Combination microwave/oven repair

The microwave part of our oven has stopped working, though the electric oven and grill work normally.
Will the magnetron or its control board be likely to have fuse separate from the main fuse? Or is this most likely a failed magnetron?
Thanks,
Daniele
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On 06/01/2019 14:05, D.M. Procida wrote:

I'm sorry I can't help you - but those who can, can better if you give a make and model number.
I just wanted to mention that if you saw the Doctor Who New Year Special last week THESE ARE NOT OFFICIAL DISASSEMBLY INSTRUCTIONS FOR A COMBINATION MICROWAVE/OVEN.
Nick
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I somehow forgot to mention: it's a Siemens Microlle Plus. The full part number is HF83950NL, though I suspect HF839 is the basic model number.
Daniele
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On 06/01/2019 16:18, D.M. Procida wrote:

Going by the price of parts shown here :-
https://www.ransomspares.co.uk/parts/brands/siemens/microwaves/hf83950nl__02/
You might be better off getting a replacement.
Ominously, most of the hits on Google are in Dutch. Was this ever an official UK import ?.
Magnetron shown if you then select Images.
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I think the equivalent is HF83951GB for the UK, and that the basic model number is HF8395.
Daniele
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On Sun, 6 Jan 2019 17:59:23 +0100, snipped-for-privacy@apple-juice.co.uk (D.M. Procida) wrote:

Be aware that there are some seriously high and lethal voltages inside the works of a microwave, that persist for a good while after it's been switched off. Unless you really know what you're doing, don't attempt a repair yourself.
--

Chris

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On Sun, 6 Jan 2019 15:05:46 +0100, snipped-for-privacy@apple-juice.co.uk (D.M. Procida) wrote:

It may have its own fuse so check that first. Some of the Dutch articles mention this model has two fuses. One at least will probably be a slow blow type to cope with the high current draw when it switches on. Failure of fuses for no other reason than stress is fairly common so if one has failed just replace it. If the replacement blows then the problem is obviously elsewhere.
Is the turntable working when the microwave part should be on?
Magnetrons rarely fail but the High Voltage diodes and the large high voltage capacitor sometimes do. Fortunately they are fairly easy to test. Because the "diode" is actually several integrated into one package in series a normal test meter diode test function won't do. There are several Youtube videos showing how to do it with a test meter and 9V battery.

https://www.youtube.com/watch?v
lrKf2PdVA and
https://www.youtube.com/watch?v=PdwbjNCK-xw
explain it all.
The HV capacitors also fail,

https://www.youtube.com/watch?v_OC3rDy8Rs

explains how to test them.
Do remember that the capacitor potentially stores a lethal high voltage (about 5,000V) high current charge so do make sure it is discharged by leaving the microwave unplugged for a day or three before prodding inside. The capacitor has a bleed resistor across it to discharge the capacitor when the oven is off but if it fails you have no indication in normal use that it has done so so its best to assume it has.
After leaving the oven unplugged take an insulated length of wire with bare ends and touch the ends across the capacitor terminals. You should have nothing happen but if the capacitor is charged the results are impressive. Having shorted out the capacitor check it is empty by using a multi meter on its highest DC range. Don't do this before shorting out the capacitor.
If it is an inverter microwave then the very heavy high voltage transformer (which rarely fails) is replaced by a switch mode power supply which can also fail. Because of the high energy bits inside failures often leave visual clues such a soot marks so a detailed visual inspection is always a first action.
There is no DIY way of testing the magnetron.
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On Sunday, 6 January 2019 18:40:08 UTC, Peter Parry wrote:

Failed magnetron is an unlikely cause of such a fault. You could help yourself by describing the symptoms exactly.
NT
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Well, it's of the type that has no turntable, but there's no sign of life from it whatsoever. The timer doesn't count down, for example, and no light comes on.
Daniele
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D.M. Procida was thinking very hard :

Likely the magnetron and etc. are fine, the issue is the control PCB and maybe just a blown fuse on that PCB - a simple fix.
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On Sunday, 6 January 2019 19:56:12 UTC, D.M. Procida wrote:

Fuse would be the first suspect. Do use the right type, and /always/ check the big shorting resistor when replacing the fuse. If that's faulty, check your interlock switches are working correctly.
'Dead' is usually the easiest fault to fix.
NT
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On 06/01/2019 14:05, D.M. Procida wrote:

There's probably a 10A slow-blow fuse hidden inside somewhere. If that's blown, replacing it may fix the problem - but it's more likely that it's blown due to something else.
Once common cause of failure is the microswitches on the door. These are to prevent the microwave from operating unless the door is fully closed. These are wired in a way which causes the fuse to blow if one of them fails.
--
Cheers,
Roger
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In this case, the cause should be found and fixed. Worn door mounting and catches are common. The manufacturers will also always replace the microswitches once they have shorted out the supply to blow the supply fuse. Worn microswitches can be the cause, but they are not designed to continue providing protection after they've shorted out the supply once.
The high voltage fuse can simply blow with age - they are very closely rated to the running current, as the transformer is not capable of providing very significantly more than this when shorted, so a wide fuse margin would not provide suitable protection. The symptoms described don't match the HV fuse as having blown though - the timer/controller would still work in this case.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
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More likely a failed power supply. Its pretty crude normally. a large transformer, some rectification block and some hv capacitors. The problem is its a very dangerous area to work on for many reasons as a day thing. Thousands of volts even when off and microwave leakage when on, and really hard to get details of the circuit, at least back in the days of yore. Brian
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On Monday, 7 January 2019 08:19:45 UTC, Brian Gaff wrote:

Nearly all microwaves use the exact same HV circuit. The diode is the most failure prone part of it, cheap & easy to replace. But the OP doesn't really want assistance as they've not provided the symptoms.
NT
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snipped-for-privacy@gmail.com formulated on Tuesday :

Actually, he has. He said something like 'no response from display, no timer running'.
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On Tuesday, 8 January 2019 15:30:58 UTC, Harry Bloomfield wrote:

you're right. Then the most likely culprit is the power fuse. Which if blown requires a check on the shorting resistor & interlock system. For an electronic controller there may also be a fuse or fusible resistor on the PCB, or a blown dropper cap.
NT
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Thanks everyone for the tips.
There were a pair of fuses on the control panel, but both had continuity.
I also found a large thermal fuse, which hasn't, so I suspect that's my culprit, pictured in the image below between a pair of hefty 5K resistors.
<https://share.icloud.com/photos/0jZKqbOP5ZakhfAc2pvt8Hk_Q
I'm not entirely sure how to read its specifications.
It's labelled:
Z430KK6E E065FF51 UK T180
and pressed into the ceramic:
Z42/43 6/250 T250
(also at <https://share.icloud.com/photos/0WjYbgCrmYFfjZrPRQvzm0fpw ).
Daniele
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On Tuesday, 8 January 2019 19:06:41 UTC, D.M. Procida wrote:

6A 250v. Maybe 180C, can't be sure. Are the resistors it monitors ok?
NT
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They seem to be. However the only way I can check continuity on anything at the moment is to press it to the contact of an electrical screwdriver (I know, I know) stuck in a socket, as I am without my usual tools.
Daniele
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