Panasonic Microwave Oven NN6850 Internal Fuse

[G3NDI]

This microwave oven is quite old – bought in March 1991 and has never given any problems. I have vacuumed the dust from the inside a couple of times after removing the cover.

Today when I switched it on the there was a loud pop after about 1 sec. and it went “dead” and the ££ signs started ticking away! Having initially checked the 13 A plug fuse I suspected there might be an internal fuse as I know my GlowWorm Hideaway gas boiler has one in the electronics box, likewise my old Armstrong 621 HiFi amplifier had one.

Having removed the cover I spotted a blackened standard 1 1/4″ fuse in clips on the “inverter” PCB. This turned out to be a 8A fuse. There was no indication that it was a slow blow type which might have been required because of the heater current surge in the magnetron; the only markings on the metal caps were SOC 8A. I found that Maplin stock various fuses, but not 8A fast blow; I used 10A and on replacement the oven switched on normally. It seems the fuse may have failed from old age running close to its rating?

On the subject of microwave oven repairs I found the following site may be of help to other members.

Rodney Fry

Mod edit, While the information is openly available on the web we would prefer that they find and act on it themselves. I have removed the microwave self repair link as we cannot ascertain anyone else’s competence. DIY repairs on microwaves is dangerous to say the least.

[electrofix]

Re: Panasonic Microwave Oven NN6850 Internal Fuse

non professionals please note

microwaves use very high voltages (5000v) you can get a shock from an electric socket and live but not from a 5000v microwave

if you dont know DONT TOUCH !!

Dave

[SAMURI]

Re: Panasonic Microwave Oven NN6850 Internal Fuse

Yes and that can happen even if it is unplugged from the mains if you do not discharge the capacitor.

But it is ok for a second person to fix it if the first one has been electrocuted as this would have discharged the capacitor 😀 .

That’s if it has not been plugged in again to the mains to charge the capacitor again.

Bob

[G3NDI]

Re: Panasonic Microwave Oven NN6850 Internal Fuse

Thanks for the comments on the HV in the ovens – I’m aware of these having made years ago a 2.5kV/0.5A power supply for an HF amplifier!

I have found subsequently that the fuse failure was not due to old age deterioration, but the fuse blows only after pressing the Start button on the oven. Checking the sequence of events I found that with a new fuse
the oven switches on and the display illuminates
the power levels are selectabel
the time intervals are selectable
the defrost levels are selectable
but placing a load, i.e. jug of water, in the oven and pressing Start button the transformer gives loud, rough hum and the fuse blows.

I suspect that I have a magnetron internal failure, or possibly the HV diode in the voltage double is faulty; there is no smell of burning or discoloration or obvious shorts on the large transformer, or anywhere else. I am quite surprised to find the magnetron, e.g. 2M210-M1 and other parts available on the internet for such an old oven, but looking at the various images it seems there is little internal difference over the years, probably the main changes between makes are in the PCB for the diplay/control buttons – there is not much one can change in the transformer/voltage doubler circuit and magnetron operating around 2.4 GHz. Even the HV capacitor marked at 2.14 uFd, 2100V, is available! As I don’t have appropriate test meters I have taken the oven to a small company offering microwave oven repairs and keeping my hopes up that it will be repaired.

Rodney Fry

[electrofix]

Re: Panasonic Microwave Oven NN6850 Internal Fuse

its a fairly simple proceedure without meters to find out whats wrong and components are cheap

we have to be careful and dont want to give the general public the idea that trying to fix a microwave is a good idea if you dont understand the risks. we had a proffessional killed in Liverpool area about 20 years ago and we think hae was trying to power 1 mag from another mirowave without realising the dc return is through the chassis

hope you fix it

Dave

[Martin]

Re: Panasonic Microwave Oven NN6850 Internal Fuse

I’m more than convinced that Rodney is fully proficient when it comes to safety procedures. He’s a radio ham after all (as am I :wink:) and very experience with high power amplifiers and such.

I’m fairly sure from what you tell us Rodney that the magnetron is the guilty party. 😀

73

[G3NDI]

Re: Panasonic Microwave Oven NN6850 Internal Fuse

I contacted a company in Thatcham, Esteem Domestic Appliances, and following my email just before Christmas 2016 and their reply, I took the oven in for assessment. There was no urgency as we had borrowed another oven. They called me in the New Year with a quote for a new magnetron and HV diode, as I had suspected. The job was completed in less than a week for reasonable price. They advised that the magnetron failure had knocked out the HV diode which was why both were replaced. Apparently another customer had bought in the same oven with a failed magnetron – so a good life out of the original component!
Now have a working oven again, so a happy customer and can recommend them.

Rodney Fry

[Author]

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