Ariston microwave + oven. Repairable fault?

There is a combined microwave/ oven fitted in the galley of a boat. The appliance was made by Ariston & the model no. is MW212K, type no. MW600, 230 V ~50Hz, serial no. 304020251. It is likely to be at least 10 years old, but is in good condition, undamaged, corrosion-free & worked perfectly until a week ago. Now the oven works & the red indicator light comes on, but if microwave only is selected the light doesn’t light & the microwave creates no heat.If both oven & microwave are selected the the boat’s mains shorepower is tripped off. I suppose this means the circuit is overloaded by a short or there’s a current leak to earth. I believe the interlock door microswitches are working OK. Is this enough information for you to advise me where the fault is likely to be and could there be an economic repair? I am keen to repair it if possible because the oven has been built into the galley and it may be very difficult to find a replacement that would fit. Thanks for your expert advice.