Repair of a Panasonic Inverter Microwave

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kb0nes

kb0nes

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Dec 11, 2009
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Location
Burnsville, MN
My girlfriends parents 10 year old Panasonic microwave quit working a while back. It emitted a noxious cloud of smoke and no longer continued to heat. While this isn't a classic microwave in any way, it is still much better built then the typical low cost junk of today. I figured I'd at least take a peek under the hood before junking it.

For the record it is a NN-T990 Inverter design from about 2000.

kb0nes++11-27-2012-01-36-20.jpg
 
Inverter board

This is the inverter printed circuit board. It takes in 120v mains AC, rectifies it into DC, then chops that at ~36Khz into the large transformer in the center of the board. The two square diodes on the right side rectify the high frequency AC and voltage double it. HV output is ~4000v DC at 300ma! There is also a smaller filament coil to supply current to the magnetron filament.

The heatsink in the back cools the line voltage diode bridge, and two IGBT (Insulated Gate Bipolar Transistors)

In the lower left standing vertically is the control and monitoring sub PCB. The white connector in front of that supplies a variable pulse signal from the front panel control board to vary inverter output power and therefore reducing RF cook power. The small black IC's are optical isolators that save the front control board if the Inverter destroys itself!

As a word of warning, 4 kilovolts at 300ma is scary power! This can and will kill you dead if you get it wrong. Panasonic officially suggests against ANY repair of the inverter board. Don't do what I did!! ;)

kb0nes++11-27-2012-01-41-8.jpg
 
Well here is your problem...

Not a lot of trouble shooting skill needed here!

The smaller IGBT has totally blown off a leg and all the magic smoke has escaped from the package. This may have something to do with the reduced power!

For the record the smaller transistor is the one in series with the transformer primary that chops the input current. The larger one in the middle is some form of a "freewheeling" control that is actually across the primary of the transformer. Later inverter models have since lost this transistor.

kb0nes++11-27-2012-01-50-8.jpg
 
New Parts

Panasonic initially had a kit out that replaced both the IGBT transistors with different, higher rated devices. Apparently they had problems with blowups on the early model inverters. I had to do a bit of searching but I was able to locate both these vintage components. The smaller one I was only able to find from an Ebay seller in Hong Kong. They were very inexpensive but they were shipped on the slow-boat-from-China! The larger IGBT was available from Mouser.

kb0nes++11-27-2012-01-57-38.jpg
 
Checking over the rest of the board

I went over the rest of the board both visually and checking what I could with a diode tester. There were a couple smaller diodes that had vaporized from the fault. Also I noticed one of the film capacitors had a crack in its cover. Not likely a problem but I swapped it out too. I got a donor inverter board from a friend who works at a local repair shop I was able to salvage some parts from.

kb0nes++11-27-2012-02-03-26.jpg
 
New transistors soldered into place

Bottom of the inverter board after soldering the IGBT's and the diode bridge back into place.

Design of the board shows wide traces and lots of cuts through the board to provide additional insulation to the high voltage. Its an AMAZINGLY light assembly considering it handles 1300 watts!

kb0nes++11-27-2012-02-09-8.jpg
 
Fan cleaned and oiled

Testing the fan here. One concern was cooling. Panasonic's latest inverter models have a different inverter design and a larger fan/improved shroud for better cooling. The "factory" fix for this model now is a "kit" that substitutes out the inverter and fan for the new and improved models.

I figured the least I could do was to clean and oil the open frame motor.

kb0nes++11-27-2012-02-12-1.jpg
 
Cross everything you have!

With fingers and toes (and everything else I could cross) crossed, I turned it on and hoped it would work. These high voltage solid state supplies can be feeble and fickle. Amazingly there was no plume of smoke nor was the room plunged into darkness! After 30 seconds the water in the cup I had inside the cavity for a load was noticeably warmer!! Success! Here is a shot of the setup to test the HV

kb0nes++11-27-2012-02-15-58.jpg
 
That should do it!

Meter shows 3.6Kv DC into the magnetron.

I buttoned it up then gave it some run time. Its looks as if the case forms part of the fan ducting so I would be careful about running one of these for too long with the case off. I brought water to a boil several times and at this point I claim a successful repair.

Another appliance saved from the landfill!

Hopefully there will be some interest in this story. Again I will reiterate that a repair like this isn't for the uninitiated. Lethal voltages like these demand great respect. Only because I have experience with high voltage supplies from tube transmitters did I feel confident tackling this. Be safe and beware!

Phil

kb0nes++11-27-2012-02-17-35.jpg
 
Excellent repair procedures and job.Glad you were able to fix your Panasonic invertor board even though Panasonic suggested against it.Could see HV experimenters going after these boards-after all those HV guys like to work with pole pig transformers!Have enough HV at the transmitter site.Yes,HV is lethal-so we have to be careful working on it-or with some playing around with it.Lokked up what some of those HV experimeters do-kinda scary!Just hope they stay careful.A couple of our transmitters ahve "invertor" type modulator b oards instead of the push pull tube modulator and large modulator transformer.The Pulse step mod boards produce both the HV for the RF power amp tube and the audio -program modulation.
 
Rex,

Indeed there are a number of experimenters that are using these boards as part of a HV plate supply for transmitters, amplifiers or just Tesla coils to see how big a spark they can make. I got a lot of useful information from a ham in Australia who reverse engineered the unit to make a 2000 volt 1Kw plate supply for a tube RF amp. Panasonic of course provides no technical information since the unit is "non-repairable".

What kind of broadcast gear do you work on? Radio/TV, AM/FM?? That game has sure changed in the past 15 years. From big air cooled tubes with handles on them to a bunch of combined smaller solid state amps. Its amazing what some of the RF transistors can do today (MRF151G comes to mind). I used to believe that anything over 100w needs a filament, but I am changing my luddite ways now.

73

Phil, KB0NES
 
fascinating!

I must admit, a lot of it was over my head. Even so, I learned some things and am most impressed with the repair. That's a handsome appliance, glad you could save it.
 
Safer??

Get rid or what, the entire Microwave? Might be safer yet to just disconnect the AC mains from the house! ;)

I don't consider anything I did to be unsafe, I took more risks driving to work today. Many of the things in life we do are aren't safe, but what would life be like if we avoided it all? Its all about knowing the risks each of us can manage in a reasonable manner.
 
Furthermore,

Kb0nes seems to know quite well what he was doing. I, for one, would never dream of cracking a microwave's case, because I know that I DON'T know.

Cracking the case of a washer, or a hand mixer, or a toaster could be dangerous, but not like a microwave or television, or any number of electronic devices.

Lawrence/Maytagbear
 
interesting

I have worked on several of these units, most of them being newer than yours. Tho because of my position, i have to replace parts as a whole, meaning the whole inverter board.

Too bad your not closer, i could use your expertise here in my area...lol
 
That's handy, having a meter capable of handling those voltages, my Avo only goes up to 1000 volts DC or 750 volts AC. The last one of those I repaired ended up being a similar , component level repair, although I had bought what was supposed to be a new inverter board on Ebay. The one I did also needed a new magnetron so it was an expensive repair but the oven was built-in and the customer was selling the apartment so had to have everything working.
 

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