Follow up to: Unit shuts off (JVM1340WW002) and blows fuses
I appreciate the initial respons on below problem description, but need further clarification. Though the fuse does blow as soon as I hit the "Start" keypad, pointing to faulty HV Cap, the way the microwave died is a suspect. As I described, it started fine and died down after few seconds - you could hear as the power just start going down. Can HV capacitor die in such a way?
Previous problem description:
The power on my microwave JVM1340WW002 started to go down after few seconds into operation (~5 sec not right away) and then shut down completely. Found that fuse was blown. After replacing the fuse it would shut down as soon as it would start, blowing the fuse again. The High power Capacitor and rectifier show no visible sign of damage. Checked door switch and thermal sensors (two of them - both fine: one is normally open). It leaves either HV Capacitor/Diode, Transformer or Magnetron (still under warranty). I would be inclined to suspect the Capacitor, but the fact that microwave was able to start and died down in few seconds raises doubts. My understanding is that HV cap is there to provide the boost on start up only. Please help.
Re: Follow up to: Unit shuts off (JVM1340WW002) and blows...
I have a similar problem in a GE Microwave. I metered the capacitor and it shows normal buildup of resistance over an extended period of time - leads me to believe it's in it's charging cycle. Following the assumption that the capacitor was good I check the triac assembly and found one of the two of them to be a dead short. I've ordered two new triac's to replace the two that are on the board and will replace them on the board - but should I again suspect the capacitor? After the microwave cuts out - the capacitor has no power retained, but I went under the assumption that the magnetron drained it since the power feed fuse blew.
I found a slight discoloration on the control board (browning) around the on board relays, and checked them. One of the relays has a path to short - which I belive is controled by the monitor switch - but shows no sign of use - I have not yet figured out the browning of the board, but all the nearby components and traces check out.
Any assistance would be greatly appreciated.
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Re: Follow up to: Unit shuts off (JVM1340WW002) and blows...
In my 20 years of working on microwaves, I have never seen a capacitor fail in any way but a dead short, but it's possible that it's failure may occur in a less abrupt manner.
The internal structure of a capacitor is essentially a rolled up sandwich consisting of two thin layers of foil separated by a very thin insulator.
While a "perfect" capacitor consumes no power, in the real world, things can happen to change that.
A defect could increase the capacitve reactance or other internal resistance and allow internal heat buildup or expansion, which could lead to an intermittent short when it's warmed up a bit.
After a while, the short could become permanent - sort of like arc welding, if you're familiar with that.
So, while it's quite possible that this is the pathology that lead to a fully shorted capacitor, I think that's what you have, no matter how it happened.
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Katherine, there is an internal fuse on microwaves. First, make sure you un-plug the microwave and then you will need to take the cover off of the unit. You will find screws on the back and sometimes the sides that hold the cover on. Once you have the cover off, look for where the power cord goes into the machine. There will be three wires coming out of the power cord, one black, one white, and one green. Follow the black wire and it will usually take you right to the fuse in a holder, or to a circuit where the fuse will be located. You will need to look at the end of the fuse to see what amperage fuse you will need. Most microwaves use either a 15 or a 20 amp fuse. Make sure you replace with the identical size fuse. If it blows again right away, that means you have further problems and should have it looked at by a regular repairman. Hope this helps!
It appears like the high voltage capacitor is shorted or the diode connected to the capacitor. You might want to check the monitoring switch as well if the capacitor and diode are good. Click here for more info on how to work around these high voltage devices.
Let me know if you need further assistance.
The fuse is internal and you will have to remove the cover to be able to access it. Unplug the unit. Once you remove the cover, follow where the power cord goes into the unit. The fuse should be in that area. Replace the fuse with the EXACT value of that of the original one.
If the fuse blows again when you put the cover back on and turn it on, then this indicates that you have a problem in the power supply or with the magenetron.
Please let me know if I can be of further assistance to you.
> My question: Could they be right about the transformer and magnetron?
It certainly could be the transformer or the magnetron, but I really don't think so.
Not that I'm recommending that an inexperienced or unqualified person should do this - and bearing in mind that any person who would do so must assume ALL liability for injury or damage - here's how I (on a VERY careful day *grin*) would find out:
- I would disconnect the oven's power cord from the wall
- I would remove the outer cover
- I would discharge the high voltage capacitor
- I would look again to be sure the plug is out of the wall
- I would make a careful note of and/or mark the exact connections of the wires going to the HV capacitor
- I would be very careful in case the connectors have positive locks and not yank on them before I push in the locks
- I would disconnect the wires from HV capacitor
- I would set my meter to a medium or high resistance scale and touch one probe to each capacitor terminal and from each terminal to the case of the capacitor
- If either reading held steady at less than infinity I would replace the capacitor, reconnect the wires, reassemble and test
- If neither reading held steady at less than infinity I would reconnect the wires
- I would then make a careful note and/or mark the exact locations of the wires attached to the transformer
- I would then disconnect just the lead which goes from the secondary of the transformer to the capacitor
- I would then remove the other end of that lead wire
- I would then insert a new fuse then plug the microwave into the wall and test it
- If the fuse blew, I would know it's not the capacitor or the magnetron and I would troubleshoot further
- If the fuse did not blow, I would know it's the magnetron or the capacitor (but I just measured the capacitor and it's not shorted!! *grin*)
- I would unplug it again from the wall
- I would install a new capacitor, hook it all back up, install a new fuse, reconnect the wires, and test it again
Sorry to be so stiff about my reply, but I do not want to advise or suggest that an untrained or inexperienced should troubleshoot or work on a microwave oven. The voltages can be instantly lethal.
Good luck and be safe!!!
Generally speaking, if the high voltage capacitor is shorted, the fuse will blow as soon as you hit the START pad. When the HV cap fails, I've never seen one do anything but short.
If it's a few seconds or so into cooking, it's usually the high-voltage transformer. There may or may not be a burning smell. The cooling fan often will dissipate the smell.
What can happen to the transformer is an expansion of the windings to the point where a couple of hot spots eventually make bare spots which touch, then the short causes the fuse to blow. When it cools, they aren't shorting anymore.
It's possible it's the mag or something else, but not too likely.
You can (carefully!) disconnect the primary leads from the HV transformer then run the oven, making sure the wires are free and clear. If the fuse blows, the problem is in the low-voltage side. If it doesn't the trouble is in the HV side.
If you broke a seal (tamper tag) or left any other evidence that you were inside the microwave, your warranty will be voided.
Some parts may be covered, but the labor warranty usually expires sooner. Be careful.