I had a similar issue with my one years ago. The problem was the surface mounted jack sockets - soldered directly onto the circuit board rather than wired to the circuit board. This, combined with a plastic case means that it flexes quite easily under normal use and cracks the joint of the socket to circuit.
It may be possible to repair, but honestly, for something that vintage, unless it contains a specific sound that you can't get anywhere else it's simply not worth it. In fact, it would probably cost more to repair than it's actually worth.
I would upgrade if I were you to something like the Zoom B2.1u - its spiritual successor, and vastly superior unit. Better build quality, better processor, better sounds all round.
Attitude II SFG; RBX-JM2; Att Std; Thumb 5 BO; Corvette Std fretless; LMII; MB 121H; Nova Dynamics; EBS Billy Sheehan; BEQ(x2); Mooer ABY; LS2; Mooer FOG; MS-60B; PT Jnr.