Hi Loch, welcome to the board!
There are several things that might be going on here.
The 1/4 jack is probably stereo; if you plug in a normal guitar cable, you will only hear one pickup. To hear both pickups, try plugging in some headphones.
If the bass has been sitting around unplayed for a while, the pots probably need exercising. Alembic uses sealed pots; the pots are self-cleaning. Try turning each of the knobs, including the selector switch, back and forth twenty or thirty times.
Each of the pots should be firmly attached to the body since they ground to the shielding paint on the inside control cavity. If a pot are loose, the knob will need to be removed; there are two allen screws holding the knob to the shaft.
Open the control cavity cover and make sure that nothing is inappropriately grounding to the shielding paint or other components; and just generally make sure everything looks ok. Look for any user modifications, sloppy soldering, messing wiring, that might indicate that the electronics have been modified.
Notice the three points where the pickups plug in to the PC board. Make sure they are firmly attached. You may want to unplug and replug them. But be careful. At the moment I can't recall which connectors are on that model year. Instructions for the old style are
here; for the new style, grab the housing and not the wire.
There are four trim pots mounted on the back plate. The two outside pots are gain controls for the pickups. You might want to make sure both of these are turned up (clockwise).
According to the Alembic web site, the price of a new power supply is $550, and the price of the cable is $190.
