Miele DGM7440 combi oven

1. Does the 230v arrives at the inverter pcb?

2. does the signal arrives at the inverter? You got a schematic from the other topic. The "noload" is possibly a error return.

3. What actualy happens? It starts but end at 10 seconds or so?

4. Are you aware of the hazards of working with high tension (1500v)? It's lethal. But if you are aware:
Detatch the 2 wires at the magnetron. Measure with a HV voltmeter the voltage when you start the magnetron. If you get HV than the magnetron is bad. If not, we have to look at the inverter or driver. Don't come anyway near the oven when you make HV measurements!! Observe from safe distance. At least 1 feet. But better not to use feet to confirm distance

5. For info i need a mail adres. I don't want to put things public. If you also like things private than i suggest you make a temporarily gmail or something to exchange info. Takes up 2 minutes to make. You can also make your email appear in your credentials here. Than it appears as a picture. Not a copypastable txt. But nevertheless, public to anyone so your choice.

6.when the inverter is malfunctioning, there is no guaranty the magnetron is not bad also. It is just not testable with a bad inverter.

7. just curious. What's your location (country) or native language?

8. I hope you have good AI translation because the info is in Dutch.

9. Does the ventilator that is attached to the inverter pcb run when you start the MW?

10. what we find a lot here is bad model numbers .Check your previous question and you find DMG7440. Now it's DGM7440. Lot of difference. That's why we allways ask for a picture of the sticker. It resolves a lot of errors and time wasted.

Thank you very much for this detailed answer!
I have put answers to your points as a comment to each

1. Does the 230v arrives at the inverter pcb?

I will test this later to confirm but there is the power supply fan turning on when microwave is enabled. So I assume the 230 V are there given that the fan seems to be supplied through voltage converter from the 230 mains in

2. does the signal arrives at the inverter? You got a schematic from the other topic. The "noload" is possibly a error return.

Would a noload fault drop an error message on the screen? As currently there are no faults reported no matter how long the thing is running.

3. What actualy happens? It starts but end at 10 seconds or so?

It starts and runs continuously but things don’t heat up. Have tried different powerlevels, same issue. Maybe twice the water got ever so slightly warm but this could be just imagination. I have done all trials with approx 200 ml of water in a cup.

4. Are you aware of the hazards of working with high tension (1500v)? It's lethal. But if you are aware:
Detatch the 2 wires at the magnetron. Measure with a HV voltmeter the voltage when you start the magnetron. If you get HV than the magnetron is bad. If not, we have to look at the inverter or driver. Don't come anyway near the oven when you make HV measurements!! Observe from safe distance. At least 1 feet. But better not to use feet to confirm distance

Aware of hazards but at home I don’t have the necessary equipment. Need to see where I can find something to measure. I guess max output voltage is much higher than 1.5kV?

5. For info i need a mail adres. I don't want to put things public. If you also like things private than i suggest you make a temporarily gmail or something to exchange info. Takes up 2 minutes to make. You can also make your email appear in your credentials here. Than it appears as a picture. Not a copypastable txt. But nevertheless, public to anyone so your choice.

Thank you, I surely understand. i have created gmail adress for this purpose dgm7440issue@gmail.com Thank you!

6.when the inverter is malfunctioning, there is no guaranty the magnetron is not bad also. It is just not testable with a bad inverter.

That makes sense. I also don’t have a history of the device. I got it as used device. That's why I am kind of clueless...

7. just curious. What's your location (country) or native language?

I’m from Germany

8. I hope you have good AI translation because the info is in Dutch.

Yes, for all I couldn’t read I will use that

9. Does the ventilator that is attached to the inverter pcb run when you start the MW?

Yes it does. I suspected the door closed detection as a first issue however if pulling the door slightly while microwave is on, this fan turns off. fan runs stable if door is closed normally so seems not to be the issue

10. what we find a lot here is bad model numbers .Check your previous question and you find DMG7440. Now it's DGM7440. Lot of difference. That's why we allways ask for a picture of the sticker. It resolves a lot of errors and time wasted.

Sorry for the confusion earlier. It is a DGM 7440 OBSW / D. I'm currently not close to the device but can share the label later if needed

I would indeed expect a error. Perhaps the magnetron is faulty. A HV measurment is therefore needed to analyse. With the manual you perhaps can read the error register.

It is crucial you check the right ventilator. I asume there is more than one. Check the one that goes to the inverter. Look for it on the schematic (mailbox (spam?)).

I made my own HV meter. Super cheap and simple. Same warning. Attach meter. Hold distance when measuring. A meter got a 10Mohm internal resistance (dubble check the specs on the used meter). Ad a 100 or 82Mohm HV resistor. It devides by 10 fold. I put it in a electrical pvc tube with tec7 on each side with wires comming out. I will look for the AliExpress link. Black meter lead to chassis. Red via 100Mohm to HV.
The official meter i got is a rebuild tv tube (30kv) meter. I eliminated 2 of the 3 HV resistors so now the range is 10kv. But these things are expensive. You can expect to max 2kv.

