microwave flashing and burning, looks spectacular but guessing it's bad news

[whenwillisleepagain]

It is bosch, possibly still under guarantee, will check. No problems till now, cooked porridge as usual this am then went to defrost frozen fruit, forgot to reduce power, but every day I defrost a few berries on low on a saucer. Spectacular flashing and burning. quickly reduced power, covered berries ad changed container. On low power one more bit of flashing. all the fruit tasted burnt. Racked our brains as to whether anything metallic had got it - but what on a clean dry plate? sigh, never not using it.

[PigletJohn]

look at the plastic window in the side or top that the waves come through. Is it dirty or burned?

[whenwillisleepagain]

Hi piglet many thanks - sorry to be dim, but do you mean the on the outside, with the slats? it looks ok, but full of dust

[SomethingAboutNothing]

I take it there was no metal in the microwave?

[PigletJohn]

no, there is a whitish plastic or mica window, inside the cabinet, on one of the walls or possibly the roof. Might be three or four inches long and two or three inches wide. The microwaves are projected through here into the cavity of the oven.