Author Topic: MWO repair: a chain of consequential electrical damage  (Read 403 times)

Offline klugesmith

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MWO repair: a chain of consequential electrical damage
« on: September 04, 2019, 06:49:34 AM »
Our household's microwave oven failed the other day.
Fixing it took a couple hours, but it would have taken more time to go out shopping for a new one. I've lost count of MWO repairs, but this case seemed interesting enough to tell the story at a forum.

History:  After the last successful heating run, there was a smell of burned electrical stuff, but no visible smoke.  Next run was aborted within a few seconds because the light came on dimly and fan ran slowly.  On the bench with cover off, same thing "sort of" happened a time or two, while I dug in with the aid of schematic diagram and multimeter.  After that, there was no light or fan action at all, while the control timer appeared to behave normally.

Looks like original problem was at one of the door interlock switches, in series with the ungrounded line to main transformer etc.  A tab contact, or tab-to-wire connection, had developed enough resistance to get very hot. Shown here "connected" and disconnected on purpose.


With the tab temporarily re-mated, the voltage was normal at that place but light, fan, and transformer appeared to have no neutral connection.  Neutral for the light and fan are switched by the blue relay on control board, via the S1 power connector.


The light and fan ran normally when I created an alternate path to neutral, using a clip lead.
Then pulling out the control board revealed some scorch marks. 

The spatter pattern on sheet metal pointed back to a short, narrow (0.4 mm wide?) trace, in the 120 V circuit, that had apparently exploded from a short circuit.  Thus protecting the main fuse. :-)
Root cause is visible in the hot-tab picture.  Not only did the black wire partly melt its insulation, it melted into the white wire enough for the conductors to touch when the wire harness was disturbed.  That made a literal hot-to-neutral short circuit.

The fused trace on circuit board was replaced with one strand from some stranded copper wire.  Holes in wire insulation were covered with tape.  The door switch was replaced with one from a junk MWO, and the blue relay appears to be undamaged.

p.s. While telling the story, I realized that the damaged wire tab terminal needs to be replaced.
« Last Edit: September 04, 2019, 07:36:25 AM by klugesmith »

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MWO repair: a chain of consequential electrical damage
« on: September 04, 2019, 06:49:34 AM »

 


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