Miele combi microwave – H6400BM – no heating action

[twicknix]

This was an interesting one here, blooming heavy that microwave oven was!

Customer reported that no microwave action happening, it is touch control unit.

Checked capacitor which is all good. Not done the diodes – in hindsight, I should have done but anyhow…

check magnetron, some discolouration and unsure if it is meant to be black inside the probe? Continuity is good on the magnetron,

checked transformer, had me slightly confused – lack of experience on my part. Both high voltage and low voltage reading were low as expected from what I gathered – high voltage was 0.1 ohm on resistance, the low voltage was similar. The red lead to ground was 86 ohms on resistance and I unsure if this should be way higher?

since it’s a Miele, I am apphrensive on ordering a new magnetron which is a lot of money, I needed to be sure.

What could be the issue here?

[electrofix]

ok you dont have to order a miele mag

i use magsells
https://magsells.co.uk/

ring them up as online prices are higher as most common mags are under £50 delivered

ok did you check mag to earth ?

did you check for bad contacts on the heater circuit ?

Dave

[twicknix]

electrofix wrote:ok you dont have to order a miele mag

i use magsells
https://magsells.co.uk/

ring them up as online prices are higher as most common mags are under £50 delivered

ok did you check mag to earth ?

did you check for bad contacts on the heater circuit ?

Dave

Re: Mag to earth – no I didn’t for the reason being it is hard wired that I wasn’t able to do this in situ. I guess I can revisit and check again to be sure especially with contacts on heaters and earth leakage.

[electrofix]

ok how much do you know about how the sytem works

do you understand the heater and the high voltage circuit and how it operates along with the safety proceedures to be able to work safely ?

Dave

[twicknix]

electrofix wrote: ok how much do you know about how the sytem works

do you understand the heater and the high voltage circuit and how it operates along with the safety proceedures to be able to work safely ?

Dave

I can work with the safety procedures to work safely. I am fully aware of it. I am aware of the heating elements on the microwave and didn’t think it is directly related to non working microwave.

So what I have done so far on the microwave, I have tested the internal fuse, capacitor, transformer (though I am confused on the result), magnetron did show signs of overheating due to discolouration. I would have posted the image up here but it would not let me due to the file size being too large.

i do know from AEG combi microwave that power module fails due to bad earth leak on heating elements. I would be surprised to see this on a Miele. I am more than happy to reeducated. Thank you for asking to check.

[electrofix]

ok your getting confused i think

a magnetron works on the same principles of the thermionic radio valves from the 50’s etc. There is a heated Cathode which emits electrons and these electrons are attracted to the high voltage anode. To make it work the transformer has 2 windings, A high voltage winding which is about 2500v AC and a low votage winding which is 12volts Ac or so. The low voltage winding connects to the 2 main terminals of the mag. it has a higher current so any bad joints in the wires or connectors will stop the cathode heating and the mag wont work. Have had bad end connectors, burnt out end connectors, bad connection between the wire and connector. If its properly made these connectors should be crimped and then soldered as the wire is single core so crimps can fail

the high voltage winding is different. one end of the winding connects to the metal chassis (earth) the other end of the winding connects to one side of the capacitor. On the the other side of the capacitor a high voltage diode connect to earth. This same connector connects to the filament anode connection of the magnetron. It works like this, An AC supply goes from zero to full +ve voltage then to zero and the full -ve voltage and back to zero 50 times a second on our supply. Starting at zero the voltage climbs and the current flows from the secondary winding through the capacitor and doide and back to the transformer winding via earth. This charges the capacitor. then the voltage changes direction. In this direction the diode blocks the current flow so the voltage which is in the capacitor adds to the transformer output. this doubles the voltage and changes it from AC to DC

see if that makes it clearer or not lol

Dave

[twicknix]

Thank you for taking the time to go through the steps. Good to be refreshed, still I would like numbers on readings on which part on the transformer.

I will contact customer and start again and test everything.

I have contacted Magsells and they have a transformer for me should I require it. All of this rest on numbers…

Matt

[electrofix]

disconnect mag

you should get very low ohm reading on the 2 terminals both across the wires you removed and the mag thats the heater circuit

hig voltage output lead from the transformer. you should get a reading from there to earth. reading does not matter unless its meg ohms

across the cap you should see the display count down or up but it should not be short

has it got a high voltage fuse ?

if yes and its ok you can rule out shorts on the high voltage side

Dave

[baron]

twicknix wrote:Customer reported that no microwave action happening

Do 220V appear on transformer? Can you select other program without microwave function? Does he works?
First of all check in settings that isn’t in “DEMO” inside “Settings – Showroom programme” …set NO

[baron]

I mean 220v on primary widing…

[twicknix]

electrofix wrote: disconnect mag

you should get very low ohm reading on the 2 terminals both across the wires you removed and the mag thats the heater circuit

hig voltage output lead from the transformer. you should get a reading from there to earth. reading does not matter unless its meg ohms

across the cap you should see the display count down or up but it should not be short

has it got a high voltage fuse ?

if yes and its ok you can rule out shorts on the high voltage side

Dave

Thanks Dave, feeling more empowered now… just few more things…

The primary and secondary leads obviously two goes out and two goes in, both wires should have low continuity resistance?

The high voltage output lead from transformer to earth, I presume you meant earth leak test?

The capacitor is easy as I have the device that test it very quickly, it’s good reading.

With having understood, all this to be completely disconnected prior to testing so that I don’t fry myself and the oven.

[twicknix]

baron wrote:
Do 220V appear on transformer? Can you select other program without microwave function? Does he works?
First of all check in settings that isn’t in “DEMO” inside “Settings – Showroom programme” …set NO

I am not aware of demo setting, but that is good to be aware of. As far I am aware that the oven works but no action on microwave. Says that you press start, nothing happens. I have a feeling that it is more likely to be either internal fuse or magnetron or relay switches on the board.

[twicknix]

Oh, will double check the diodes with the battery and multimeter.

[baron]

twicknix wrote:

Says that you press start, nothing happens. I have a feeling that it is more likely to be either internal fuse or magnetron or relay switches on the board.

But, what’s happen with time? did he starts counting down without activating transformer? Do appear sounds of transformer?
Connect a lamp instead transformer and see if he’s lighting… If yes you have on High-voltage part problem if not than PCB(relay) or thermostat or door switches…

[electrofix]

if it does nothing just double check all the door microswitches are activating

Dave

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