Thank you for your quick help and for the documents - received them

I have measured the magnetron the other day. The resistance between the contacts was low, as expected so the heating seems okay. Also the to-case resistance is high so okay when measured with a multimeter. I am not sure how to diagnose more sophisticated internal damage.

I would not expect the inverter to have accurate current measurements. So I guess it could only detect fully open-circuit on the output and since the heating of the magnetron seems still intact this most likely would not trigger.

As another thought: Do you have experience how a damaged stirrer motor for the antenna would show?

I will order some parts later for measuring the high voltage part too! Thanks for the tip!

The antenna:
It does not really make a difference if testing with a glas of water. With food it may heat on some side a bit harder. It does not stop heating if it stops turning. Just not good spreading. So not your specific problem.
Normaly you can unscrew the micromotor and lay it on it's back so you can observe the turning (or not turning) shaft.
Usualy 230vac model so you can test with a power cord. Carefully....

You can visualy inspect the magnetron (get it completely out) to see if one of both magnets are cracked or the antenna has damage (front pin).

The only thing you can measure is if there is HV present. So with 2 wires loose or attached to the magnetron. (HV!!)

What you also can do is measure power. Put a current meter or watt meter in place. Mains. The power should be around 1600watt if magnetron starts

Tip: get a random wire (like 1,5mm2...) Put one side somewere between the chassis (screw between two plates for example). Use the other side to check the HV for charge. Do it when you stop the microwave. Unplug from mains. Wait 10 seconds for natural discharge (inverter so it goes quick). Then touch the HV component with the other end of the wire. The wire is grounded so no need for HV gloves. The residual charge is now 100% gone. Working (unpluggrd from mains) is now safe.

Simple test: If you connect Microwave tube, you listen with screwdriver with metal part to internal weak Iron of transformer and plastic part till your ear, Doe you hear when starting ticking sound or nothing? When ticking it is tube, transfromer or HV rectifier, No ticking sound then inverter is not starting. No alarms on screen of Microvave control panel??

Thank you all guys for your support so far!
Little update: I have checked the state of the door switches they seem fine in the maintenance mode. They are recognised correctly.
There are also no faults in the fault memory at all.
EDIT: Does anyone have access to a list of the error codes? Does it say anything if there is an error code for the no-load fault?

Further I have measured power consumption. During running the microwave at 1 kW setting consumes only 36 Watts. So obviously something is not right here ? As soon as the microwave program is finished the power consumption goes down to 27 W (as the main fan continues running and the power supply (fan) turns off). I could not see if there are any short peeks right in the beginning when it starts up due to the device used for power measurement.

Since the magnetron pin - pin resistance is very low as per my measurement last week, this would mean for me that the Inverter does not start up at all? Or would we expect such low power consumption too for some kind of a funny condition in the magnetron despite it having low impedance on the input?

I am still waiting for my High Voltage Resistors to be delivered... Seems like they only arrive in the new year ?

@smd There are no alarms shown on screen and also non in the memory.
Do you mean doing this test while turning on the microwave?

Only when starting up microwave section, in beginning it gives 3x pulse if no you can measure if PWM signal is arriving at Inverter. It looks like a problem with firmware due some reason because you won't get any alarms, that is a strange trigger for me...

Ok, took the magnetron out. It is obviously damaged. How ever also the inverter seems to have got hit. So I fear there is no point of repairing unless anyone knows a good place to order the power supply?

Ok, start to replace magnetron tube, then check HV diodes around transformer. Sometimes one is shorted duebad tube

I measured the diodes. Either they failed open circuit or they did not fail. They definitely are not shorted. I will need to build a little testing circuit as the forward voltage drop of the diodes is very high and I am not sure if my multimeter is able to apply a high enough voltage to bring them into conduction mode (Vf nominal is around 10V and goes down to somewhat 5V at low current)

I had a company guy coming in who got a magnetron (new Miele original part). But it wasn’t quite the right part number (it was I believe a 2m282, which is somehow listed on the Miele website as the official spare part for dgm7440). However the magnetron which was installed originally is 2m303. He said it wouldn’t matter but I am not quite sure if it is really that easy. He didn’t really have knowledge about electronics..
Even though the inverter is the same, I am not sure if the pwm frequencies and therefor output voltage differences are stored in the firmware according to magnetron model originally used

Nevertheless- it didn’t heat up still so he took his magnetron with him again. He offered full replacement (new magnetron + inverter) for an unreasonable amount of money.

Ok. So test of magnetron did not help? Then Inverter is broken also. Best way to start to measure IGBTs, and are all voltages present around driver chip,measure with scope the signals if they are arriving at IGBTs

The magnetron fits in a duct/tunnel. If there is debris in that tunnel from the magnetron radiator/antenna than a new working magnetron (and inverter) can be damaged also. So clean the tunnel very thorough.

The service info indeed states a 2M282. No info if it is compatible with 2M303. Normally Miele info is correct. To be shore you need to contact Miele.

Perhaps you can find a 282 online?