[londons_explore]

Probably burnt paint from the inside of the waveguide.

Happens frequently when the mica sheet that covers the injection port gets moisture from steam (who'd have thought - steam in a microwave?!?)

Simple fix is to replace the mica (a few cents from AliExpress) and use steel wool to get rid of any carbon residue around the injection port.

[xattt]

It sounded like there was no magnetron load on the power supply, because the cooling fan was spinning a little faster and the interior was a little brighter. This was during the few seconds between the smoke show and from me pulling the plug.

There was indeed a char mark on the mica sheet, but the beryllium terror at the time was enough for me to chuck it.

[londons_explore]

You would expect less load on the power supply if there was an arc causing a short in the waveguide.