Why needs to calibrate.
The CV1 input of the mixers are set for a gain of 1.00. There is probably no easy way to detect that you mustrecalibrate this input. You can measure it.
How difficult is calibrating the CV1 input
This is quit easy, you need a voltage meter to do this, Each CV1 Mixer input has his own trimpot.
Checking and Adjusting the CV1 input
Put a voltage meter between the CV1 input and the output. Measure and memorize the reading.Connect a voltage (Fi the output of the CPR) to the CV1 input.The change in reading may max 5mV. Fi: without no voltage at the CV1 input you read 105 mVand with a voltage to CV1 connected you read 108 mv. The change is 108-105= 3 mV.
If you choose to recalibrate the only thing you do is adjust the trimpot so that the reading in the case that CV mixer 1 isconnected is close to the original reading.
6e PCB board with Mixer 1,2 and 3.