Microwave works perf3ctly and but convection runs for a while and then cuts out.

What MAKE and what MODEL are you talking about? Respondents are not mind readers! I wish people would realize that!

you have a short circuit.
test the a) HV rectifier b) Capacitor HV c) HV transformer d) Magnetron e) test the swich for door for short.f) circuit board main is short.
see the diagrams and proceed.

If the fan seems to run even after the microwave had completed the cycle, then it is possible that the drive to the fan is having a time delay. Also it has to be made sure that the magnetron has been cut OFF power or else it is not a good situation. You will have to remove the control panel and check on the board, check the drive to the fan, since the processor cuts off the fan and magnetron at the same time the fan circuit must be traced. Look to the circuit after identifying the fan wiring and back check using a multimter if you are able to trace. If not take help as this can be harmful in case the microwave works till the fan cuts off. A faulty capacitor on the board can create this time delay.

My microwave runs for 10 seconds and then cuts out. it now says look in manual before use.

It sounds like the magnetron may have either failed or is not being supplied with the extremely high voltage required to run it.

This is a job for a professional.

Hope that helps.

sounds as either inverter or magnetron is faulty difficult to diagnose although when mag fault usually runs for about 22 secs inverter fails sooner around 5 secs.

you need to buy a new one because it is broken.

This may be due to the microwave section not working. Check the high voltage diode if it is ok, replace the Magneton.

You most likely a shorted capacitor which is causing the fuse to blow. This is part of the microwave HV system and isn't in use during convection cycles.