Re: Inverter microwave ovens - reliable?

sam@xxxxxxxxxxxxx (Samuel M. Goldwasser) wrote in

Rick <rickajho@xxxxxxx> writes:

Mike WB2MEP wrote:

On Mar 3, 2:28 pm, Franc Zabkar <fzab...@xxxxxxxxxxxxxxxxx> wrote:

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:

Here are several exploded views and parts lists:

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

- Franc Zabkar
Please remove one 'i' from my address when replying by email.
I don't think they're as reliable as conventional
transformer/diode/ cap
power supplies. "Inverter" is just a switching power supply that
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
Somebody donated an old (1984) Panasonic with a transformer power
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
may be using switching power supplies, just not using the
"Inverter" name.
Last time I checked, most of the microwaves were still using
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
on display, and buy the heaviest one.

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. :)

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
The MWs of today don't have as tall a cavity as my Sharp's.

Jim Yanik