Maker Pro
Maker Pro

Inverter microwave ovens - reliable?

F

Franc Zabkar

Jan 1, 1970
0
My brother's Panasonic model NN-ST756W inverter microwave oven failed
after one year, just out of warranty. I found a short circuited HV
diode, D702, UX-C2B. The associated 8200pF 3kV capacitor appears OK,
as do the inverter transistors.

The inverter PCB (240V version) is similar to the one in these service
manuals:

http://www.eletrodomesticosforum.com/downloads/microondas/Panasonic_NN-G62BH.pdf
http://rc5.ru/upload/panasonic_mw_NN-C2000_(MWO).pdf

Here are several exploded views and parts lists:

http://www.prime-electronics.com.au...t=MICROWAVE&model=NNST756W&view=OVEN ASSY.jpg
http://preview.tinyurl.com/bre9kh

http://www.prime-electronics.com.au...Submit=Search&search=search&PRODUCT=MICROWAVE
http://preview.tinyurl.com/ahr8s8

I'm wondering if inverter microwaves are any less reliable than
traditional transformer/capacitor types. They seem to be overly
complicated for what they do. I understand that they cook more evenly
in low power modes as a consequence of not having to pulse on and off
like conventional microwaves, but I'd rather not pay a reliability
penalty for this feature. BTW, once the microwave is sitting on my
bench top, the extra weight of a traditional mains transformer is of
no consequence.

The Microwave Oven Repair FAQ appears to bear me out:
http://www.repairfaq.org/REPAIR/F_micfaq6.html#MICFAQ_019

- Franc Zabkar
 
G

GregS

Jan 1, 1970
0
My brother's Panasonic model NN-ST756W inverter microwave oven failed
after one year, just out of warranty. I found a short circuited HV
diode, D702, UX-C2B. The associated 8200pF 3kV capacitor appears OK,
as do the inverter transistors.

The inverter PCB (240V version) is similar to the one in these service
manuals:

http://www.eletrodomesticosforum.com/downloads/microondas/Panasonic_NN-G62BH.pd
f
http://rc5.ru/upload/panasonic_mw_NN-C2000_(MWO).pdf

Here are several exploded views and parts lists:

http://www.prime-electronics.com.au/trade/cgi-bin/display.pl?brand=PANASONIC&pr
oduct=MICROWAVE&model=NNST756W&view=OVEN%20ASSY.jpg
http://preview.tinyurl.com/bre9kh

http://www.prime-electronics.com.au/trade/PRICEANDAVAIL/frametest.asp?Model1=NN
ST756W&Submit=Search&search=search&Model=NNST756W&Model1=NNST756W&Submit=Search
&search=search&BRAND=PANASONIC&Model1=NNST756W&Submit=Search&search=search&PROD
UCT=MICROWAVE
http://preview.tinyurl.com/ahr8s8

I'm wondering if inverter microwaves are any less reliable than
traditional transformer/capacitor types. They seem to be overly
complicated for what they do. I understand that they cook more evenly
in low power modes as a consequence of not having to pulse on and off
like conventional microwaves, but I'd rather not pay a reliability
penalty for this feature. BTW, once the microwave is sitting on my
bench top, the extra weight of a traditional mains transformer is of
no consequence.

The Microwave Oven Repair FAQ appears to bear me out:
http://www.repairfaq.org/REPAIR/F_micfaq6.html#MICFAQ_019

- Franc Zabkar

I didn't know much about it. I bought one maybe 3 years ago. Small lightweight,
and the most powerfull one I have ever had. I use it everyday. The only problem, the light
does not come on when you open the door. My brother uses one in a resturant,
and they have failed after much use. That one the commercial job, costs a lot
more. Like I said, it small, powerfull, and cheap. My Sears GE microwave is
big, not nearly as powerfull, also has turbo oven, but I use the Panasonic
to heat fast. By other brother has a Samsung which has failed. Replaced
diode and found the transformer is bad. After two months still waiting for transformer.

greg
 
H

hr(bob) [email protected]

Jan 1, 1970
0
I didn't know much about it. I bought one maybe 3 years ago. Small lightweight,
and the most powerfull one I have ever had. I use it everyday. The only problem, the light
does not come on when you open the door. My brother uses one in a resturant,
and they have failed after much use. That one the commercial job, costs alot
more. Like I said, it small, powerfull, and cheap. My Sears GE microwave is
big, not nearly as powerfull, also has turbo oven, but I use the Panasonic
to heat fast. By other brother has a Samsung which has failed. Replaced
diode and found the transformer is bad. After two months still waiting for transformer.

greg- Hide quoted text -

- Show quoted text -

Maybe the reason the light does not come on is because it is burned
out.
 
