Make a tiny incision on the heat shrink in the back of low side optocoupler (with your xacto knife), near the the leads of the opto’s led. No longer than 1mm long. In calibration mode, with the harmonic mix knob on the low side, look to see of the opto’s green led is lighting up. If not, turn the low side trim pot all the way up (clockwise 20 times) to see if you can see it light up. If it does light up then you have a problem with the analog signal path, if it does’t light up there is a problem with the led or it’s driving circuit.
Let me know what you find.
-Brach