Panasonic Microwave Oven Repair (Comes With Video)
This microwave is our personal unit & so repairing it quickly was a priority if I wanted to stay in my wife’s good books!
She had explained that during cooking, the food was not getting hot and there was a strange smell. It appeared that most of the functions were working, but heat was not one of them. I carried out the usual test – a glass of water put on high for one minute. Sure enough, my wife was correct (not that I doubted her…..) the water remained cold.
Some other microwaves that I have repaired in the past have used a high voltage linear transformer; however this unit uses an inverter.
The component that creates the microwave is a magnetron. Unfortunately, in the case of a linear transformer, it is either on or off and this is where earlier microwave ovens struggled to efficiently cook a meal. Even when set to ‘50% power’, the microwave oven is actually going through cycles of delivering 100% power, followed by a period of no power.
With an inverter, cooking is able be more thorough due to variable frequency control over the magnetron.
Difference Between Traditional and Inverter Microwave Ovens
Traditional microwave ovens
Conventional microwave ovens use linear power transformers to increase the line voltage (240 VAC at 60 Hz) to a level high enough to operate the magnetron. The magnetron generates the microwaves that cook the food. Unfortunately, operating at a low frequency of 60 Hz, the transformer is relatively inefficient:
- Power is lost (through heat dissipation) in changing the line voltage to the higher required magnetron voltage.
- The transformer operates at a constant power that can only be changed by switching the power on or off constantly.
Inverter Technology
In inverter equipped microwave ovens, the linear transformer is replaced by electronic circuitry, which changes the 60Hz line frequency to a variable rate of 20 KHz to 45 KHz. Only a reasonably small transformer is needed to provide the high voltage required by the magnetron. By changing the pulse width, the output power can be controlled to provide better cooking, retaining nutrients within the food and more even temperature control.
On With The Job…
These Panasonic microwave ovens suffer from some common faults, including door switches and faulty magnetrons.
But as usual, don’t guess – test!
The Panasonic Genius has software that allows fault code storage. The following process will allow access to the fault codes.
Self-Diagnostic Display
To activate the mode, the following steps must be taken:
- TEST MODE – Plug in oven, press Timer/Clock twice, press Start once, Micro Power once
- Keep pressing Timer/Clock for more than 2 seconds until the buzzer sounds
- Press Start twice
- Error codes will be displayed
Note: Up to 3 error codes are kept in memory. To scroll through codes, press Start.
However, in my case, no codes were stored. These are only used as a guide and parts should not be replaced based solely on the codes.
First things first – a good visual should be taken of the entire appliance. No fuse was blown and the unit would start up straight away after shutting down after about 20 seconds. This is a designed protection system built within the circuitry.
Please be aware that these transformers can provide the magnetron with up to 4,000 Volts! Safety first! Before carrying out any testing with the power turned off, ensure that the main filter capacitors have been drained.
It was noted that one of the door switches had burn marks on the terminal.
After carrying out a visual check and testing it was decided that this was not of concern.
An example of a burnt door switch is shown below.
Continuing with the visual inspection, it was noted that the plastic surrounding the magnetron had been extremely hot, melting in several areas. This was clear evidence that the magnetron had been very hot at some stage.
As already mentioned, these transformers can supply up to 4,000 Volts, which is well beyond the testing limit of an average Multimeter. A high voltage probe must be used if any voltage testing is to be done. But…..there is an alternative. Testing for current in the system can provide the diagnostic path we need to isolate the fault. This can be done following the steps below:
Procedure to test the Inverter using an Ammeter
- Place a glass of water into the microwave oven.
- Disconnect the 2 wires that connect from the inverter to the magnetron.
- Set the oven at high power for 1 minute and press start. The microwave oven will operate for approximately 15 seconds and then stop, showing the error code H98. During the operating time the AC line current should be between 1A and 1.7A.
- Unplug the 3 pin connector from the inverter to the display control board.
- Set the oven at high power for 1 minute and press start. The microwave oven will operate for approximately 27 seconds and then stop, showing the error code H97. During the operating time the AC line current should be between 0.4A and 0.8A.
But due to the obvious signs seen on the magnetron, I felt that this should be a basic starting point. Remember, all other operations of the oven worked, including display, turntable rotation, fan and other functions.
Testing the Magnetron is probably one of the easiest tests to do on the oven. Firstly testing the filament, (not under load) with an Ohmmeter should read 1 Ohm or less. This tested fine.
Secondly, it is important to make sure there are no shorts to ground – a shorter path for the current to take. Testing between the filament terminals and the body should show ‘OL’ on the Multimter, or no continuity. In this case it showed only 121.1 Ohms of resistance – very dangerous!
Clearly, this is where the fault lay. Just to prove my theory, I tested a spare magnetron that I have and no continuity was shown on the good unit.
It was a simple task of replacing the Magnetron and carrying out the ‘glass of water for 1 minute’ test and this showed success in the repair.
However, days after, the Magnetron sat on my desk, eyeing me, begging me to pull it apart to see what had gone wrong. Finally I could take it no more and decided to carefully pull it apart for inspection.
