The most common problem with Roland juno-106's is one or more of the voices going dead or that keep hanging. When they fail, they typically cause either a loss of sound from the affected voice, or a crackle that usually gets worse as the synth warms up. From factory they put a protective coating over the chip, which over time can become conductive. Cold solder joints can also be the problem. However, since the voice chip is dead anyway there is a chance to bring it back to live by desoldering and removing the coating with acetone (that can take some time...), clean everything on that pcb and reflow all solder joints.