Armed with a replacement Capacitor/Diode combination, I headed over to my son's house this morning to finally have it out with that cursed microwave. With the help of my grandson, we took it down from the cabinet and set it on the kitchen table. I removed the cover and got to work.
First step was to verify where the problem was originating. A microwave troubleshooting guide suggested disconnecting the primary side of the transformer to help isolate the problem. So I unplugged it and powered up the microwave. Upon opening the door while it was running, as has become the usual M.O., the fuse blew.
Right there that tells me the problem is not with the microwave generation circuitry, so my capacitor/diode purchase wasn't going to fix anything. I confirmed that by replacing the fuse and heating a glass of water for one minute. It heated fine with no issue, so it was time to go back to the door switches; it had to be either them or some of the circuitry they're attached to.
Examining the operation of the switches, it became apparent that the one labelled the Monitor switch switches state first when opening the door. The Primary switch goes next, with the Door Sensing switch either immediately after or about the same time as the Primary switch.
I examined the design of the switch mechanism carefully and verified that the middle switch is definitely designed to change first, before the other two. Pulling lightly on the door, just when you start to encounter resistance is when that Monitor switch connects (it's a NC switch). I wanted to verify that sequence was what was causing the problem so I unplugged it and plugged in a spare switch. While holding the switch down with my finger and having the door closed, I plugged in the microwave (kids, don't try this at home!) I put some time on it and started it heating a glass of water. After letting it run a few seconds, I released the button on the switch I was holding and sure enough, that blew the fuse.
Since the family was leaving I enlisted my grandson to help me remount the microwave in the cabinet since I can access everything I need to regarding the door switches with it in place. Having run out of ideas, I buttoned it back up and told them they could use it as long as they never open the door when it's running (the fuse was blown when I got to their house because my granddaughter can't seem to follow instructions and opened the door while it was running).
I was still pondering the whole issue as I started on the half hour drive back home. One thing I realized was that someone had previously worked on that microwave because one of the two NO switches was a different brand and color, so it's been replaced in the past. I was thinking about perhaps replacing the switch mounting assembly since that's what's responsible for the sequencing of the switches, even though I couldn't find anything that looked like it was wrong with the original one.
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That's when I had a Eureka! moment and realized that maybe the sequencing is working exactly the way it should, but what if someone put the switches back in the wrong position? That would mean the monitor switch putting a dead short across the power lines and blowing fuses exactly as I saw, but if instead the Primary switch went first then there wouldn't be an danger of blowing the fuse.
+
That's when I had a Eureka! moment and realized that maybe the sequencing is working exactly the way it should, but what if someone put the switches back in the wrong position? That would mean the monitor switch putting a dead short across the power lines and blowing fuses exactly as I saw, but if instead the Primary switch went first then there wouldn't be any danger of blowing the fuse.
I was at the halfway point but I went ahead and turned back around and returned to my son's house. Opening up the control panel took just a minute and since I had already replaced all three switches with NO/NC type switches I didn't have to swap the switches themselves, just change which plug goes to which switch.
I plugged it back in and tested. Heat worked fine, lights work correctly and - glory hallelujah - the fuse did NOT blow when I opened the door as it was running! So yeah, all that time and effort because the last guy who worked on it replaced the switches in the wrong positions. I'm unclear as to why it seemed to work fine for the several years they've been using it, unless perhaps they had a higher amperage fuse (it calls for 20A) and because it's on a 15A circuit breaker, I have to wonder if the circuit breaker has been going and my son just didn't think anything of it and reset it up until it finally blew the fuse and I put in the right amperage fuse that blew much easier than whatever was in there. That's my working theory, but at this point I don't care enough to follow up on it; the microwave is working now and I don't have to deal with it any more.
TL;DR version: Last person who replaced a door switch swapped the position of the NO Primary switch with the NC Monitor switch. Putting the switches back in their proper locations has fixed the problem.
Armed with a replacement Capacitor/Diode combination, I headed over to my son's houses this morning to finally have it out with that cursed microwave. With the help of my grandson, we took it down from the cabinet and set it on the kitchen table. I removed the cover and got to work.
+
Armed with a replacement Capacitor/Diode combination, I headed over to my son's house this morning to finally have it out with that cursed microwave. With the help of my grandson, we took it down from the cabinet and set it on the kitchen table. I removed the cover and got to work.
First step was to verify where the problem was originating. A microwave troubleshooting guide suggested disconnecting the primary side of the transformer to help isolate the problem. So I unplugged it and powered up the microwave. Upon opening the door while it was running, as has become the usual M.O., the fuse blew.
Right there that tells me the problem is not with the microwave generation circuitry, so my capacitor/diode purchase wasn't going to fix anything. I confirmed that by replacing the fuse and heating a glass of water for one minute. It heated fine with no issue, so it was time to go back to the door switches; it had to be either them or some of the circuitry they're attached to.
Examining the operation of the switches, it became apparent that the one labelled the Monitor switch switches state first when opening the door. The Primary switch goes next, with the Door Sensing switch either immediately after or about the same time as the Primary switch.
I examined the design of the switch mechanism carefully and verified that the middle switch is definitely designed to change first, before the other two. Pulling lightly on the door, just when you start to encounter resistance is when that Monitor switch connects (it's a NC switch). I wanted to verify that sequence was what was causing the problem so I unplugged it and plugged in a spare switch. While holding the switch down with my finger and having the door closed, I plugged in the microwave (kids, don't try this at home!) I put some time on it and started it heating a glass of water. After letting it run a few seconds, I released the button on the switch I was holding and sure enough, that blew the fuse.