Caution:
Care must be taken to avoid harming the pink insulators on the tube as they could be made of beryllium oxide, which is hazardous if inhaled as dust from a broken insulator.
Removing the frame and cooling fins allowed access to the filament core. After cutting the terminal connections at the filter end, the damage became obvious. A burn mark showed where the filament was shorting to ground.
Needless to say, even though my wife wasn’t as excited to see the filament damage, I was once again back in the good books with a useful, repaired microwave oven!
If you are interested in this repair, you can see a video on my channel following the link below:
This article was prepared for you by Mark Rabone from Australia.
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Note: You can check out his previous repair article below:
https://jestineyong.com/citroen-fan-resistor-module-repair/
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Parasuraman
January 29, 2018 at 9:09 pm
Oh my God! So much in a microwave oven! News to me! Many thanks for sharing your valuable experience!
Parasuraman
January 29, 2018 at 9:15 pm
Video is too good and enjoyed your professional presentation. Hilarious too!
Mark
January 30, 2018 at 5:30 am
Thanks Parasuraman,
I have been repairing microwave ovens for a number of years now, but back when I started, had no idea what actually happens inside. I have learned a lot about not only the workings of the microwave, but also the history - fascinating!
Thanks for your support 🙂
Gary Gemmell
February 5, 2018 at 6:39 am
So have I ... NOW!!!
Cheers Max always interesting.
Henrique Jorge Guimarães Ulbrich
January 29, 2018 at 11:17 pm
For those interested in best knowing the inverter technology applied to microwave ovens, I´ve found an interesting publication from Panasonic at http://educypedia.karadimov.info/library/Inverter.pdf
Happy reading!
Mark
January 30, 2018 at 1:17 pm
Hey Henrique,
Thanks for sharing an excellent article!
Henrique Jorge Guimarães Ulbrich
January 29, 2018 at 11:20 pm
Very good, Mark. Congratulations for your sense of humor. And by the great video too.
Mark
January 30, 2018 at 1:19 pm
Hmmmm....... not sure my wife would call it a sense of humour!
Suranga Electronics
January 30, 2018 at 1:16 am
Thanks Mark-good article!
Albert
January 30, 2018 at 7:58 am
I liked your repair instructions very much since this was a completely new kind of microwave oven to me. I only knew much about fixing the linear transformer types. Don't know though if measuring the AC input current thing and the Panasonic fault messages will be of much help on fixing other manufactured ovens.
And I surely appreciate your warning to watch out for inhaling any beryllium oxide. (The same kind of dangerous material can be found in HF/RF transistors!).
Mark
January 30, 2018 at 1:22 pm
Thanks Albert,
I haven't come across one that has successfully given correct fault codes. I guess it isn't yet up to the standard that the automotive industry has with regard to fault codes - that's mainly what I deal with.
Jim Whalen
January 30, 2018 at 8:28 am
Super nice video thanks.
Where did you ever find the schematics for the microwave?
Mark
January 30, 2018 at 1:29 pm
Hey Jim,
I couldn't find a schematic for this particular microwave, but there are quite a few on the net that deal with Panasonic microwaves in general.
That being said, Henrique has given us a link in his comments above to an excellent article similar to what I used.
http://educypedia.karadimov.info/library/Inverter.pdf
Yogesh Panchal
January 30, 2018 at 1:09 pm
Mark,
Thanks for wonderful! article.
Mark
January 30, 2018 at 1:30 pm
Thanks Yogesh
Mihai
January 30, 2018 at 1:29 pm
Thank you for the article and video!
Keep the good work !
Mark
January 30, 2018 at 3:44 pm
Hey Mihai,
Thanks for the feedback - I especially enjoy making the videos 🙂
But I get so much from the articles on Jestine's blog - there is so much experience to learn from!
Robert Calk
January 30, 2018 at 7:57 pm
Good job Mark. Very entertaining! I hope that pesky gecko never bites you.
Jumbe mateyu
February 4, 2018 at 2:49 am
Very educative article. I have learnt a lot from this article. Thanks so much for sharing.
Muhammad Ashfaq
February 14, 2018 at 3:35 pm
Hallow . sir its a very very remarkable video for invertor type microwave repairing
thanks a lot.
wency gonsalves
August 26, 2020 at 9:00 pm
My Panasonic Microwave 44Litre oven NN-S754WF MODEL
Touch pad "START" NOT working/rest all is good. can you suggest how i can solve this.
Ryan Reynolds
March 5, 2023 at 3:47 am
Great article.
I will add that the other potential hazard within the magnetron is the thoriated filament (thorium is radioactive). It takes work to get direct access to the filament, but if the back insulation tube breaks you would be looking at the filament. Not a worry otherwise. Some scrap yards here in the states do not accept microwave ovens because of this small radioactive element. Probably because it sets off the radiation detectors they are required to have monitoring incoming scrap. A buddy brought a flatbed trailer of microwave ovens and they said never do that again.