how do i reset my manual timer on baumatic range bt2710ss?

since i had my oven professionally cleaned my manual ticktock timer does not work. The timer ticks for 3 mins or so then slowly fades and stops - can anyone help me please. I have been back to the technician that valeted my oven but they say its nothing to do with them and their insurance doesnt cover knobs even though he removed all of them to clean them. they state that their insurance only covers the internal parts of the oven that are cleaned.