Hello! I've just got an used bass (active preamp with J pickup in neck position and Musicman at bridge). It has a problem: if I use the neck pickup it sounds good but if I use the MM pickup only or both combined the output volume goes down. The output goes down as soon as the MM pickup volume is set at maximum, if I use it at 3/4 it works.
So it seems that the MM pickup is the problem. It seems that all the wires
are ok and pot works fine. What could it be? A phase problem? A faulty
pickup? Any advice appreciated.

Hello!How many wires your MM Pickup have ?If he have a 4 wires, i think it's a phase problem in this pickup. In this case the pick up coils may
be wired uncorectly. 2 of 4 wires are joint together. Simply unconnect there and join one of this wire with another one(you have a two options) and check. If your Pickup have a 2 wires there is another problem.

I had the bass checked by a friend and it seems you were right... now it works flawlessly :rolleyes:
Thanx a lot for your advice!