"Run away !"
from this thread http://applianceguru.com/forum1/21915-1.html
I did find someone's repair info on the Inverter Board,
along with some oscilloscope waveforms,
but I don't think you want to go there ...
here's an excerpt:
"The Control signal turns out to be a TTL-level 220Hz square wave where the duty cycle determines the “Power” from the Magnetron.
The Status signal from the PSU is a 110Hz square wave with a fixed 50% duty cycle.
This signal seems to be present when there is a current drain, probably to signal that the Magnetron is warm and operating.
The Control and Status signals are synchronised to each other, but bear no relationship to AC mains frequency or phase.
1. At low power levels, the PSU starts at a higher power level (50%) until the Status signal appears, then drops back to a lower level.
This could be because the Magnetron filament requires at least the 50% power level for it to heat up, then it sustains itself from back bombardment once RF is being produced.
2. At low levels, the PSU is cycled on and off at the 40% power level over a 22 second period to reduce the average power output."