Since I always do pretty much the same work on every Juno and have posted about it 7 times, this time I’m going to go more into depth about two specific, very common problems a Juno 106 chorus/jack board can have.
1. Symptom: you only hear one channel of the stereo chorus in the mono mix (“Mono/Left Output”), but it sounds normal through headphones. Your left output will sound like instead of a proper chorus you just have a yucky sounding, sort of steppy pitch modulation.
This often happens just because of cold solder joints on the solder tabs of the switching jacks for left and right output. These jacks are supposed to send the right channel of the chorus in to mix with the left channel right on the left jack when there is no cable plugged into “Right Output.” All you have to do is reflow the solder on the tabs of those jacks.
2. Symptom: you hear distortion when chorus is turned on and your VCA/filter output is reaching a high level (VCA gain is set high and/or envelope is at a high point). It will sound like overdrive or “analog clipping.” This means your chorus circuit is being overdriven because its bias is incorrect. The solution is to adjust the chorus bias trimmers on the jack board according the procedure in the service manual (Step 11 in the Adjustment procedure).
If you don’t have a signal generator and oscilloscope though, it is possible to do it by ear. First, plug a cable into each output jack (left and right) but only plug the left one into an amp. Make sure you have an oscillator waveform selected, turn VCF Env all the way up and VCA Gain all the way up and turn on a chorus. Play a chord and adjust trimmer 1 on the jack board to the point where you hear the least distortion at the high point of the envelope (hopefully none).
Repeat the procedure with only the right output plugged into the amp, turning trimmer 2 on the jack board until the high point of the envelope is not distorted.