Repair of a Panasonic Inverter Microwave

[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