Since the family was leaving I enlisted my grandson to help me remount the microwave in the cabinet since I can access everything I need to regarding the door switches with it in place. Having run out of ideas, I buttoned it back up and told them they could use it as long as they never open the door when it's running (the fuse was blown when I got to their house because my granddaughter can't seem to follow instructions and opened the door while it was running).
I was still pondering the whole issue as I started on the half hour drive back home. One thing I realized was that someone had previously worked on that microwave because one of the two NO switches was a different brand and color, so it's been replaced in the past. I was thinking about perhaps replacing the switch mounting assembly since that's what's responsible for the sequencing of the switches, even though I couldn't find anything that looked like it was wrong with the original one.
That's when I had a Eureka! moment and realized that maybe the sequencing is working exactly the way it should, but what if someone put the switches back in the wrong position? That would mean the monitor switch putting a dead short across the power lines and blowing fuses exactly as I saw, but if instead the Primary switch went first then there wouldn't be an danger of blowing the fuse.
I was at the halfway point but I went ahead and turned back around and returned to my son's house. Opening up the control panel took just a minute and since I had already replaced all three switches with NO/NC type switches I didn't have to swap the switches themselves, just change which plug goes to which switch.
I plugged it back in and tested. Heat worked fine, lights work correctly and - glory hallelujah - the fuse did NOT blow when I opened the door as it was running! So yeah, all that time and effort because the last guy who worked on it replaced the switches in the wrong positions. I'm unclear as to why it seemed to work fine for the several years they've been using it, unless perhaps they had a higher amperage fuse (it calls for 20A) and because it's on a 15A circuit breaker, I have to wonder if the circuit breaker has been going and my son just didn't think anything of it and reset it up until it finally blew the fuse and I put in the right amperage fuse that blew much easier than whatever was in there. That's my working theory, but at this point I don't care enough to follow up on it; the microwave is working now and I don't have to deal with it any more.
TL;DR version: Last person who replaced a door switch swapped the position of the NO Primary switch with the NC Monitor switch. Putting the switches back in their proper locations has fixed the problem.
Armed with a replacement Capacitor/Diode combination, I headed over to my son's houses this morning to finally have it out with that cursed microwave. With the help of my grandson, we took it down from the cabinet and set it on the kitchen table. I removed the cover and got to work.
First step was to verify where the problem was originating. A microwave troubleshooting guide suggested disconnecting the primary side of the transformer to help isolate the problem. So I unplugged it and powered up the microwave. Upon opening the door while it was running, as has become the usual M.O., the fuse blew.
Right there that tells me the problem is not with the microwave generation circuitry, so my capacitor/diode purchase wasn't going to fix anything. I confirmed that by replacing the fuse and heating a glass of water for one minute. It heated fine with no issue, so it was time to go back to the door switches; it had to be either them or some of the circuitry they're attached to.
Examining the operation of the switches, it became apparent that the one labelled the Monitor switch switches state first when opening the door. The Primary switch goes next, with the Door Sensing switch either immediately after or about the same time as the Primary switch.
I examined the design of the switch mechanism carefully and verified that the middle switch is definitely designed to change first, before the other two. Pulling lightly on the door, just when you start to encounter resistance is when that Monitor switch connects (it's a NC switch). I wanted to verify that sequence was what was causing the problem so I unplugged it and plugged in a spare switch. While holding the switch down with my finger and having the door closed, I plugged in the microwave (kids, don't try this at home!) I put some time on it and started it heating a glass of water. After letting it run a few seconds, I released the button on the switch I was holding and sure enough, that blew the fuse.
Since the family was leaving I enlisted my grandson to help me remount the microwave in the cabinet since I can access everything I need to regarding the door switches with it in place. Having run out of ideas, I buttoned it back up and told them they could use it as long as they never open the door when it's running (the fuse was blown when I got to their house because my granddaughter can't seem to follow instructions and opened the door while it was running).
I was still pondering the whole issue as I started on the half hour drive back home. One thing I realized was that someone had previously worked on that microwave because one of the two NO switches was a different brand and color, so it's been replaced in the past. I was thinking about perhaps replacing the switch mounting assembly since that's what's responsible for the sequencing of the switches, even though I couldn't find anything that looked like it was wrong with the original one.
That's when I had a Eureka! moment and realized that maybe the sequencing is working exactly the way it should, but what if someone put the switches back in the wrong position? That would mean the monitor switch putting a dead short across the power lines and blowing fuses exactly as I saw, but if instead the Primary switch went first then there wouldn't be an danger of blowing the fuse.
I was at the halfway point but I went ahead and turned back around and returned to my son's house. Opening up the control panel took just a minute and since I had already replaced all three switches with NO/NC type switches I didn't have to swap the switches themselves, just change which plug goes to which switch.
I plugged it back in and tested. Heat worked fine, lights work correctly and - glory hallelujah - the fuse did NOT blow when I opened the door as it was running! So yeah, all that time and effort because the last guy who worked on it replaced the switches in the wrong positions. I'm unclear as to why it seemed to work fine for the several years they've been using it, unless perhaps they had a higher amperage fuse (it calls for 20A) and because it's on a 15A circuit breaker, I have to wonder if the circuit breaker has been going and my son just didn't think anything of it and reset it up until it finally blew the fuse and I put in the right amperage fuse that blew much easier than whatever was in there. That's my working theory, but at this point I don't care enough to follow up on it; the microwave is working now and I don't have to deal with it any more.
TL;DR version: Last person who replaced a door switch swapped the position of the NO Primary switch with the NC Monitor switch. Putting the switches back in their proper locations has fixed the problem.