Mike Tomlinson writes:
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 exactly did it trip?
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
This would be correct for a DMM. You need to test it with a power
supply since the forward drop is more than 6 V.
* the magnetron reads a dead short across the filament terminals
It will be very low ohms though not quite 0.
* 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.
Only thing would be if it fails at full voltage.
* 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.
A shorted magnetron or diode would probably not pop the breaker. A
shorted cap might.
When it popped the breaker would be significant. If it happened exactly
when the heating was called on to stop, could be a triac failure also.
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.
Yep.
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