Discussion:
Microwave oven fuse
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D.M. Procida
2025-01-28 20:22:48 UTC
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My microwave oven, now 15+ years old, blew its fuse yesterday.

It's a slow-blow fuse (T10 AL 250V P).

I'm going to replace it tomorrow, but how likely is it that whatever caused it
to go is just going to do it again straight away?

Daniele
The Natural Philosopher
2025-01-28 23:40:43 UTC
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Post by D.M. Procida
My microwave oven, now 15+ years old, blew its fuse yesterday.
It's a slow-blow fuse (T10 AL 250V P).
I'm going to replace it tomorrow, but how likely is it that whatever caused it
to go is just going to do it again straight away?
Daniele
Quite likely.

I am surprised the high voltage fuse didn't blow. That's what happened
to mine - the magnetron had an anode cathode short and had to be
replaced . That and a failure of the high voltage capacitor or the high
voltage diode are really the most common problems, and are trivial to
replace.

It is possible to remove the HT fuse mostly, and if the thing doesnt
blow the mains after that, there are only 5 components on the HT side
to check - the magnetron, the HT fuse itself, the diode the capacitor
and the HT transformer. If THAT goes you are in trouble with likely
magic smoke coming out.

You can check the magnetron off power by putting a resistance meter
between cathode heater and ground. Should be open circuit. The
capacitor you can use a resistance meter to see if the needle jumps and
then goes back to open circuit.

The fuse is obvious to test but the diode is not - its stacked diodes in
series and needs a few volts to turn on.

If the magnetro9n has gone its generally around 50% of the unit cost, or
more, (somewhere in the £30-£150 range) and you should consider
scrapping the unit.

Diodes fuses and capacitors are all sub £5.

Apart from that I found mine easy enough to fix, with the proviso you
absolutely dont want to power it up with the covers off.


All in all its not worth the faff for a sub £100 unit, but mine was a
£250 catering unit, and a £75 magnetron and a 90p fuse was in budget.
--
"The most difficult subjects can be explained to the most slow witted
man if he has not formed any idea of them already; but the simplest
thing cannot be made clear to the most intelligent man if he is firmly
persuaded that he knows already, without a shadow of doubt, what is laid
before him."

- Leo Tolstoy
D.M. Procida
2025-01-29 17:54:54 UTC
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On 28 Jan 2025 at 23:40:43 GMT, "The Natural Philosopher"
Post by The Natural Philosopher
Post by D.M. Procida
My microwave oven, now 15+ years old, blew its fuse yesterday.
It's a slow-blow fuse (T10 AL 250V P).
I'm going to replace it tomorrow, but how likely is it that whatever caused it
to go is just going to do it again straight away.
I am surprised the high voltage fuse didn't blow. That's what happened
to mine - the magnetron had an anode cathode short and had to be
replaced . That and a failure of the high voltage capacitor or the high
voltage diode are really the most common problems, and are trivial to
replace.
<useful advice>
Thanks for all that, interesting and informative. It did indeed blow the fuse
again, with a an angry buzz, so I am just going to take it to a shop - if such
a thing exists any more - and see if someone else can repair it.

Daniele
Andy Burns
2025-01-29 21:02:39 UTC
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Post by D.M. Procida
It did indeed blow the fuse
again, with a an angry buzz, so I am just going to take it to a shop - if such
a thing exists any more - and see if someone else can repair it.
Economically?
RJH
2025-01-30 09:07:56 UTC
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Post by Andy Burns
Post by D.M. Procida
It did indeed blow the fuse
again, with a an angry buzz, so I am just going to take it to a shop - if such
a thing exists any more - and see if someone else can repair it.
Economically?
There is a cost to /not/ getting it fixed . . . and indeed benefits to fixing
it. Not always all about money . . .
--
Cheers, Rob, Sheffield UK
The Natural Philosopher
2025-01-30 09:45:01 UTC
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Post by RJH
Post by Andy Burns
Post by D.M. Procida
It did indeed blow the fuse
again, with a an angry buzz, so I am just going to take it to a shop - if such
a thing exists any more - and see if someone else can repair it.
Economically?
There is a cost to /not/ getting it fixed . . . and indeed benefits to fixing
it. Not always all about money . . .
Well there is a good You tube video about testing and fixing the high
voltage side which is most likely what is gone.

The circuits are pretty much all the same - a high power high voltage
transformer that drives the magnetron, with a winding for its heater,
then a diode to rectify and voltage double the HT voltaqe and a fat
capacitor to smooth it., and usually a fuse in it.

