Author Topic: MWO repair: a chain of consequential electrical damage  (Read 137 times)

Online klugesmith

  • High Voltage Enthusiast
  • *
  • Posts: 45
  • Karma: +2/-0
    • View Profile
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 »

High Voltage Forum

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

 


* Recent Topics and Posts

post Re: IR wavelength in photoelectric smoke detectors?
[Light, lasers and optics]
klugesmith
November 19, 2019, 11:15:31 PM
post Re: coil Tesla in the space
[Dual Resonant Solid State Tesla coils]
Mads Barnkob
November 19, 2019, 11:28:10 AM
post Re: RF Amplifier for Cubesat plasma thruster
[Electronic circuits]
Weston
November 19, 2019, 08:56:49 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
jturnerkc
November 19, 2019, 06:25:49 AM
post Re: my first DRSSTC is not working properly
[Dual Resonant Solid State Tesla coils]
davekni
November 19, 2019, 06:17:03 AM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
davekni
November 19, 2019, 06:03:37 AM
post Disc launcher attachment
[Induction launchers, coil guns and rails guns]
MRMILSTAR
November 19, 2019, 05:51:39 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
davekni
November 19, 2019, 05:47:34 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
jturnerkc
November 19, 2019, 12:49:52 AM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
babass
November 18, 2019, 09:19:26 PM
post my first DRSSTC is not working properly
[Dual Resonant Solid State Tesla coils]
zanikk
November 18, 2019, 06:56:18 PM
post Re: IR wavelength in photoelectric smoke detectors?
[Light, lasers and optics]
Uspring
November 18, 2019, 02:38:14 PM
post Re: coil Tesla in the space
[Dual Resonant Solid State Tesla coils]
Uspring
November 18, 2019, 01:46:06 PM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
Mads Barnkob
November 18, 2019, 12:21:53 PM
post Re: IR wavelength in photoelectric smoke detectors?
[Light, lasers and optics]
shrad
November 18, 2019, 09:01:41 AM
post Re: RF Amplifier for Cubesat plasma thruster
[Electronic circuits]
shrad
November 18, 2019, 08:57:05 AM
post Re: IR wavelength in photoelectric smoke detectors?
[Light, lasers and optics]
klugesmith
November 18, 2019, 04:48:39 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
jturnerkc
November 18, 2019, 03:30:02 AM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
davekni
November 18, 2019, 02:53:29 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
davekni
November 18, 2019, 01:59:24 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
jturnerkc
November 18, 2019, 12:07:38 AM
post Re: IR wavelength in photoelectric smoke detectors?
[Light, lasers and optics]
klugesmith
November 17, 2019, 07:15:01 PM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
babass
November 17, 2019, 04:06:01 PM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
davekni
November 17, 2019, 06:26:58 AM
post Re: Measuring permanent magnets
[Electronic circuits]
klugesmith
November 17, 2019, 03:02:45 AM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
babass
November 16, 2019, 09:20:41 PM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
Mads Barnkob
November 16, 2019, 08:03:58 PM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
babass
November 16, 2019, 07:27:30 PM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
davekni
November 16, 2019, 06:55:16 PM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
thedoc298
November 16, 2019, 05:19:05 PM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
babass
November 16, 2019, 01:33:17 PM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
davekni
November 16, 2019, 04:56:44 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
davekni
November 16, 2019, 04:13:35 AM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
babass
November 16, 2019, 01:21:10 AM
post Re: coil Tesla in the space
[Dual Resonant Solid State Tesla coils]
Alexis
November 15, 2019, 08:14:00 PM
post Re: coil Tesla in the space
[Dual Resonant Solid State Tesla coils]
Mads Barnkob
November 15, 2019, 05:40:33 PM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
jturnerkc
November 15, 2019, 03:59:48 PM
post coil Tesla in the space
[Dual Resonant Solid State Tesla coils]
Alexis
November 15, 2019, 03:35:38 PM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
davekni
November 15, 2019, 05:18:19 AM
post SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
babass
November 14, 2019, 05:40:58 PM
post Re: Welcome new members, come say hello and tell a little about yourself :)
[General chatting]
babass
November 14, 2019, 05:13:01 PM
post Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
[Transformer (ferrite core)]
T3sl4co1l
November 14, 2019, 01:50:30 PM
post Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
[Transformer (ferrite core)]
Uspring
November 14, 2019, 11:45:15 AM
post Re: Problems with my first DRSSTC
[Dual Resonant Solid State Tesla coils]
davekni
November 14, 2019, 05:35:19 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
davekni
November 14, 2019, 05:25:57 AM
post Re: How to measure voltage output of AC flyback transformer
[Transformer (ferrite core)]
davekni
November 14, 2019, 04:49:57 AM
post Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
[Transformer (ferrite core)]
davekni
November 14, 2019, 04:32:50 AM
post Re: How to measure voltage output of AC flyback transformer
[Transformer (ferrite core)]
profdc9
November 13, 2019, 06:00:58 PM
post Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
[Transformer (ferrite core)]
profdc9
November 13, 2019, 05:51:27 PM
post Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
[Transformer (ferrite core)]
Mads Barnkob
November 13, 2019, 05:04:30 PM

Sitemap 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30