When we come to set the required elapsed time on the timer, the adjusting knob and the display do not relate; for example on trying to set the timer to sound after say 30 minutes, the figures in the display just jump about, giving random numbers unrelated to what we are looking for. The process of elimination produces the lapsed time required, but it would be better if we just rotated the knob to set the required time. Any ideas on the remedy, please?