Microwave -Can not identify Burning Resistor in Switch mode power supply section

[nhekborin]

Hi, I have a basic electronic skill.
My microwave oven is not working, no display on the screen. fuse is ok, but I found a resistor is burning in power supply section on the board.
if you anyone can help to identify what is the value of this resistor or is there other shorted components?

[fzabkar]

I would suspect the Viper 12A (?) IC.

https://www.st.com/resource/en/datasheet/viper12a-e.pdf

What is the model number of the microwave oven?

[pepona]

Hi, this resistor is in parallel with L1 wich are part of the Pi-Filter together with C20 and C21.
The value of R38 is about 3900R. 
Most of the times this resistor burn out because due to the dry of C20 and C21 the PWM controller fails and is in dead short.
Also must check de input diodes.
The pcb looks so ugly, with a lot of sulfate.
Has the fuse blown?

Excuse for my bad English

Regards
Marcos.

[nhekborin]

Yes, that chip is Viper12A KN8026
it is Russell Hobbs Model: RMW232DG.
It happens when I turn on and run a few seconds then it smells like something burning suddenly lost power. then I unplug the power cable and I found the cable is eaten by mice. I think this short cause by the power cable makes connection with the case. I already check the fuse is ok.

[nhekborin]

Hi, this resistor is in parallel with L1 wich are part of the Pi-Filter together with C20 and C21.
The value of R38 is about 3900R. 
Most of the times this resistor burn out because due to the dry of C20 and C21 the PWM controller fails and is in dead short.
Also must check de input diodes.
The pcb looks so ugly, with a lot of sulfate.
Has the fuse blown?

Excuse for my bad English

Regards
Marcos.

The fuse is fine.
It happens when I turn on and run a few seconds then it smells like something burning suddenly lost power. I found the cable is eaten by mice. I think this short cause by the power cable makes connection with the case. is it possible? please see the picture in my reply.

[FreddieChopin]

Yes, that chip is Viper12A KN8026
it is Russell Hobbs Model: RMW232DG.
It happens when I turn on and run a few seconds then it smells like something burning suddenly lost power. then I unplug the power cable and I found the cable is eaten by mice. I think this short cause by the power cable makes connection with the case. I already check the fuse is ok.

You are living in UK right? (Guess because of plug). If mouse is problem then you can legally hunt plink them with airgun and you'll have no mouse problem anymore.

[wraper]

Resistor seem to be connected in parallel to inductor L1 between smoothing caps. I think source of fault is inductor failing open (like wire breaking off from a terminal) and all current flowing through this tiny resistor burning it. Check resistance of inductor, should be below 1 ohm, otherwise faulty.
According to schematic in app note resistor should be around 1.5k. But it's not critical, should work even without it at all. Could be that resistor is not even electrically faulty, just paint burned off. Burning smell when powered off suggests it might be the case. In that case you can just desolder it and measure resistance.
https://www.st.com/content/ccc/resource/technical/document/application_note/97/e1/cc/b6/e7/8f/44/4d/CD00077926.pdf/files/CD00077926.pdf/jcr:content/translations/en.CD00077926.pdf
Also there is corrosion on some parts, you should clean it.

[wraper]

Hi, this resistor is in parallel with L1 wich are part of the Pi-Filter together with C20 and C21.
The value of R38 is about 3900R. 
Most of the times this resistor burn out because due to the dry of C20 and C21 the PWM controller fails and is in dead short.

Among what you mentioned the only thing which can actually cause resistor burning is dead C21. In any other case such as VIPER12A failing short, it will survive as long as L1 does not fail open because it will take most of the DC and low frequency AC current.

[pepona]

Hi wraper, your are correct, i didn't know that the fuse and the Viper12 was ok.
I think the problem could be L1.

[nhekborin]

Resistor seem to be connected in parallel to inductor L1 between smoothing caps. I think source of fault is inductor failing open (like wire breaking off from a terminal) and all current flowing through this tiny resistor burning it. Check resistance of inductor, should be below 1 ohm, otherwise faulty.
According to schematic in app note resistor should be around 1.5k. But it's not critical, should work even without it at all. Could be that resistor is not even electrically faulty, just paint burned off. Burning smell when powered off suggests it might be the case. In that case you can just desolder it and measure resistance.
https://www.st.com/content/ccc/resource/technical/document/application_note/97/e1/cc/b6/e7/8f/44/4d/CD00077926.pdf/files/CD00077926.pdf/jcr:content/translations/en.CD00077926.pdf
Also there is corrosion on some parts, you should clean it.
(Attachment Link)

The inductor is about 12ohm, any suggestion for the next step?
Thanks you

[wraper]

The inductor is about 12ohm, any suggestion for the next step?

I guess it's wound with very thin wire if shorting leads of multimeter shows resistance close to zero. It any case 12 ohm is enough to route away almost all of DC current from that resistor. I assume C21 is faulty then. If you don't have replacement, you can exchange C20 and C21 and see what happens. If it starts working, order new capacitors.

[fzabkar]

Inductors often develop dry solder joints. I always made it a practice to reflow all the magnetic parts in anything I repaired.

[nhekborin]

The inductor is about 12ohm, any suggestion for the next step?

I guess it's wound with very thin wire if shorting leads of multimeter shows resistance close to zero. It any case 12 ohm is enough to route away almost all of DC current from that resistor. I assume C21 is faulty then. If you don't have replacement, you can exchange C20 and C21 and see what happens. If it starts working, order new capacitors.

Sorry for long holiday in my country, now I am back.
Yes, it is wound with a very thin wire. So I temporary soldering back the inductor. I replace both C20 and C21 because the old caps have liquid damage on each side.

I replace the burning resistor with new 1.5k. Every things good Then i turn on the power for testing but there is no sign of voltage.

after measuring with the multimeter I inspect the resistor(R5) skin is crack. it is 5 band resistor. I send it to an electronic shop they send me a back 38ohm resistor they said it can replace.

Then after i replace with 38ohm resistor and power on, boom! the new R5 is exploding.
so what next?

[pepona]

Hi, please  see Reply#2. Your pwm controller is probably dead.

[nhekborin]

Thanks for all support, I finally fixed it by disconnect the broken power supply module then attach with 12v 0.5Am adaptor to the board.
One thing that I am not sure about is How many Am which the old power supply module product? Is it enough Am(0.5am) to run this microwave's control board stability?