You test the capacitor for leakage with an ohm meter.
You test the fuse for breakage with an ohm mketer.
Unless you have 9V plus batteries in your meter you cant test the diode
as its forward drop is about 7V (Its a stack of lower voltage diodes in
series) but you can fake it with a 9V battery and a resistor and a
current meter.,
You test the transformer for winding continuity and no shorts between
windings or grounded wiring.
If it is the transformer you are in for a hefty bill and trouble getting
a spare.

Also be absolutely careful to see if the broken parts are in fact
obtainable - a 15 year old machine is somewhat likely to not have a
great range of spares.

And 'taking it to a man' is likely to incur a repair bill up towards
£200 or a 'I cant fix that' response.
--
The biggest threat to humanity comes from socialism, which has utterly
diverted our attention away from what really matters to our existential
survival, to indulging in navel gazing and faux moral investigations
into what the world ought to be, whilst we fail utterly to deal with
what it actually is.
The Natural Philosopher
2025-01-30 09:48:26 UTC
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Post by The Natural Philosopher
Post by RJH
Post by Andy Burns
Post by D.M. Procida
It did indeed blow the fuse
again, with a an angry buzz, so I am just going to take it to a shop - if such
a thing exists any more - and see if someone else can repair it.
Economically?
There is a cost to /not/ getting it fixed . . . and indeed benefits to fixing
it. Not always all about money . . .
Well there is a good You tube video about testing and fixing the high
voltage side which is most likely what is gone.
The circuits are pretty much all the same - a high power high voltage
transformer that drives the magnetron, with a winding for its heater,
then a diode to rectify and voltage double the HT voltaqe and a fat
capacitor to smooth it., and usually a fuse in it.
You test the capacitor for leakage with an ohm meter.
You test the fuse for breakage with an ohm mketer.
Unless you have 9V plus batteries in your meter you cant test the diode
as its forward drop is about 7V (Its a stack of lower voltage diodes in
series) but you can fake it with a 9V battery and a resistor and a
current meter.,
You test the transformer for winding continuity and no shorts between
windings or grounded wiring.
If it is the transformer you are in for a hefty bill and trouble getting
a spare.
I forgot...

You test the magnetron for no short between the cathode/heater and
ground, and for heater continuity.
Magnetrons are also expensive
Post by The Natural Philosopher
Also be absolutely careful to see if the broken parts are in fact
obtainable - a 15 year old machine is somewhat likely to not have a
great range of spares.
And 'taking it to a man' is likely to incur a repair bill up towards
£200 or a 'I cant fix that'  response.
--
The urge to save humanity is almost always a false front for the urge to
rule.
– H. L. Mencken, American journalist, 1880-1956
ajh
2025-01-30 11:09:24 UTC
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Post by The Natural Philosopher
Magnetrons are also expensive
Do they lose vacuum over time?
The Natural Philosopher
2025-01-30 15:07:57 UTC
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Post by ajh
Post by The Natural Philosopher
Magnetrons are also expensive
Do they lose vacuum over time?
I dont think so. The article I read and my own magnetron failed with a
soft short between anode and cathode. It also showed signs of rough
treatment before I got it. I suspect metal was being deposited from the
cathode onto whatever insulates it all inside, and eventually it arced
over.

50 year old valves still work.
--
The urge to save humanity is almost always a false front for the urge to
rule.
– H. L. Mencken, American journalist, 1880-1956
Jeff Layman
2025-01-31 08:26:17 UTC
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Post by The Natural Philosopher
Post by RJH
Post by Andy Burns
Post by D.M. Procida
It did indeed blow the fuse
again, with a an angry buzz, so I am just going to take it to a shop - if such
a thing exists any more - and see if someone else can repair it.
Economically?
There is a cost to /not/ getting it fixed . . . and indeed benefits to fixing
it. Not always all about money . . .
Well there is a good You tube video about testing and fixing the high
voltage side which is most likely what is gone.
The circuits are pretty much all the same - a high power high voltage
transformer that drives the magnetron, with a winding for its heater,
then a diode to rectify and voltage double the HT voltaqe and a fat
capacitor to smooth it., and usually a fuse in it.
Don't forget there's usually a bleed resistor to discharge the
capacitor. I've never heard of one going short-circuit, but you never know.
Post by The Natural Philosopher
You test the capacitor for leakage with an ohm meter.
Not without first making sure it's fully discharged! If the bleed
resistor has gone o/c and the capacitor's decent it could keep a charge
for quite a time. At best you'd destroy the meter, at worst... :-(
Post by The Natural Philosopher
You test the fuse for breakage with an ohm mketer.
Unless you have 9V plus batteries in your meter you cant test the diode
as its forward drop is about 7V (Its a stack of lower voltage diodes in
series) but you can fake it with a 9V battery and a resistor and a
current meter.,
You test the transformer for winding continuity and no shorts between
windings or grounded wiring.
If it is the transformer you are in for a hefty bill and trouble getting
a spare.
When an old microwave oven went belly-up (magnetron died), I wondered if
I could use the transformer. however, it seemed so firmly fixed in I
gave up (perhaps a bit of self-preservation clicked in as well!).
Post by The Natural Philosopher
Also be absolutely careful to see if the broken parts are in fact
obtainable - a 15 year old machine is somewhat likely to not have a
great range of spares.
And 'taking it to a man' is likely to incur a repair bill up towards
£200 or a 'I cant fix that' response.
Yep. When I tried to source a replacement magnetron I found I was
looking at £50+.
--
Jeff
Tim+
2025-01-31 20:25:20 UTC
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Post by Jeff Layman
Post by The Natural Philosopher
Also be absolutely careful to see if the broken parts are in fact
obtainable - a 15 year old machine is somewhat likely to not have a
great range of spares.
And 'taking it to a man' is likely to incur a repair bill up towards
£200 or a 'I cant fix that' response.
Yep. When I tried to source a replacement magnetron I found I was
looking at £50+.
If you get the model number of the *magnetron* rather than the microwave
you may well find that a compatible magnetron is still available. When our
44 year old Toshiba microwave had a hiccough and panicked me into looking
for a new magnetron, I found a compatible replacement without too much
trouble (and it wasn’t particularly expensive as far as I can recall).