J

Jim Yanik

Jan 1, 1970
0
Since they're a relatively new technology (at least at the consumer
level), my suspicion would be that, yeah, they are a bit less
reliable.


There's also a bit of savings in lower freight costs from the lower
weight.

the savings is in not having to build an expensive,big transformer.
all that copper is expensive,too.
And smaller people (women in particular) tend to put some
emphasis on the weight of their kitchen appliances, even if (as you
point out) it's often completely irrelevant once you get the thing up
on the counter at home.

transformers,being far less complicated,are more reliable.
Inverters have a lot more components,particularly electrolytic caps,that
increase the odds of failure.
Many electrolytics are only rated for 3000 hrs or so of operation.
and it seems that surface-mount electrolytic caps are even less long-
lasting.
 
M

Mike WB2MEP

Jan 1, 1970
0
My brother's Panasonic model NN-ST756W inverter microwave oven failed
after one year, just out of warranty. I found a short circuited HV
diode, D702, UX-C2B. The associated 8200pF 3kV capacitor appears OK,
as do the inverter transistors.

The inverter PCB (240V version) is similar to the one in these service
manuals:

http://www.eletrodomesticosforum.co...rc5.ru/upload/panasonic_mw_NN-C2000_(MWO).pdf

Here are several exploded views and parts lists:

http://www.prime-electronics.com.au...brand=PA....http://preview.tinyurl.com/bre9kh

http://www.prime-electronics.com.au...etest.asp...http://preview.tinyurl.com/ahr8s8

I'm wondering if inverter microwaves are any less reliable than
traditional transformer/capacitor types. They seem to be overly
complicated for what they do. I understand that they cook more evenly
in low power modes as a consequence of not having to pulse on and off
like conventional microwaves, but I'd rather not pay a reliability
penalty for this feature. BTW, once the microwave is sitting on my
bench top, the extra weight of a traditional mains transformer is of
no consequence.

The Microwave Oven Repair FAQ appears to bear me out:http://www.repairfaq..org/REPAIR/F_micfaq6.html#MICFAQ_019

- Franc Zabkar

Franc,

I don't think they're as reliable as conventional transformer/diode/
cap
power supplies. "Inverter" is just a switching power supply that
steps
up the voltage, and consumer-grade SMPS in a variety of products have
a
rather high failure rate.

There were several threads on SER a few years ago when Panasonic
first came out with the Inverter oves, about failures of the Inverter
circuit. At that time, Panasonic wouldn't sell replacement parts for
the Inverter power supply, or even provide a part number for the
switching trasistors that were failing. You had to replace the whole
power supply, which cost almost as much as a new oven.

The service manual for the NN-C2000 you linked to does have a parts
list for the Inverter board, so maybe they are considered repairable
now.

3 or 4 years ago, we needed a new microwave for the break room at
work.
Somebody donated an old (1984) Panasonic with a transformer power
supply,
and shortly thereafter a fancy new stainless-steel Panasonic Inverter
microwave was purchased. The two were used side-by-side for just over
a
year, then one day I came in and saw the Inverter oven set outside for
trash pickup.

So, I would definitely avoid the Inverter microwaves. Other
manufacturers
may be using switching power supplies, just not using the "Inverter"
name.
Last time I checked, most of the microwaves were still using
transformer
power supplies, but once the cost of a switching supply becomes less
than
a conventional supply, I would expect most of them to change over to
SMPS.

When you go into the store, just lift up the right side of each
microwave
on display, and buy the heaviest one.

Mike
WB2MEP
 
S

Samuel M. Goldwasser

Jan 1, 1970
0
Franc Zabkar said:
My brother's Panasonic model NN-ST756W inverter microwave oven failed
after one year, just out of warranty. I found a short circuited HV
diode, D702, UX-C2B. The associated 8200pF 3kV capacitor appears OK,
as do the inverter transistors.

The inverter PCB (240V version) is similar to the one in these service
manuals:

http://www.eletrodomesticosforum.com/downloads/microondas/Panasonic_NN-G62BH.pdf
http://rc5.ru/upload/panasonic_mw_NN-C2000_(MWO).pdf

Here are several exploded views and parts lists:

http://www.prime-electronics.com.au...t=MICROWAVE&model=NNST756W&view=OVEN ASSY.jpg
http://preview.tinyurl.com/bre9kh

http://www.prime-electronics.com.au...Submit=Search&search=search&PRODUCT=MICROWAVE
http://preview.tinyurl.com/ahr8s8

I'm wondering if inverter microwaves are any less reliable than
traditional transformer/capacitor types. They seem to be overly
complicated for what they do. I understand that they cook more evenly
in low power modes as a consequence of not having to pulse on and off
like conventional microwaves, but I'd rather not pay a reliability
penalty for this feature. BTW, once the microwave is sitting on my
bench top, the extra weight of a traditional mains transformer is of
no consequence.

