Post by plutronus on Jan 8, 2020 10:02:53 GMT
Today, as I was getting ready to cook a chimichanga (a tastey little shreaded steak & cheese burrito) in my 1100 Watt Magic Chef microwave oven, I cut off the end of the bag, set the timer for 1.5 minutes & hit start. About 45 seconds into the cooking period the bluish-green vacuum flourescent digital timer display went dark, and the microwave quit cooking. I thought out loud, "Aw shit! It pop'd another fuse!" I like this microwave appliance configuration and I spent aroung $250 for it at Homebre Depot, discounted as it had a cracked plastic bezel corner. It normally sold for around $350. Durring the 11 years I've had it, I've repaired it several times. The first few repairs, were preceded by a stinky fried phenolic aroma. Something inside the guts was a cook'n. Unplugged unit, and muscled it to my work bench in my lab. Upon inspection, the four case screws were tamper-screws, square drive with a with blocking center-pin. I pulled out my specialty screw-driver set, selected the proper drive, and removed the case.
To prevent me from killing myself I discharged the 4,000Vdc high voltage filter capacitor by using a pair of aligator zip cords to short the lugs. I shorted across the capcitor lugs a 115,000 Ohm, 15W power resistor, to drain the capacitor without damaging it.
In the photo below, lower right corner, see the yellow aligator zip cord shorted across the 'Cap'icitor lugs.
The smokey smell originated on the AC mains filter board. Apparently one of the solder lugs of the PCB inline fuse holder was not properly soldered at the factory and was likely a so-called 'cold-joint'. The solder around the lug on the printed-circuit-board (PCB) had become so hot that it melted and dripped down onto the bottom of the case! Appx 350°F/176,6°C.
And the phenolic board was charred black.
I repaired it by sucking all the remaining solder off the fuse lug, jumped the smoked PCB trace, wrapped each end with a 18Ga solid copper wire around and crimped onto the lug and slathered it in solder. That fixed it for the past five years.
A couple of months ago, it began to pop fuses infrequently, each time associated with a loud humming sound which was emitting from the power-transformer. The magnetron was going bad. I bought a replacement magnetron for $30 and replaced the 11 year old magnetron. I also installed an exterior 30A 250VAC fuse holder, so if the fuse pop'd again, I could replace it without having to remove the case each time.
The microwave has been working great until today!?
A microwave oven only has a few cooker parts, --a timer, power-transformer, a high-voltage rectifier/diode, a high-voltage filter capacitor and a magnetron. Thats all. If it starts popping fuses, its likely either the capacitor is shorting or the diode has shorted. Intermittent fuse blowing/popping is very likely the capacitor, whereas diodes when they go bad, they either blow open or short. So replaced the capacitor and put in a new fuse. It turned on and then when I opened the door, the new fuse popped. Aw shit!
There is another part, all consumer microwave ovens are manufactured with a special door safety-interlock. It is a configuration of switches that are interlinked with each other mechanically are designed to pop the fuse should any of the interlock switches fail to operate correctly. The main purpose of those door interlock switches is preventing the microwave from operating with the door open. One of the microswitches had become stuck in the 'ON' position and the interlock system blew the fuse.
I replaced the switch, and now the microwave is working good again!
plutronus
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