Tuning problems with ZOOM BFX 708

[Mottlefeeder]

I've noticed that my Zoom BFX 708 Mk I tuning section is a few percent out compared with my other two tuners (Korg and Sabine).

I am competent enough to take it apart and solder the input/output sockets back onto the printed circuit board - so is there an adjustable component within the unit that I can use to bring this tuner into line with my other two (without any test equipment other than a voltage meter!).

I am assuming that it would not be worth taking it to a repair agent.