Dissecting a malcontent (and moist?) microwave oven

November 14, 2023

0 Views 0

SaveSavedRemoved 0

Back in February 2015, my wife and I purchased Whirlpool’s WMC30516AB 1.6 cubic foot microwave oven from Amazon:

It lasted a bit more than four years. Toward the end, various segments of the four-digit, seven-segment per-digit display would flake out, then return, and random buttons on the front panel would also stop working only to revive later. Eventually, the resurrections ceased and, although the remainder of the microwave oven presumably still functioned fine, there no longer was any way to control it (notably via the no-longer-responsive “Start” button).

So, we hauled it to the dump and replaced it in July 2019 with…another Whirlpool (what can I say, my wife likes the brand, or should I say liked it), the familial WMC30516HV, this time for more than twice the price of its predecessor at Home Depot:

Stop me if you’ve heard this before: a bit more than four years later, and around a month ago, the replacement unit failed in exactly the same way. And if you search online for reviews of either model, you’ll find that plenty of other owners’ units have suffered the same fate.

We’ve subsequently purchased a Panasonic 2.2 cubic foot NN-SN966S in like-new claimed condition from Amazon’s Warehouse sub-site:

I’m happy to report that it was as advertised; I don’t think it had even been taken out of the box by the original purchaser (list price: twice what we paid) prior to being returned for resale. But given that we had two related-family Whirlpools die exactly the same way at near-exactly the same time, the engineer in me suspected a fundamental design flaw somewhere. So before taking this one to the dump…you guessed it…I decided to take it apart, in part because more generally I’ve always been curious about what’s inside one of these things.

My upfront suspicion was that moisture accumulation resulting from poor ventilation flow through the unit’s interior while in use was backflowing into the electronics area of the system and eventually causing something(s) on the PCB to fail. Part of this, I admit, might be our “fault”. I’m such a tightwad that if my wife doesn’t finish a Starbucks drink, I toss it in the fridge and heat it up and drink it the next morning as part of my daily caffeine intake. Further to that point, brewed coffee that we don’t finish goes into a carafe for me to later reheat and consume, too. Then there’s corn in the cob, soups, and plenty of other moisture-rich food and drinks that regularly find their way to the microwave for cooking and otherwise heating up…

But I’m not willing to shoulder all, or even most, to be blunt, of the blame. For one thing, I don’t think our usage pattern is all that atypical. And for further evidence of a potential fundamental design flaw, check out this case study example review of the WMC30516AB, complete with submitted photo and titled “Major steaming problem, and no help at all from Whirlpool”, that I found on Amazon in the process of finalizing this writeup:

This Microwave had a major problem with steaming up, even with small cook loads like a few slices of microwave bacon. This soon led to obvious streaking and spotting on the inside of the (non-cleanable) viewing window. Whirlpool customer service was the worst part. They insisted that this steaming situation is “normal performance,” though I’ve never seen another microwave steam like this. I requested a service call to evaluate the Microwave but Whirlpool refused.

Come to think of it, I’d noticed seemingly excessive interior condensation accumulation with our two units, too.

Enough preparatory chatter; let’s get to the teardown. Here’s our patient (for the bulk of this project, I’ve moved from my usual office desk to the workbench directly below it in the furnace room for perhaps-obvious available-space reasons, although the lighting’s not as stellar there):

Note that, unlike with my new Panasonic, there are no outgoing airflow vents (either passive or active) on the left side. Hold that thought:

Speaking of airflow, the back’s where the bulk of the action is:

Air is forced into the unit by a fan behind the ventilation hole mesh at left. It flows through the electronics, from there passively transitioning (theoretically, at least) into the main cooking cavity of the microwave oven, then again passively out vent holes on the opposite upper side and back corner of the interior (upper right side and back right corner, from this photo’s perspective). And how does the air exit the microwave oven? Through those passive vents you see at top and on the right edge in the photo, all at the back and on the right (again, from this rearward perspective, at least) half of the unit, mostly making a 90° turn in the process, again, versus directly out the opposite side with the Panasonic approach.

Before continuing, a couple of close-up sticker shots:

Now for the right side, those aren’t actual air vents, by the way, only cosmetic metal “trenches”:

Top:

And finally, the bottom:

Note that there are functional passive air vents here, too, but their locations are curious. They’re predominantly on the air-outflow half of the microwave oven, but since the air will be heated (albeit moist, therefore heavier than when it entered) at this point, and since hot air rises, not falls, I question just how functional they really are.

Back to the front; let’s now pop open the door:

The inside of the door is conventional for a microwave oven, as far as I can tell from my limited, elementary experience with these devices (and shielded, of course, for obvious reasons):

Again, the airflow direction through the interior is right to left from this front-view perspective. That metal plate on the right side is a cover for the waveguide, called a mica plate:

A couple more sticker closeups before continuing:

Before diving in, I decided to satisfy my curiosity and see if the microwave oven’s several-week sojourn sitting downstairs unplugged and awaiting “surgery” had left it reborn, as had happened (temporarily, at least) in the past. Nope:

The “8” front control panel button still worked, so you can tell which segments (of which digit) failed:

But many of the other numerical and functional buttons remained non-responsive…again, including the all-important “start”. Oh well.

