Repair of a Panasonic Inverter Microwave

[kb0nes]

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]

Under the hood

Here you can see the important bits. This is my first time I ever confronted an Inverter design. No big heavy iron transformer or large oil filled filter capacitor.

 

[kb0nes]

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]

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]

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]

Heatsink Assembly

Here is the heatsink all cleaned up with the new transistors installed. The rectifier bridge tested fine so it was reused.

 

[kb0nes]

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]

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]

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]

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]

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

 

[tolivac]

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.

 

[kb0nes]

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

 

[marthalover4eve]

safer

would it not be safer to get rid of it

 

[westingman123]

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.

 

[kb0nes]

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.

 

[maytagbear]

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

 

[super32]

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

 

[fido]

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.

 

[kb0nes]

HV Probe

Phil, I agree the HV probe is a handy addition to any normal DMM. Its just a super insulated probe with a large resistive voltage divider in it. It simply reduces the AC or DC voltage by 1000 times to therefore read 3.6Kv as 3.6 volts. You could use the probe with any 10Meg Ohm input DMM. The one I have is a 40Kx Fluke model but there are a number of other makers that sell them for far less, I got a great deal on the one I have on Ebay.

 

[cfz2882]

leakey cap possible cause of blown transistor

i have found high frequency inverters that blow a drive transistor usually have a leakey capacitor between the blown transistor and the winding driven by that transistor-good you changed that one that had a crack,it may have been bad...At the frequency these inverters run at,just a little leakage through the capacitor is basicly a short circuit. Whenever i find a blown drive transistor in an inverter i always replace the coupling capacitor too-i have found they are almost always leakey in those cases.BTW nice fix on that microwave oven-kinda reminded me of fixing the damaged control board in my neptune.

 

[tolivac]

Phil: Used to work on AM,FM and TV transmitters.Mostly RCA-and Gates/Harris and CCA.At the present site-a Govt plant-its a short wave broadcast site that can broadcast to anywhere in the world.It involves transmitters that can provide up to 500Kw of RF output power.And they are varying ages-3 of them are 60+ years old(Continental electronics).Another three are 45 yrs old(GE)The later transmitters are 27 years old.AEG and Brown Boveri.Lots of VERY large tubes here-you need a small hand cranked crane to pull and install tubes here.Primary voltages the transmitters run on are 4160V three phase.The Brown Boveri transmitter uses the pulse step modulator-supply modules.Each has a Gate Turn On device in them-sort of like a large high frequency triac.The modules switch at like 37Khz kinda like the microwave oven supply.The modulation is derived from pulse width modulation to the modules-provided by a specific purpose computer driver-it couples to the modules via fiber optic cables-the modules are at up to 14Kv DC.Each module has its own three phase recitifier from a special power transmformer.Two of these transformers are used.Each one has one 4160V 3 ph primary and 16 650V three phase secondaries.This is recified and provides the voltage for the module to operate from.32 modules in all.They are wired in series each one 90VDC-and up to 1800VDC max.A low pass filter allowws the modulation to the plate of the PA tube-along with the DC derived from the modules.All of us here have rebuilt the switching modules at one time or another.The GE transmitters use a conventional push pull tube modulator for the 250Kw PA stage.The Continentals use a triode Doherty 250Kw power amp.Each Continnetal uses two PA stages into a combiner for 250Kw-can be tricky to tune!So we use both tubes and solid state here.A joke for us here-"50Kw AM?-thats a DRIVER stage to us here!"Indeed-the GE transmitters use a 50Kw stage as a driver for the 250Kw PA.

 

[arbilab]

Nice photostory b0nes. Illustrates that appliances can be repaired at component level rather than module.

Inverter u'wave is very clever. Much lighter, cook better, more conservative of resources. Instead of alternating max/off, proportion continuous power.

Last thing that shocked me was 400V 650W 100kHz inverter. Working on these, use extreme caution and wear eye protection. When those FETs explode they throw shrapnel.

 

[philr]

That's a very interesting thread Phil. Thanks for showing us! I have friends who have an over-the-range version of this Panasonic inverter and they had problems with it just two years after they got it... I think they paid almost $200 to have it repaired, which is a lot more than the price of a new cheap counter-top microwave. 

 

[foxchapel]

I am impressed. Those of you who know how to make these repairs are truly fortunate. The rest of us have to toss the dead microwaves to the landfill and buy a new one.

We recently bought a Panasonic inverter microwave and really like it. I hope it lasts ten ears, given the build quality of current appliances.

 

[gansky1]

Very interesting! I also have a Panasonic Inverter microwave - our daily driver in the kitchen and it's performed great in the five years we've had it. We have the smaller version with the dial control. For $100 at Costco, it's been the best microwave I've ever used. If it quits working, I'd likely shed a crocodile tear, have to toss it and look for another Inverter model.

Would never attempt a repair like this unaccompanied by a trainer , I'd surely be dead! Thanks for the introductory course and responsible safety warnings. Fascinating.

 

[kb0nes]

Inverter Microwaves

Just like other switching power supplies the high voltage inverter power supplies were feeble at first. Even more so because faults at HV cause catastrophic failures to solid state electronics instantly.

They have finally started to become highly reliable and they offer advantages of cost, weight, efficiency and control. The early Panasonic oven such as the one I repaired has gone through several inverter design changes. They uprated the transistors at first, then a few years later they totally redesigned the inverter to make it simpler and more reliable. A service shop today would swap out the old board for the new design along with a bigger fan.

I agree with you Greg that these ovens are very decent daily drivers. They seem to "haul the mail" just fine and they offer lots of control. Unlike the old pulsed HV designs you can simmer in these without the intermittent boiling.

At work we have one of the very early Whirlpool glass touch microwaves that is in excellent condition. I'm not sure any of these Panasonic microwaves will still be serviceable in 30 years but they do seem to do fine today for a pretty reasonable cost.

 

[arbilab]

Apartment supplies 1100W MW. Too high on P3/intermittent. I fall back on my 1982 (30yo) GE 600W. Wouldn't be necessary with inverter. OTOH, nothing made anywhere near today would last 30 years.

What I don't get is why they didn't find these component weaknesses in development and upgrade as needed before releasing them. Doesn't ANYone test ANYthing any more before they sell it?

 

[spookiness]

great post

I don't understand anything you did, but I just wanted to add another voice in favor of Panasonic microwaves. I have had mine for maybe almost 10 years now. I've considered replacing it because although it works perfectly, the interior is wearing a bit and paint is coming off in one place. Otherwise, I love it. The automatic defrost settings truly work, and the "sensor cook" function is spot-on accurate, I use it especially for frozen foods and they always come out just right.

 

[davetranter]

Panasonic Inverter

I have just discovered that I have one of these supplies recovered from a damaged oven. Does anyone have a schematic, or information on the input (control) connections to this unit??

Many thanks

Dave T