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Kenmore microwave (Samsung) model #401.80089700-- dead.


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#1 Nords

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Posted 02 December 2009 - 04:15 PM

Tenants complained that the microwave (18 months old) was dying. We visited the next morning and it was dead.


During the 2-3 days before the microwave totally died, the display would randomly shut off. It would do this when the appliance was idle (not in use, nobody even in the kitchen or lights turned on, let alone any electrical equipment being used) and when it was in use. Sometimes unplugging the microwave and plugging it back in would restore the display, and the tenants could use the control pads to program the cooking cycle, but when they'd shut the door the display would blank out again.


That's a verbatim description of the tenant's observations/troubleshooting. I couldn't tell whether the door really had anything to do with the symptoms, or if that was just a vibration issue.
My troubleshooting:
No blown circuit breakers. 120V is present at the line (input) terminals to the noise filter. The 250V 20A fuse appears to be fine and 120V is coming out of the noise filter's load-side (output) terminals. (The fuse's glass cylinder is wrapped in a white plastic that makes it impossible to see the filament, but it has zero resistance when pulled out of the board and checked with an ohmmeter. Besides, there's 120V at the noise filter's output.)


The control board looks OK-- no visible scorch marks, slagged solder, or other damage visible on either side. However the control panel display does not light up or make noises when the buttons are pushed, the microwave's interior light doesn't come on when the door is opened, and the vent fan doesn't run. Everything's deader'n a doornail.


I was able to measure an AC volt or two going onto the control board at the "Hi power secondary relay #2" and the "Low power secondary relay #1". So I'm assuming that voltage is getting to the control board.


Upon really close inspection with a magnifying glass, I finally noticed a small burn spot on the LED display module mounted at the end of the control board. It appears to be a sealed, integrated, & evacuated component. The scorch mark is a small 1/8th-inch circle on the lower right-hand side of the display next to the word "TIMER". The photo shows a couple arrows pointing to the scorch.


I think I'm going to need a new control board because this display module appears to have all 27 of its pins soldered directly to the board. The only way to replace it would be to unsolder the old display, yank it out of the board, clean out the solder holes, stick in the new display, and solder 27 pins. I doubt I could get it right on the first try, although my technique would probably be a lot better by the 27th pin.


Before I go board-shopping, is there any other troubleshooting I should check?  Would any other part have been destroyed by this fault?  Does anyone know if this control board has a recall or some sort of tech service bulletin or out-of-warranty repair guarantee? Is it worth replacing this control board, or has this microwave been "cooking" a lot of them?


I know that Kenmore is Sears' cover name for the actual manufacturer (in this case Samsung) but I'm having trouble figuring out how to search/order the "primary control board" or "smart board". What number do I use here? The microwave's model # (401.80089700), the card's label (00040RAS-SM7MGV-04), or the card's "part number" (DE41-00352A)?

Eventually I'll put something witty here.

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#2 Samurai Appliance Repair Man

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Posted 03 December 2009 - 05:06 PM

Intermittents are the hardest to troubleshoot. Before you change it out, run the cord to another outlet on a different circuit using an extension cord if you have to. Leave it like this for several days or until the problem re-appears. I've seen this very same problem and it turned out to be a poor connection at the outlet. Don't try to be clever and "checked the outlet and looked OK." Run the cord to another outlet on a different circuit to eliminate all variables.

#3 Nords

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Posted 03 December 2009 - 07:26 PM

[user=1]Samurai Appliance Repair Man[/user] wrote:

Intermittents are the hardest to troubleshoot. Before you change it out, run the cord to another outlet on a different circuit using an extension cord if you have to. Leave it like this for several days or until the problem re-appears.
Run the cord to another outlet on a different circuit to eliminate all variables.

I hear that.

Although it's just out of warranty, the microwave doesn't seem to have a lot of customer complaints and it matches the gas stove.  Rather than have the tenant hovering over me asking "Is it fixed yet?!?" I replaced the whole microwave oven with a newer version of the same model and hauled away the old carcass.  They can make more popcorn right away, they love their landlord, and I get to troubleshoot on my own leisure time with less pressure.

When I got the carcass back to my home and plugged it in, it continued to play dead.  All my troubleshooting was done at home, not at the tenants.  So whatever intermittent problems they were dealing with (and however well they reported them), by the time I got my hands on the microwave I think the problem had gone from intermittent to constant.

Not looking forward to getting an entire new card.  Sears parts support is OK here but the shipping charges are pretty stiff.

Eventually I'll put something witty here.

