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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Note: You can check his previous repair articles below:
You can also check on Parasuraman’s article on LG Microwave Oven Not Heating Problem: