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Daniel, Home Appliance Technician
Category: Appliance
Satisfied Customers: 17410
Experience:  20+ yrs. experience in white goods repair. NVQ Level 2 electronics manufacturers training courses.
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I have a NEFF H7871S0GB/03 Microvave combination oven. The

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I have a NEFF H7871S0GB/03 Microvave combination oven. The main 10a fuse blows when I set max (800w) power. I replaced it and the smaller fuse has now blows. I am an electrical/electronics engineer, but have no circuit diagram. I suspect the following: capacitor, transformer or magnetron. Any help appreciated

Have you tested the diode that goes from the high voltage cap to earth?
As if the small fuse is blowing this can be the cause of it more then the magnetron.
If you have a megger or a good insulation meter then test the diode both ways on you meter as you should only get a reading one way. If you get a reading both ways then the diode is faulty.
Let me know what you find.
Customer: replied 3 years ago.

I've only got the front control panel open. I need to strip the casing to get at the magnetron if it is in there. Can you tell me where the diode I'm looking ? I've seen a part that is described as a diode, but seems to have a cable 'tail'. Is that what I'm looking for, please?

Ok when you access the capacitor you will see a diode going between the 2 tags on there. Its not that but going from 1 tag on the capacitor to the metal casing is the diode. You cannot miss it as its the only part that's attached to the metal frame and the cap from there.
But not sure if you know already that microwaves capacitor can hold up to 4500v even when un plugged so make sure you discharge it first to earth before touching it.
Customer: replied 3 years ago.

Daniel, I've found what you described, it looks like a high wattage resistor. There is another in parallel across the cap. I'm getting a reading of about 97 ohms both way on the diode.

Ok if theres a reading both ways then its faulty as with a diode you should only get a reading one way as it blocks the return. So as you have a reading both ways then its faulty and is getting a return feed from earth which is whats causing the fuse to blow.
You need to replace the diode and fuse in there and then retry and see what happens. Now fingers crossed that's sorted it but if its still blowing then magnetron can still be faulty causing this. Test the mag to earth and check there isn't an earth fault on the mag.
If the mag test ok then just replace the diode and fuse and fingers crossed that's got it.
Microwaves can be trickey to fault find as theres that many parts that can cause this and with the high voltage it produce you cannot do any live testing at all.
Customer: replied 3 years ago.

OK, I've been exploring further and found the writing on the diodes (it helps to know which way round they are!), I've also checked the one across the cap. It's completely gone! There was no charge on the cap which made me explore more. It looks like the pair have gone. The cap looks ok, but I'm only using a digital meter.

I've reconnected the diode to earth, then tried across the mag to earth and I'm getting exactly the reading of the diode, Just tried across the mag and getting a near full short circuit. I don't know what I should read across a magnetron. Can you help there?

The on across the mag is a bleed resistor which is ment to discharge the cap when its turned off so to be honest if that fails it wont really cause much of a problem.
Were as the diode that goes from the cap to earth that's the main part of it. But normally to test the diodes you need a megger to pump the 5000v across it to test it.
But what I find on microwaves when it keeps blowing the fuse like this its the diode mostly followed by the mag. Very very rarely is it the transformer or the capacitor that causes this.
Also another thing to check this the door switches. As if any of them are faulty they will cause the fuse to blow as well as all door switches need to be activated to work. If one isn't it can cause the fuse to blow as well. Quite rare this is the fault but worth checking as well.
Customer: replied 3 years ago.

I'm fairly confident that the diode has gone, as it is only showing about 98 ohms to earth when the diode is reconnected and it is the same both ways. I've got a moving coil meter I can give it a second go with, but not got a megger.

My wife is asking:

Would you fix it or replace it (it's 13 years old)?

If it was replace, do you know what would fit the same size (built in) hole?

Would you go or another brand? (That's me asking)

Ok the diode itself is about £14 from Neff so possibley worth a gamble.
As to a replacement I don't have the sizes to hand on this but just measure it and see what you find online as usually they are aout the same sizes in appliances.
As to a brand then to be honest on microwaves they are pretty much the same as they all use different brand parts microwaves as you see by the sticker on the mag.
Customer: replied 3 years ago.

Thanks, ***** ***** been a great help. It's the 1st time i've used this site, but I'll certainly be back if I need help again.

Best regards.

Your very welcome. Glad to have helped you.
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