I am attempting to diagnose a Sharp combination microwave oven, it's a
model R-7E53(W)M. This is a conventional/microwave oven and grill in
one unit. The microwave output power is 850W. I am in the UK.

After cooking some food last week using dual-cook (convection and
microwave), the breaker for the circuit that the oven was plugged into
tripped. On resetting the breaker, I found the oven worked normally -
when asked to cook, the fans run, the oven light illuminates, the
turntable turns, but no microwave energy is produced and the food does
not heat.

On opening the oven, I discovered the following:

* the fuse (M8A 250V) in the feed to the HV transformer primary is
blown. The main fuse (13A 250V) is okay.
* the HV diode reads infinity both ways using a DMM
* the magnetron reads a dead short across the filament terminals
* the HV cap reads neither open nor short. If I set my DMM to the 20M
ohm resistance scale, the resistance reading climbs up slowly. I assume
this means the capacitor is charging and is thus likely OK.
* the door interlock switches check out okay.

I am following the Microwave oven guide in the S.E.R. FAQ.

Question 1: the blown HV fuse is marked M8A 250V. Is the M
significant? I'm familiar with T fuses (slow blow), F fuses (fast
blow), but not M. A google has but turned up anything. Can I replace
this fuse with a standard 8A fuse, and if so, should I select fast or
slow blow?

Question 2: Is this likely to be a magnetron failure? The magnetron is
made by Sharp and is marked "RV-MZ A165 WREO" on one line, and "2M226
(16)" on another. Googling, this appears to be a type 2M226-16
magnetron.

Thanks for any insight. If this were a standard cheapo microwave-only
oven, I'd probably chuck it out. As it's a combination oven, it may be
worth some effort to fix.

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