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Panasonic Inverter Microwaves - Advanced or retrograde?

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Posted: Thu Oct 22nd, 2009 08:27 pm

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41^st Post

iceman
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un·re·pent·ant

(

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nt)
adj.
Having or exhibiting no remorse.

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Posted: Thu Oct 22nd, 2009 08:40 pm

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42^nd Post

RegUS_PatOff
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Posted: Sun Nov 8th, 2009 10:23 am

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43^rd Post

Konnan
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grindit wrote:
Hi Iceman-

Read your post with interest as I do have a countertop Panasonic Inverter Microwave Model NN-S760WA that I bought in late 2000. It just died this week, so yes, it did last over 5 years. It will turn on briefly but shut off after a 4-5 seconds, not enough to heat anything.

I checked the interlock switches and they are OK, but that's about as far as I got. It was a nice unit while it lasted. Not knowing much about Inverter ovens, any diagnostic suggestions while I have the patient open prior to sending it to microwave heaven?

Hi Master iceman,

God of the mighty Panasonic Inverter Microwave, could you help me ?

I have exactly the same problem as the guy I quoted above. Microwave will work for a few seconds at a time at most.

It was slightly arcing due to excess grease, because of the bad integrated hood fan design, grease manage to get inside the microwave electronics.

Managed to fix the arcing problem with a good cleaning, but obviously something is damaged.

The high voltage power diode looks to be burnt, due to blackened PCB in the area... I wonder why it still works for a few seconds on the other hand... could it be only a fan problem then the microwave enter in protection mode ?

Diode is UX-C2B made by SANKEN Electric, now discontinued part worth 30$ on eBay with no SANKEN Electric equivalence, but a local store can supply an equivalent from another brand for 5$.

Should I also look at the high voltage capacitor or somewhere else ?

Thanks alot Master iceman,

If anyone else wants to help me, I'm all ears.

Thanks !
Konnan

Last edited on Sun Nov 8th, 2009 10:25 am by Konnan

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Posted: Sun Nov 8th, 2009 05:37 pm

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44^th Post

iceman
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Hi Konnan,

To quote our erudite leader

"Never replace a part unless you have proof that the part is bad."

http://fixitnow.com/wp/2005/09/23/samurais-12-laws-of-appliance-repair/

Let us consider. You have used a special tool to remove tamper proof fasteners to gain access to the POWER Printed circuit board of a device designed to emit MICROWAVES. Presumably you read the prominent safety warnings on the unit as you did so. Suppose that you did manage to replace enough parts to make it work. Lets postulate some follow-on questions:

1) Was the arcing caused by grease or by the normal failure mode of this product?

2) How long will it work for before it fails again?

3) Are you comfortable being near this device while it is operating under your fresh repair? How about your kids?

4) Could your repair result is a fire when you are not at home?

OK, let's assume that you have decided to disregard the cautions above and are prepared to assume full responsibility of the consequences of your repair or losses resulting therefrom. Here are the logical steps the I would follow if I had decided not to just replace the unit.

1) Try to obtain a schematic of the power converter board.

2) Failing that, write down all the part numbers of the power devices (FETs, Diodes, BJTs). The really damaged ones might be illegible

3) Get the data sheets for the defective devices.

4) De-solder all power devices and test them.

5) Replace the defective ones.

Once again, I STRONGLY recommend that you dispose of this microwave and go get a new one. That is what I did, and I am very satisfied with it. This project has exponentially diminishing returns.

Oh, and don't forget the flip Panasonic the bird.

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Posted: Sun Nov 8th, 2009 06:34 pm

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45^th Post

Goosegog
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Further to my post of April 21 2008 we have finally got fed up with a microwave that only operates for 23 seconds at 1/3 power. Because of the vent and trim kit issue I have purloined another Pana (NNT795S - CAN$220 at London Drugs) which seems to be the only one that fits. It's been there 3 days and still works, so it looks very promising.

Interestingly, it's almost impossible to fit the existing vent kit sheet metal to the new unit, as Pana has changed the security screws on the back from a "star" pattern to a "star-with-a-projecting-prong-in-the-middle" pattern, for which I do not have the proper tool. Vice-grips rule! It's only one screw, so I hope this will not invalidate the warranty.