The Microwave Oven Repair FAQ appears to bear me out:
http://www.repairfaq.org/REPAIR/F_micfaq6.html#MICFAQ_019

Aside from the reliability issues which have been addressed in other
replies, it's possible that the HV diode died for reasons unrelated
to the inverter.

What does the oven do when you go to "cook". If nothing at all, then
there are almost certainly other bad parts, a fuse at the very least.

--
sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.
 
F

Franc Zabkar

Jan 1, 1970
0
Aside from the reliability issues which have been addressed in other
replies, it's possible that the HV diode died for reasons unrelated
to the inverter.

I'm not sure where to look except for the magnetron.

Page 20 of the following service manual has a detailed circuit diagram
of the inverter:
http://www.eletrodomesticosforum.com/downloads/microondas/Panasonic_NN-G62BH.pdf

Page 42 of this manual has a block diagram of the controller chip:
http://rc5.ru/upload/panasonic_mw_NN-C2000_(MWO).pdf
What does the oven do when you go to "cook". If nothing at all, then
there are almost certainly other bad parts, a fuse at the very least.

I didn't test it in case something else broke, but my brother tells me
that everything appears to work except for the actual heating. That
is, the control panel works, the turntable turns, and the fan spins. A
shorted HV diode (there are two in a voltage doubler arrangement)
would mean that one of the 8200pF caps is now connected directly
across the secondary winding. I expect this would overload the
transformer and presumably the inverter would sense this fault
condition via the current transformer in the primary circuit. I'm told
that the oven shuts down after a time. Anyway I've ordered the diode
and I'll report back once I've tried it.

- Franc Zabkar
 
J

jakdedert

Jan 1, 1970
0
GregS wrote:
The light comes on when its cooking. I usually carrry a flashlight so I can see the
condition of the food.
That seems kind of obvious: door switch...probably gunked up with food
residue.

jak
 
G

GregS

Jan 1, 1970
0
GregS wrote:

That seems kind of obvious: door switch...probably gunked up with food
residue.

jak

It never worked. I don't see any reason to not believe thats the way its
designed. I need to hear from those who have these.
But, one day I'll open er up and change it.

I remember way back on an old Norelco microwave, the controller
failed, so I got out the old mechanical timer just like the one
on my first Heathkit microwave. I can't stress how this simple and easy
to use feature is missing on todays ovens. My father loved it.
I think on better models they used aother timer to cycle
the defrost. Those older models had at least 3 fail safe
microswitches on them, but the switch took the full current.
I'm sure in a fail safe mode, at least one microswitch in
ALL microwaves take the full current.

greg
 
F

Franc Zabkar

Jan 1, 1970
0
There were several threads on SER a few years ago when Panasonic
first came out with the Inverter oves, about failures of the Inverter
circuit. At that time, Panasonic wouldn't sell replacement parts for
the Inverter power supply, or even provide a part number for the
switching trasistors that were failing. You had to replace the whole
power supply, which cost almost as much as a new oven.
The service manual for the NN-C2000 you linked to does have a parts
list for the Inverter board, so maybe they are considered repairable
now.

The transistors are now offered as "A691E4V10GP Transistor Kit Series
2".

Here are the datasheets for the GT30J322 (75W) and GT60M303 (170W)
IGBTs:
http://www.toshiba.com/taec/components2/Datasheet_Sync//361/4133.pdf
http://www.toshiba.com/taec/components2/Datasheet_Sync//361/4169.pdf

The oven has a rated output of 1100W. If the SMPS has an efficiency of
90%, then I expect that the transistors would be dissipating around
100W between them, although the mass of the heatsink appears
relatively small for such a high heat load.

- Franc Zabkar
 
D

David Lesher

Jan 1, 1970
0
jakdedert said:
That seems kind of obvious: door switch...probably gunked up with food
residue.

My sister had one like that; light on during cook -- no light with the
door open. Dumb design.

I added a relay to solve the issue and was the big hero...
 
F

Franc Zabkar

Jan 1, 1970
0
Anyway I've ordered the diode
and I'll report back once I've tried it.

I replaced the HV diode and all appears well so far.

