Hitachi Built-in IH Cooking Heater HTW-4SF It was working normally until recently, but the switches for the left and right heaters have become unresponsive. The power turns on. The center heater and grill function normally. If the switches for the left and right heaters can be operated, the heaters themselves heat up normally. Therefore, I believe the problem is only with the switch board for the left and right heaters (both the manufacturer and Hitachi service personnel shared the same opinion). Is there anyone who can repair this? There is some dirt from use on the top plate, the ventilation opening at the back, and inside the grill.