Onward. You may have already noticed the large Torx head screw at the bottom of the right side of the unit, and the four additional ones around the edges of the back side. Let’s get those off:

With them removed, the unified right-top-left panel slides right off the back:

From boring-to-exciting (IMHO) order, here’s the now-exposed left side:

Top side:

Complete with warning (hah!) sticker closeup:

And the right side, where all the electronics action happens:

Your eye will likely be immediately drawn to the cavity magnetron at the center, behind which (not shown) is the aforementioned waveguide:

That metal shroud to the right draws in ambient air from the outside to keep it cool. Speaking of which, this doodad perched above it:

is, I’m assuming, a temperature sensor to ascertain whether the magnetron is overheating due to, for example, using the microwave oven with nothing inside it or with metallic contents.

Below the magnetron is a hefty transformer:

And to its right is an equally formidable capacitor:

In the upper right of the earlier overview shot is a small PCB:

Presumably, particularly given the diminutive size of the onboard fuse, it does AC/DC conversion for only a subset of the entire system circuitry.

And at far right is the fan:

Now let’s move to the left side of that earlier overview shot. First off, here’s the light bulb, which shines through the passive air inflow vents to illuminate the interior:

To its left and below are three components whose purpose wasn’t immediately obvious to me:

until I purposelessly pressed the latch to open the microwave door and noticed that they’d also transformed in response:

These are, I believe, triple-redundancy switches intended to ensure that the magnetron only operates when the door is closed.

Last, but not least, let’s look at the main system PCB at far left, which is the upfront intended showcase (not to mention the presumed Achille’s Heel) of this project:

Here’s a slightly tighter zoom-in:

First step: unhook the various bits of cabling connecting it to the rest of the system:

Two screws are immediately visible along the left edge. But removing them:

didn’t free the PCB from its captivity:

Looking again, I found another one hidden among the connectors, capacitors, and such on the right side of the PCB:

That’s more like it:

Left behind, among other things, is the oddly-varying-contact-color ribbon cable that originally routed between the PCB and the front control panel:

And here’s one more wiring mention; referring back to earlier airflow comments, I at first thought that the two wires heading underneath might be going to a ventilation fan, intended to pull cool air into the body cavity from the outside to the underside:

But after pulling off the bottom panel to expose the otherwise unmemorable under-interior to view, I realized that they were instead connected to (duh on me) the glass turntable motor:

Now back upstairs, where the lighting’s better, for the rest of the PCB analysis:

Let’s stick with this latter side for the first closeup shot set. Here’s that faulty-segments display:

and the exposed portions of this side of the PCB, dominated by solder points and traces:

Did you notice, though, what looks like one corner of an IC sticking out from under the display, further exposed after slipping off the surrounding gasket?

Let’s see what some side views reveal:

Yep, there’s definitely a large lead count chip underneath. Fortunately, by unclipping two of the plastic “legs” from the bracket surrounding the display, I was able to swing it out of the way, revealing both its underside and the remainder of this side of the PCB:

The glossy finish atop the IC makes it very difficult to read (far from photograph) the product markings, so you’ll need to take me at my word that it’s a M9S8AC16CG microcontroller, containing an 8-bit S08 CPU, 16 KBytes of flash memory and 512 bytes of SRAM, and still with its original Freescale Semiconductor vendor logo stamp atop it (the company, therefore product line, were later merged into NXP Semiconductors).

Let’s now flip the PCB over to its other side, starting with some side views. Check out, for example, that circular “beep” piezo transducer near the middle:

And, wrapping up, a couple of full-on closeups, starting with the top half:

The two ICs you see at left are an I-core AiP24C02 2 Kbit EEPROM (what an EEPROM is doing in a microwave oven is beyond me, unless it’s used for operating dataset fine-tuning on the assembly line, or something like that) and, below it, an unknown-supplier LM358 dual 30V 700-kHz op amp.

Now for the other (lower) half:

The clutch of ICs in the lower right corner comprises two chips oriented 180° relative to each other and strangely stamped:

1730, preceded by an upside-down 7 in a larger font size
817 C
WK

and below and to the left of them, Power Integrations’ LNK364 AC/DC converter.

No obvious failure culprit emerges from my visual inspection of the PCB; see anything, readers? It kills me that the likely moisture- or heat-induced (another potential side effect of poor ventilation, of course) failure of a single inexpensive component on this board is likely what caused the demise of the entire expensive microwave oven, but that’s our “modern disposable society” for you, I guess…Sadly, even if I could fix it, I’d be reluctant to pass it on to someone else without a plethora of upfront qualifiers, because it’d likely only be a matter of time before the unit died again, due to its innate shortcomings.

With that, I turn it over to you for your thoughts in the comments!

—Brian Dipert is the Editor-in-Chief of the Edge AI and Vision Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the company’s online newsletter.

Related Content

<!–
googletag.cmd.push(function() { googletag.display(‘div-gpt-ad-native’); });
–>

The post Dissecting a malcontent (and moist?) microwave oven appeared first on EDN.