.e. caps, transistors, chips (MN3101, MN3009)? Is it possible something upstream is failing?

Thanks in advance for any advice,
Kevin L

2

I found the problem, and fixed this myself. The problem was a bad transistor and a bad chorus clock chip (MN3101) that were failing when the unit heated up. I replaced these parts, and everything sounds clean again.

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