Breville B25B7DCS Microwave

[Crackle]

I have been asked about a non-working microwave by a neighbour and I volunteered to look at it for them.

All seems to be working (clock, grill etc.) except the microwave part.
On investigation the 0.8A HV fuse has blown, after breaking several fingernails I got the fuse holder apart and found there is no cartridge just what must have been a fuse wire held by a spring.

I wonder if someone can confirm my findings with the HV diode.
   
It is type CL04 -12 RG706 I checked it using a 9 volt battery, and in the forward position the voltage measured was 5.5v, in the reverse position the voltage measured was 9.8mV The circuit I used is here as seen on a video on youtube.
   
My results were similar to the youtube test so does this prove the diode is OK

The capacitor was tested next. At first in the machine I thought it was faulty as it had a resistance of 10M but when it was removed I saw it had a 10M discharge resistor.
   
I tested it on my reformer and at 370 volts the capacitor passes 35uA this is accounted for by the 10M resistor and as the capacitor is measuring spot on 1.05uf so I am assuming it is OK.

This leads me to believe that the fault lies with the magnatron.
Is there any way of testing a magnatron?
I found this youtube video https://www.youtube.com/watch?v=Be906D3sxFI
The person in the video claims the resistance should be 1 ohm, but he is using one of those £3 multimeters and he did not check the resistance of the test leads first, so I think the test is suspect.

On my meter the short circuit resistance is .09 ohms and the heater measures .12 ohms.
Is this indicating a fault do you think.

I found a circuit on the inside of the cover
   
Thanks
Mike

[ppppenguin]

I've seen claims that the filament resistance should be less than 1 ohm. It's hard to see how it would have failed near enough short circuit.

I'm no expert but it could just be the fuse. What does the blow look like? Is there vapourised metal on anything? If so, it blew with violence and there's probably another fault. If it's just parted then it could simply be old age.

I'm sure that Mike is aware of the hazards of a microwave oven but it's worth repeating that the voltages involve are lethal. Over a kilovolt with lots of current available. The capacitor may be holding a charge after switch off. Never assume the bleed resistor is OK.

[Crackle]

Hi Jeffrey
Thanks for the tips.
The fuse does not look like it vaporised much but then there is not much of it left either.
I don't know how long the actual fuse element would have been.
This is the inside of the fuse
   
The spring is obviously there to ensure when the fuse breaks there is no chance of a prolonged arc.
Mike

[ppppenguin]

That fuse certainly didn't blow violently so I reckon it's worth a replacement if they don't cost too much.

[`FRANK.C]

A hairline crack in the magnatron's magnets will cause the magnatron to be a short circuit across the EHT.  The crack can be so small that it is not possible to see so difficult to diagnose.
I would think that the heater is OK . The heater is also the cathode and its resistance is usually low.
As Jeffrey has said try a fuse, it most likely just got tired.

Frank

[Crackle]

I made a temporary test fuse link with a strand of wire the same thickness as the original.
Put a small amount of water in a bowl and gave it a 5 second test, the water was definitely warm.
So it looks like the original fuse was weak.
The microwave is only just out of its 1st year guarantee, what a pity something like this can go wrong, normally the microwave would probably have been dumped.

New fuses look to be about £10 from UK supplier and £3.5 from China.

Mike

[boater sam]

Flash over because of metallic non self items in the cavity will blow the fuse, gently, so try a new one. The cartridge type can be substituted, they are special, very long. Most just have leads soldered on and 2 layers of heatshrink sleeving but spaced away from any earthed metalwork.

[ppppenguin]

The microwave is only just out of its 1st year guarantee, what a pity something like this can go wrong, normally the microwave would probably have been dumped.

Consumer Rights Act gives you significant rights up to 6 years from purchase. If it was bought from a decent place such as John Lewis or Argos there's normally no problem enforcing your rights. Alas little hope now after you've been inside it.

https://www.moneysavingexpert.com/shoppi...nge/#goods

A few years ago I helped a friend with an 18 month old faulty TV get a repalcement from Argos. All it took was a well worded letter showing that she knew her legal rights.

[Crackle]

I have been wondering if the wrong size fuse was fitted by mistake, does the fuse rating of .8 amp sound correct for a 900 watt output microwave.
If the voltage is 1000 as mentioned above then it would need a slightly larger fuse.

Mike

[Refugee]

You are supposed to throw away the fuse assembly and replace it with another. They come with the spade terminals and leads fitted. They do not have to look physically the same but it is easier if they do.