Music Man Bongo 5 HS+piezo

Hi Bass Gang! I have a Music Man Bongo 5 HS+piezo. I just replaced the PCB pre-amp assembly with a new one (old one was fried from a previous owner trying to unsuccessfully replace the pickups). The pickups are now factory/stock. I’m confident all the colour coded wire connections are soldered correctly to the pre-amp as per the attached wiring diagram with all the necessary grounding wires connected. My issue is that the humbucker is humming/buzzing really loud when it is dialled in with the control and actually amplifies if I touch the humbucker pickup housing without touching the strings. Touching the strings does not reduce it. The ground is connected correctly in this case. The pickup works but is unusable with the background hum/buzz. The chin stroking problem is if I roll off the humbucker completely using the pickup balance control to 100% neck single coil pickup, that works perfectly and is completely silent in terms of interference or hum/buzzing (as is the piezo if that is dialled in without the humbucker), exactly the opposite to what you might expect from a humbucker and a single coil. The humbucker and housing only has two wires (white/start+black/finish) and no other grounding. Perhaps the humbucker pickup is faulty but I don’t know how to test it. If anyone has any ideas / suggestions to try and remedy this issue, they will be gratefully received! Cheers in advance!!

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