Panasonic Microwave Oven Not Heating- HV Diode Faulty
This is my personal Panasonic microwave oven model: NN-C2000P which my wife complained that it is not heating on microwave mode. This model has dual mode of heating i.e with the conventional heating element combine with microwave heating. I tested the oven on conventional heating and it is working fine. All the front panel buttons were able to switch on as normal but on microwave mode it is not heating the food. This functional test confined that the main control board (including the front panel switches) and the safety features or switches for the door are functioning normal.
I suspect the problems could be either the faulty magnetron or the high voltage inverter board .I started isolating the fault by removing the connector CN702 from the inverter board and measure the voltage with the microwave function switched on (photo 1). There is a drive voltage of 240Vac which re-confirmed that the control panel and the door safety switches are normal. With the same setup i.e with the multi-meter probes still connected to CN702, you can also test the door continuity switches by switching to ohm range. When the door is closed the resistance is infinity and when open it is zero.
Next, I check the magnetron since it is very much easier to trouble shoot than the HV inverter board. For safety reason, the capacitors around the HV rectifiers must be discharged before unplugging the magnetron filament connector (CN703). Using the ohm range of multi-meter, the filament of the magnetron measured to be normal i.e less than 1 ohm (photo 3). You can also check for insulation resistance between the magnetron casing and filament for leakage and it should show infinity (photo 4).
Since the magnetron does not show any defect, next I focus my attention on the high voltage inverter output stage which consists of control board, HV capacitors and rectifiers (UX-C2B). The HV inverter board need to remove from the microwave oven unit for easier access and repair. One of the HV rectifier was test to be shorted (photo 5) and the other showed infinity resistance in the forward and reverse biased testing. The voltage on the multi-meter output lead is not high enough to turn on the HV rectifier due to the higher forward voltage of around 2 voltage. In order to test this HV rectifier , I connect a battery with voltage of around 7.2V in series with it(Photo 6). When the rectifier is turn on there is a voltage drop of 1.7 v across it.
Even though this HV rectifier was tested to be good, I order 2 units just in case it fail when high voltage is applied to it. I feel that is is not necessary to proceed with further checking of the components since there is no sign of catastrophic failure. Microwave oven calibration is not an easy task and moreover, it can only be done with expensive and sophisticated equipment so it is not recommended to mess around the inverter control board if can be avoidable.
The inverter board was re-installed back with a new HV rectifier and ready for testing. The easiest way to test is to place a glass of water on the rotating glass plate of the microwave oven and switch it on. After one minute the water in the glass is warm indicating the repair is successful.
This article was prepared for you by SK Wong from Malaysia and he graduated from London with a degree in Electronics and Electrical Engineering. He has more than 40 years of experience in the consumer product manufacturing. With the passion to repair electronic products from young , he has been doing it part time until now.
Currently, he can provide turnkey solutions for consumer products including services from idea or concept to design , prototyping , finished products (including electronic,mechanical and plastic parts) and manufacturing.
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You can also check on Parasuraman’s article on LG Microwave Oven Not Heating Problem:
https://jestineyong.com/how-to-repair-microwave-oven-not-heating/
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Yogesh Panchal
October 5, 2016 at 2:29 pm
Good Job Sir,
Thanks for sharing.
Donald
October 5, 2016 at 3:53 pm
Interestingly
Albert van Bemmelen
October 5, 2016 at 5:40 pm
Very nice presented solution on how to test the HV Diode(s) Mr.SK Wong!
The HV transformer in your article though seems to be a totally different model as the ones that are used in our European Microwave Ovens. There are also not always 2 Diodes present in EU Microwave Ovens. Probably the second Diode is used as over-voltage safety protection Diode.
skwong
October 6, 2016 at 10:51 am
The high voltage circuitry after the HV transformer is a HV doubler circuitry. The working principle is the same with some variation in the design from different manufacturers.
Albert van Bemmelen
October 8, 2016 at 2:53 am
Okay, that probably also explains the different looking Transformer that looks more like a TV Transformer. Our EU Transformers produce about 4KV without any Doubler circuit. I thank you for the information.
Albert van Bemmelen
October 8, 2016 at 3:00 am
PS: according to an answer I got from Paris Azis, it is possible to make a dirtcheap AC isolation Transformer with 2 disgarded Microwave Transformers. By simply connecting the HV outputs together. Which creates a very cheap High Power safety Transformer for testing 110/230 Volt AC powered devices. I haven't tested this yet. Because I now only have one Transformer.
Albert van Bemmelen
October 8, 2016 at 11:49 pm
I recently got an confirming answer from Paris Azis (our Transformer Expert!) involving the idea of placing two of these European High Power 4KV Microwave Transformers in anti-serie connected HV to HV, which gives us a very cheap high power Safety Transformer 110V/230V AC in- and output. Ideal for experiments on the to the AC Mains connected Devices. Of course with the HV Fuses still connected too. And I was however told not to use the 10 Amps low voltage Microwave Filament coils.
I haven't tested this yet because I now only have one Big Transformer. But that is why I keep any disgarded Microwave Transformer I do get. These Safety Transformers aren't exactly cheap normally and after all Safety above all ! And the 1200 Watt, or so, of Power makes a great Tool for our electronics Lab when this works.
Parasuraman
October 5, 2016 at 11:39 pm
Great, as usual! The mixture of thorough knowledge of theory, backed by year long dedicated and devoted experiences are your assets!
Bulent NUR
October 6, 2016 at 12:19 am
Good job, thanks for share, I seen first time inverter in microvawe oven!
Robert Calk
October 6, 2016 at 12:40 am
Good job, SK. Thanks for the article.
Ulises Aguilar Pazzani
October 6, 2016 at 10:42 am
great experiences tech job Mr .SK
Humberto
October 7, 2016 at 1:00 am
Good article. We always learn with every article posted in Mr. Jestine Yong's blog.
Andre Gopee
October 7, 2016 at 9:20 pm
Excellent job Sir. Keep up the good work.
Ruben
December 9, 2016 at 11:29 pm
hi, why should exactly to put 9 volt bat.in a series to turn on the hv rectifier ? , it 's my first time to watch the trouble-shooting of a micro oven , well done ! thank you for the article. w/ best Ruben