I just finished my Tubbutec j66 and all seems to work apart from some digital noise when moving sliders that happens once in awhile. I also had the extra wires and I’d say it is safe to assume they aren’t intended to be used.
Regarding the noisy scratchyness, sounds like you have a bad solder somewhere. Also, if you have no envelope it might be another bad solder on the CPU. I don’t have the service manual with me at the moment to look but likely there is one pin that is the source for firing the envelope.