Can you post the model number of the microwave please. It will be on a sticker on the inside of the door as you open it and starts E-nr.
Thanks for that.
Is there something like a bit of card board were the sparking is comming from?
Ok that part is what we call the splitter and the wave guide. As when the magantron fires up it sends the microwaves waves through there and the splitter bounces the waves around inside the oven itself.
Now check there and make sure you have now bare metal exposed there as if there is then the waves arch on there and causes the flashes.
But if thats all fine and given the fact you have a burning smell then the tip of the magantron itself could have burnt out. As if it does then it also sends sparks in there. Now for this the oven will need to be removed and the magantron can be replaced from there.
So first ceck the spitter at the top and check fro any bare metal were the coating has come off and if thats all fine then its possibley the maganron itself at fault.