If the mains filter board & wiring burnt out then I would say that the control board for the top drawer was also damaged at the same time. To prove it you can take the control board out of the bottom drawer and swap it over with the top one. If the fault now comes on the bottom drawer then the control board is going to have to be replaced. More expense but one of the pitfalls of DIY repairs.
Lincoln Appliance NZ, January 2012