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Posted: Sun Nov 8th, 2009 07:05 pm

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46^th Post

Konnan
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iceman wrote:
1) Was the arcing caused by grease or by the normal failure mode of this product?

1) Try to obtain a schematic of the power converter board.

Oh, and don't forget the flip Panasonic the bird.

Thank you for this nice reply, iceman, Master of the Panasonic !

Thank you for the link to the 12 laws... should have read that before hehe !

1) Well, removing grease removed arcing between power board and a ground screw when the microwave is working, but yes I guess it's just delaying its death a few more years...

1) Getting a schematic for an inverter power board from a discontinued Panasonic microwave ? Pretty much impossible, isn't it ?

You have many valid points, but these damn large microwaves with integrated hood fan vents are several hundred $$$ expensive... so I may want to try replace that 5$ diode... I will not play with the magnetron or waveguide, don't worry, be happy !

As you said, if that doesn't work, it will be lost time, lost money and a lost cause, so I need to further think about it. That diode is probably not the only damaged part.

Agreed on the "flip the bird" part... ;-)

Thank you again for your insight !
Konnan

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Posted: Sun Nov 8th, 2009 07:39 pm

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47^th Post

iceman
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Hi GG,

They sell those kind of Screwdrivers at Princess auto in a set of 6 for cheap. Part 2920902.

http://www.princessauto.com/tools/hand-tools/torx-keys/2920902-6-pc-torx-screwdriver-set

I got 'em for $6.99 on sale. Which they frequently are.

Hi Konnan,

Earlier in this thread Rob Stercraw solved your problem a little bit more safely. Please consider his solution. He got another power board.

Otherwise:

Broke the secret screw
Found the power board and fixed
One kid has three eyes!

Last edited on Sun Nov 8th, 2009 09:49 pm by iceman

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Posted: Mon Nov 9th, 2009 12:46 am

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48^th Post

Konnan
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Hi Mr. iceman,

You are again right, but I believe those power boards are not an easy find.

I will try to look on eBay unless you know a better place... we never know.

Thanks again !
Konnan

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Posted: Mon Nov 9th, 2009 01:01 am

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49^th Post

RegUS_PatOff
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Microwave model number ?

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Posted: Thu Nov 19th, 2009 08:54 pm

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50^th Post

Konnan
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Hello RegUS_PatOff

Sorry for the huge delay...

It's : NNH264W

EDIT :

I think the W is only for color white, so number is really NNH264 or NN-H264 which is similar to NN-S254 if I looked carefully... sorry !

Thank you very much,
Konnan

Last edited on Thu Nov 19th, 2009 09:05 pm by Konnan

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Posted: Thu Nov 19th, 2009 09:44 pm

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51^st Post

RegUS_PatOff
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"Run away !"

from this thread http://applianceguru.com/forum1/21915-1.html

RegUS_PatOff wrote:
I did find someone's repair info on the Inverter Board,
along with some oscilloscope waveforms,
but I don't think you want to go there ...

here's an excerpt:

"The Control signal turns out to be a TTL-level 220Hz square wave where the duty cycle determines the “Power” from the Magnetron.

The Status signal from the PSU is a 110Hz square wave with a fixed 50% duty cycle.

This signal seems to be present when there is a current drain, probably to signal that the Magnetron is warm and operating.

The Control and Status signals are synchronised to each other, but bear no relationship to AC mains frequency or phase.

Notes:

1. At low power levels, the PSU starts at a higher power level (50%) until the Status signal appears, then drops back to a lower level.

This could be because the Magnetron filament requires at least the 50% power level for it to heat up, then it sustains itself from back bombardment once RF is being produced.

2. At low levels, the PSU is cycled on and off at the 40% power level over a 22 second period to reduce the average power output."

http://www.mediafire.com/file/zf3ymc2tmdm/panasonic_inverters.pdf

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Posted: Thu Nov 19th, 2009 09:54 pm

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52^nd Post

Konnan
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You are making me sad !

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