magnetron fuse blown

[Gazz]

Got a kenwood ST/AL25 microwave, about 3 years old, fitted a new turntable motor about 2 years ago but appart from that it's been fine,

this morning, no cooking power, well, very slight warming of a bowl of porridge after about 15 minutes in there (GF kept putting it back in for another 3 minutes each time it came out cold)

I've taken the cover off, checked the overheat cutouts, fine, big capacitor is reading just over 1uF and not shorted, but the big fuse from the transformer to the magnetron has gone,

it dosent look like it blew violently, glass isnt blackened, fise wire is intact for 3/4 of the length, then a break,

i'll get another fuse or 2, but was just wondering if these fuses are likely to pop just for fun so to speak, i.e. a little power surge whilst cooking, or power dropping out for a second, anything to check before i re-fuse it and power up??

i seem to remember the door interlock switches short something out when the door is opened, so if the door is pulled open at an angle and one of the switches closes before the other, it pops a fuse, but if that were the case wouldent the fuse show signs of a violent rupture?

[Andrew Gabriel]

There can be one on the secondary. It will tend to blow if the diode or capacitor short out (or I guess the magnetron, if that's possible). I guess insulation breakdown on the HV side could also do it.

Not sure if it might also be possible to blow that fuse by running the microwave empty. I know you can write off the magnetron that way.

[Gazz]

there are 2 fuses that i can see, a standard 1/1/4 inch fuse rated at 250 volts, got a white ceramic centre so cant see the fuse wire, but that's intact, and is on the filter board that the mains lead attatches to before going off to the rest of the system,

then there's the big transformer, 2 small-ish wires into it at the bottom on uninsulated spae terminals, spaced close enough i'd expect anything over 1kv to jump the gap. Then 2 high voltage wires out (thick cloth like insulation), one goes direct to the magnetron, the other is on a spade terminal, this 2 inch 5kv fuse, then goes to the capacitor, then to the magnetron,

i think there's another wire from the output of the transformer that goes to the interlock switches,

thing is, the fuse that's blown hasnt blown violently as youd get with a short, 3/4 of the fuse wire is intact, if i think about it, it's more like a stress failure,

google brings up lots of people saying that just after the 2 year warrenty runs out, this microwave stops heating up, most just buy a new microwave, but some look inside and find the big fuse gone, replace it (at about 2 quid a go, or 10 quid if you buy one from kenwood) and it works again, (some wrap some copper wire around the fuse and get it working, claiming it to be safe)

i guess i shall just replace the fuse and see what happens, just i like to find out what caused the fuse to go before i do that usually, but i'm guessing as the fuse is mounted direclty on the transformer, each time the tranny powers up it does so with a thunk, many thousands of thunks over the years has fatigued the fuse wire till it let go,

[Andrew Gabriel]

The secondary one won't blow violently - the transformer will limit the fault current to a much lower value than is available on the primary side (hence the HRC fuse on the primary).

Out of interest, what's the current rating of the 5kV fuse? I would think the transformer would have difficulty blowing it if it's much more than the half an amp it uses, and that may mean the HV side is very closely fused.

Might want to visually check the HV diode too. I don't know if the HV diode will lend itself to being checked easily with a test meter.

And obviously, this is all done with the unit unplugged and the HV capacitor discharged.

[NT]

As its just a secondary fuse, You could just bridge it with a _very_ thin bit of wire, like a single strand from extraflex.

NT