- Franc Zabkar
 
W

William R. Walsh

Jan 1, 1970
0
Hi!
I remember way back on an old Norelco microwave, the controller
failed, so I got out the old mechanical timer just like the one
on my first Heathkit microwave. I can't stress how this simple
and easy to use feature is missing on todays ovens.

I'm not sure I follow what you are saying. Are you saying that the
microwave, when its controller went bad, failed in an "on" position?
If I understand you correctly, I think it would be a far better idea
to have the oven fail in an "off" position.
I think on better models they used aother timer to cycle
the defrost.

Older microwaves sometimes had multiple timers. My grandfather has a
Litton microwave oven with two timers. One is the main timer, which is
an analog "knob" on the front. The other "timer" is engaged and used
in conjunction with the main timer when DEFROST mode is turned on. It
consists of a slow turning motor that drives an irregular cam. When
the cam rises up, it turns on a microswitch that enables the high
voltage and magnetron.

Today, the controller board in a "typical" oven has two relays...one
that runs the fan, turntable and light and another that can be turned
on and off to cycle the magnetron.

I'm not sure how long that technique was used, as I have a slightly
newer Litton microwave oven that is largely the same internally but
has a "rolling digits" timer and a solid-state control board for the
defrost and low power modes.
Those older models had at least 3 fail safe microswitches on
them, but the switch took the full current.

Newer microwaves also have multiple microswitches. (At least I have
yet to see one that does not, and that's on ovens manufactured as
recently as a year ago.) However, in normal operation, the microswitch
that interrupts the oven's operation when the door opens is actually a
low voltage signal to the controller board.

William
 
S

Samuel M. Goldwasser

Jan 1, 1970
0
Rick said:
Heh - there's a Buyer's Guide tip you will never see in Consumer
Reports. <g>

My favorite microwave oven is the Sharp "Carousel II" I pulled out of
the trash in 1994. Replaced an open fuse for about 50 cents and it's
been working fine ever since. I used that Sharp to replace a newer,
smaller model with less capacity and never regretted that decision.

It's a shame how many of the older models I've seen tossed that only
needed a fuse replacement.

Still use our Sharp "Carousel II" from around 1987 and it's never even
needed a new fuse. :)

--
sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.
 
J

Jim Yanik

Jan 1, 1970
0
[email protected] (Samuel M. Goldwasser) wrote in
Still use our Sharp "Carousel II" from around 1987 and it's never even
needed a new fuse. :)


My Carousel from early 1980s blew the HV capacitor after about 20
yrs,replaced it and the HV diode for $25 several years ago,it's still
running.
The MWs of today don't have as tall a cavity as my Sharp's.
 
W

William Sommerwerck

Jan 1, 1970
0
The MWs of today don't have as tall a cavity as my Sharp's.

But I suspect it's not as wide.
 
J

Jim Yanik

Jan 1, 1970
0
But I suspect it's not as wide.

15.5" wide,10.25" high,and 16.5" deep,just measured it.

most MWs today are sandwich warmers....

I like to put a 2qt.pitcher in the MW and heat water in it for tea.
 
W

William Sommerwerck

Jan 1, 1970
0
My dad bought it a few years ago for their motor home, and never
opened the box. It was in the huge pile of stuff they left here when
they sold their house in Florida and moved north. I haven't opened it,
because most of what I use a microwave for warns you not to use less
than a 1 KW unit.

The only thing "wrong" with a 700W unit is that it won't heat things as
quickly. Which -- except for bacon and a few other things -- is all that
microwaves are good for -- heating and re-heating.
 
W

William Sommerwerck

Jan 1, 1970
0
The only thing "wrong" with a 700W unit is that it won't heat things as
Some items are marked, Do not use a microwave rated under 1 KW,
to prevent possible food posioning from under cooked meat.

I kinda, maybe, see the logic in that.

One should not be cooking meat in a microwave. (Other than bacon, I'm not
sure what sorts of mean /can/ be successfully cooked.

If you're heating up already-cooked items -- such as chicken nuggets -- the
only way you /might/ have a problem would be if you stuffed the oven with
several dozen, and the food stayed at an incubatory temperature long enough
for the bacteria to grow.

Even this is highly unlikely, as we're talking about pre-cooked food.

I have three that are 1 KW or more, so why open the box?

No obvious reason, other than that you raised the issue. (Nothing personal.)

There are times I feel so bad I have to use a frozen dinner, and
I don't want to take chances. Other times I cook with one of my 5.5
quart crock pots.

You'll probably be upset to learn that, when I eat frozen dinners at work, I
let them completely defrost before shoving them in the microwave. It's
quicker, saves a bit of energy, and I'm not dead -- yet.
 
Top