Tim
--
Please don't feed the trolls
The Natural Philosopher
2025-01-31 20:58:44 UTC
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Post by Tim+
Post by Jeff Layman
Post by The Natural Philosopher
Also be absolutely careful to see if the broken parts are in fact
obtainable - a 15 year old machine is somewhat likely to not have a
great range of spares.
And 'taking it to a man' is likely to incur a repair bill up towards
£200 or a 'I cant fix that' response.
Yep. When I tried to source a replacement magnetron I found I was
looking at £50+.
If you get the model number of the *magnetron* rather than the microwave
I started with that, and then the seller asked me what microwave it was for!

A lot of magnetromss sent back because they 'didn't fit' apparently
Post by Tim+
you may well find that a compatible magnetron is still available. When our
44 year old Toshiba microwave had a hiccough and panicked me into looking
for a new magnetron, I found a compatible replacement without too much
trouble (and it wasn’t particularly expensive as far as I can recall).
I found lower power ones as low as £35 and higher power ones in he £75-£110
area. I think mine was £85 from an Ebay seller. But it was a pukka
branded Samsung unit.
Post by Tim+
Tim
--
"It is an established fact to 97% confidence limits that left wing
conspirators see right wing conspiracies everywhere"
The Natural Philosopher
2025-01-29 21:24:51 UTC
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Post by D.M. Procida
On 28 Jan 2025 at 23:40:43 GMT, "The Natural Philosopher"
Post by The Natural Philosopher
Post by D.M. Procida
My microwave oven, now 15+ years old, blew its fuse yesterday.
It's a slow-blow fuse (T10 AL 250V P).
I'm going to replace it tomorrow, but how likely is it that whatever caused it
to go is just going to do it again straight away.
I am surprised the high voltage fuse didn't blow. That's what happened
to mine - the magnetron had an anode cathode short and had to be
replaced . That and a failure of the high voltage capacitor or the high
voltage diode are really the most common problems, and are trivial to
replace.
<useful advice>
Thanks for all that, interesting and informative. It did indeed blow the fuse
again, with a an angry buzz, so I am just going to take it to a shop - if such
a thing exists any more - and see if someone else can repair it.
Daniele
Simply not worth it, Parts and labour likely to be more than its worth
--
I would rather have questions that cannot be answered...
...than to have answers that cannot be questioned

Richard Feynman
ajh
2025-01-30 11:07:17 UTC
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Post by D.M. Procida
Thanks for all that, interesting and informative. It did indeed blow the fuse
again, with a an angry buzz, so I am just going to take it to a shop - if such
a thing exists any more - and see if someone else can repair it.
our local repair cafe sees quite a few faulty microwaves and fix over half.
Roger Mills
2025-01-29 00:04:49 UTC
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Post by D.M. Procida
My microwave oven, now 15+ years old, blew its fuse yesterday.
It's a slow-blow fuse (T10 AL 250V P).
I'm going to replace it tomorrow, but how likely is it that whatever caused it
to go is just going to do it again straight away?
Daniele
Very likely!

There some micro-switches on the door which are designed to make sure
that the microwave cannot possibly operate with the door open. If the
microswitches fail or are out of adjustment, the whole thing is designed
to create a short which blows the fuse.

So you need to sort out the micro-switches before replacing the fuse.
--
Cheers,
Roger
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