Hello Jim,
There is one other possibility that has not been mentioned in this OT discussion thus far and that is the very basic protection mechanism implemented by a combination of limit switches in the door interlocks.
It is designed to blow the fuse should the door interlock NOT function correctly.
You will almost certainly require the schematics for your particular microwave oven to facilitate debugging this unless you are able to easily access said limit switches inside the door interlock assemblies. There should be 3 of those micro-switches and, as you slowly open the door, you should never measure a short-circuit across the 240VAC supply when measured from the feed side to the transformer (of course, with the latter disconnected.
I hope this helps should you have exhausted all other possibilities (bad resonant cap, bad diode, shorted transformer or bad magnetron).
I had both of our microwave ovens fail in this precise manner in the last 6 months.
These micro switches take a fair beating over the years, especially if you have teenagers in the house ?
Cheers,
Rick
Melbourne, AUSTRALIA
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On 13 Dec 2021, at 12:05, Jim Ford <james.ford@...> wrote:
?Hi, everybody. Has anybody else seen this issue: one day recently, out of the blue, our Kitchen Aid microwave oven stopped working. One day it was fine, and the next day the display was blank, and the keypad wouldn't respond to anything. So I attempted to open it up, thinking a catastrophic fault like this might be easy to fix. I had to go to Home Depot to get a set of those Torx drivers with the hole in the center because the screws holding the cover on are Torx with the pin in the center. Not that I mind buying a new tool, of course! So I opened it up yesterday, and sure enough, the 20 amp fuse was blown. Really, really blown - the metal inside was splattered all over the glass!
I checked YouTube, watched a few videos, and decided to discharge and measure the high voltage capacitor, a known weak point. So I grabbed some alligator clips, some banana leads, and a 2 ohm, 25 W wirewound resistor I had lying around. I didn't think discharging the cap, a 0.95 uF job, with a dead short was a good idea. The cap measured about 130 ohms and 0.00 V, but the ohms reading was very unstable. I realized the HV transformer was still in the circuit, so I disconnected the cap leads (very hard to get the clips off, BTW) and measured again. Megohms this time and about 0.2 V (I chalked that up to dielectric absorption).
I figured it wasn't the cap, and maybe the original fuse was faulty or a power surge had fried it. I hoped for the best and made another trip to Home Depot and bought a ceramic fuse. Actually 2 in a pack. Put one in the microwave oven and was pleased to hear it beep and get all zeros on the display. Short lived, though, because the kitchen lights dimmed, and the display went blank. Sure enough, the fuse was open now.
So, anybody have an idea what to check next? Or is the cap indeed blown and in need of replacement? I'd sure hate to have to trash this appliance after only about 4 years (1 year warranty, of course!). And I'd hate even more to have to spring for another one. IIRC, it was about $750 new.
Thanks.
Jim Ford in Southern California, USA
momemeca