1. If a bridge rattles against the soundboard when you pluck firmly — what players call a jumping bridge — the string tension may be too low, the vibrating length too short, or the bridge too low. First check that the bridges are in their numbered positions, then move that bridge slightly to the left and tighten the string.
2. A sandy or growling tone on one string usually means the notch in the bone cap at the top of the bridge is pinching the string and damping its vibration. Widening both sides of the notch with a triangular file solves it.
3. If a string produces no sound, or a sandy sound, twist the nylon winding between your fingers. If it turns, the winding is loose and is damping the string — the string is finished and must be replaced.
4. If the instrument rocks, check that the floor under the stand is level and level the stand. If one leg really is longer, saw off the excess. With a folding stand, place a rubber pad under the low foot at one end and under the diagonally opposite foot.