#4 Cactus Bob

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Posted 04 December 2009 - 05:24 PM

the display part of your board is not bad . first its not a LED or an LCD  it is a fluorescent tube display . these are made like an old vacuum tube  . the tube is sealed then a small GETTER PLATE (this is what you are seeing ) is flashed , it goes off like a flash bulb and turns the glass black . its purpose it to use up any leftover gasses (air) inside this makes the display last longer .... these displays are great , long lasting and start to get dim years before segments start to go out  . lifespan is 15 years + ...... i dont know what on the board your problem is but its not the display tube

SORRY ABOUT THE SPELLING , I FIX GREAT , I DON'T SPELL WELL

#5 Nords

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Posted 04 December 2009 - 06:14 PM

Dang, I'm used to the troubleshooting being over when charring is found.

I'll start taking a look at the other components.

Eventually I'll put something witty here.

#6 Cactus Bob

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Posted 04 December 2009 - 07:07 PM

if i was going to guess .i would look at the caps in the power supply part of the board . most if not all of the board is DC powered . if there is ANY RIPPLE in the DC supply when first powered up and self check runs and ripple is there the board will shut down . so look at the caps tops and see if there bulged , there bad ! if not you will need a scope to look at the dc . you are looking for ANY RIPPLE IN THE DC . if you have it you need to find out why ,if not you should stop . problems in the digital part of the board are hard to trace without proper test equipment and knowledge of the boards timer circuits
SORRY ABOUT THE SPELLING , I FIX GREAT , I DON'T SPELL WELL

#7 Nords

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Posted 26 March 2010 - 11:57 AM

Still in progress.

Ordered a new control board in December, and then ordered it again in January when the first order got "lost". Now it's been nearly three more months, with claims that LG has taken over manufacture and is backlogged.

Guess I'll try Sears. We pay dearly for shipping out here, but that's better than not getting anything.
Eventually I'll put something witty here.

#8 appl.tech.29501

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Posted 26 March 2010 - 01:09 PM

Almost every display I have seen on microwaves has that burn mark..most likely not your problem..
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#9 Nords

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Posted 18 April 2010 - 12:27 PM

OK, I'm back on this job. As you rightfully pointed out, the control card was not the problem. It's also fully refundable, although that type of demand makes me wonder how long it's going to be until the control card really does burn out.


I started over. I'm able to verify that 120V AC is indeed getting into the microwave cabinet from the power cord. The power-supply fuse seems fine because 120V AC is also at its output (and the fuse has zero resistance). According to my schematic diagram, that also means the bottom temperature cutout relay (or "bottom thermostat") is fine too.


I think that the fuse for the high-voltage transformer is also fine because it too has zero resistance and there's voltage at the input to the secondary relay on the control card. But I haven't checked the transformer... yet.


I think I'm done with the "It's completely dead" portion of the checklist at http://fixitnow.com/...agnostic-guide/ and from here on I'd just be easter-egging it. Back to the diagnosis flow chart helpfully provided at http://fixitnow.com/...tic-flow-chart/


Starting at the top of the chart, the display is not lit and is indeed blank. (The microwave's light doesn't come on when the door is open, either.) Next, the fuse is not blown. I can't actually see the wire inside the fuses, but they have zero resistance.


That leads me to the box starting with "check 120V supply". I eventually traced the two wires that lead from the power cord to the control board and the other components. On my microwave oven's schematic diagram that's a small card called a "noise filter". It has 120V AC going into it, and 120V AC is going out.


Next in the flowchart box is "MGN CO & FLAME SENSOR". This may seem like a dumb question, but what exactly does this mean? Which components is it referring to? The magnetron is buried way down in there behind a bunch of other components and structural interference.


Getting ahead of myself in the dumb-question category, what's the meaning of the verbiage underneath that says "CONTROL PRIMARY CONTROL PRIMARY FMR VOLTS ALL OK?" I understand how to read a schematic and to take readings with a multimeter, but I'm not familiar with this vocabulary. The microwave's schematic actually has a test and specs for the transformer, but at this point I'd be taking apart nearly the entire damn thing to get at the transformer terminals.


A final dumb question (for this post anyway): I have this microwave laying on its back on a counter while I work on it. There's not some kind of "tilt sensor" that shuts it down when it's not upright and resting on its base, is there? The "parts layout" diagram of this oven's tech sheet isn't very detailed on each little component.


Pulling my head out of the flowcharts & schematics for a minute, the microwave it totally dead yet power seems to be getting inside it and getting through the fuses to the components. At this point I don't understand what single component failure could take out everything-- the inside light doesn't even come on when the door is open, let alone there's no power to the control pad. I'd love to operate the vent fan or the counter light attached to the bottom of the microwave cabinet, but their switches are electronic touchpads on the control pad. The "totally dead" symptom makes me suspect that there's something wrong with the fuses or the voltage supply to the control card, even though they seem to test out fine.


Thanks again for putting up with these newbie confusions.  Any other ideas?

Eventually I'll put something witty here.

#10 Nords

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Posted 24 April 2010 - 12:29 PM

*Bump.*

Any interpretation assistance or other ideas?  Anyone?

Eventually I'll put something witty here.

#11 RegUS_PatOff

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Posted 24 April 2010 - 12:35 PM

can you post the schematic diagram somewere and link to it ?

 

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#12 Nords

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Posted 24 April 2010 - 08:47 PM

[user=3641]RegUS_PatOff[/user] wrote:

can you post the schematic diagram somewere and link to it ?

 

I'm going to try to get the board software to let me upload three PDFs... a schematic, a parts layout, and a detail page of specific circuits.  They're all overlapping scans of the same page but they're finally under 200 KB.

Attached Files


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#13 Nords

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Posted 24 April 2010 - 08:48 PM

Parts layout:

Attached Files


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#14 Nords

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Posted 24 April 2010 - 08:49 PM

Circuit details:

Attached Files


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#15 RegUS_PatOff

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Posted 24 April 2010 - 09:19 PM

To power the Display, you need 120v at the L.V.T. (Low Voltage Transformer) on the Display Board.

Neutral is direct from the Noise Filter

and L is from the Noise Filter and through the Cavity T.C.O. & the MGT T.C.O. (Magnetron TCO)

 

 

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#16 Nords

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Posted 25 April 2010 - 07:51 AM

[user=3641]RegUS_PatOff[/user] wrote:

To power the Display, you need 120v at the L.V.T. (Low Voltage Transformer) on the Display Board.

Neutral is direct from the Noise Filter

and L is from the Noise Filter and through the Cavity T.C.O. & the MGT T.C.O. (Magnetron TCO)

 

 

Ah, thank you.  AC never occurred to me; I thought the card was getting rectified DC from somewhere "not shown" on the diagram.

I had the same symptoms with two different control cards so it's probably not the transformer itself.  I'll have to trace the wires and see where the voltage disappears.

They mounted that TCO about as far away from the rest of the components as they could get...

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#17 RegUS_PatOff

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Posted 25 April 2010 - 08:02 AM

[user=34694]Nords[/user] wrote:

... They mounted that TCO about as far away from the rest of the components as they could get...

to sense the temperatures of the Magnetron and the MW cavity ..
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#18 Nords

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Posted 28 April 2010 - 01:48 PM

Thank you again for all the help!  I've learned more about microwaves & electronics troubleshooting in the last week than I have in the last decade.

I traced the visible wiring behind the control panel and verified that indeed a couple AC volts were getting to the control board's transformer.  I just couldn't get a multimeter probe into the top or side of the microwave at the connectors for the cavity & magnetron TCOs, let alone eyeball them.  I finally stopped being timid and pulled the top/side panels off the microwave.  This time I hope the troubleshooting really does end when charring is found.

Lucky there wasn't a mist of hot oil or a puddle of grease in the vicinity when the cavity TCO died, and I'm surprised it didn't trip a breaker.  It didn't just scorch out-- it also melted the lug cover and an inch of the wire's insulation.  I'm going to have to see if I have enough slack to cut out the damage and crimp on a new lug or else solder in a couple inches of new wire & lug.

The photo shows the cavity TCO lying next to the scorched/melted lug cover with the retaining bracket in the background to the right.

The magnetron TCO looks nice & clean.  Once I replace the cavity TCO I bet the control board & display will power right up.  After I put everything back together I'll have a spare matching microwave to store for the next breakdown.

After I fix the damage and while I still have the microwave torn apart, is there any other maintenance or inspecting I should do?

Eventually I'll put something witty here.

#19 Nords

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Posted 05 May 2010 - 07:12 AM

Soldered in a new connector/cover and put in the new cavity thermostat.  The old control panel powered right up and everything's working fine. 

Parts:  $8.50
Troubleshooting apprenticeship:  Priceless.

Actually I guess you can put a price on that.  Thanks again for the troubleshooting help, and a love offering to the United Samurai Beer Fund is on the way!

Eventually I'll put something witty here.

#20 appl.tech.29501

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Posted 05 May 2010 - 02:21 PM

Yep that'll do it every time. I had one yesterday with the same exact issue except mine wasn't melted visibly. You actually had to unplug the wire to see the connector melted internally. Thing drove me nuts...kept makin and breakin contact until I finally caught it
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