Likely either a high voltage supply or magnetron issue. I doubt you'll be able to find anyone stocking spares for an old, budget microwave. The cost of professional repair is probably more than a replacement of similar calibre. It's also EXTREMELY DANGEROUS to work on a microwave EVEN IF ITS UNPLUGGED. There are capacitors used which can store enough charge for a lethal shock, even after the thing has been unplugged.
Nip down to Currys etc and get a new one. With a guarantee.
Ggkgntn, February 2025
