This Roland JP-4 had one voice board where the filter behaved differently. I had gone through the entire calibration procedure in the service manual but I could not get the VCF Envelope Balance correct. The other three voice boards had no issues. When calibrating the board in question I just could not get it into range. The good boards were possible to calibrate between 400-2500Hz, but the troublesome one was only possible to adjust between 500-560Hz. The trimmer would reach the end point.
I suspected that IC18 (BA662A) was causing the issues. But as original BA662 from Roland tend to be quite expensive I decided to try with the Open Music Labs BA662 clone and it turned out fine to my ears at least. I bought these from synthcube.com
The symptom was that when playing and rotating between the four voice boards 3 of them would sound the same and then there would be a slight difference in the fourth one. In the samples below I play the same note four times and the JP-4 rotates between the voice boards.
After replacement (and calibration)