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Kenmore (Samsung) microwave oven 401.80089700 runs but does not heat

Kenmore microwave

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#1 Nords

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Posted 02 June 2012 - 06:47 PM

I'm back again with a different problem in a different unit of the same model.

We have two of this model of microwave oven because one of them is mounted over a matching gas range at our rental property. When one microwave breaks, we put in the good one and bring the broken one home to troubleshoot. In an archived thread from 2009 (http://appliantology...180089700-dead/ ) you guys helped me fix a bad magnetron TCO. That microwave oven is running fine again.

Now the other microwave is broken, and this time it's kicking my okole. It's just under three years old.

I started with the diagnostic guide at http://fixitnow.com/...agnostic-guide/ and got to "Microwave seems to run but does not heat". I used that block and the component tests (http://fixitnow.com/...components.html ) plus the Kenmore/Samsung tech sheet included with the microwave.

The magnetron's terminals had a resistance of less than an ohm and infinite resistance to ground. I haven't pulled it out of the oven to check its seal.

The capacitor seems good-- both terminal-to-terminal and terminal-to-case resistances are infinite. I have a cheap Radio Shack digital multimeter so I might not have been able to see the "Momentarily indicates several ohms and then gradually returns to infinite". I just got "OL" on the display.

The control board? Hard to tell. That sentence refers to the magnetron control board with the noise filter, right? It looks fine-- no scorches or discoloration, no ugly capacitors, both fuses still OK. I'm able to program all the microwave's features and make the display do whatever it's designed to do. I'm not sure how to tell whether the actual magnetron control board is bad.

I checked the rectifier's resistance with my cheap multimeter and got "infinite" both times. So I used the 9v battery tip (http://fixitnow.com/...microwave-oven/ ) to get 0 VDC and 5 VDC. I didn't get the 7 VDC indicated in the chart. Is 5 VDC considered "pass" or "fail"?

Finally I checked the transformer. As near as my multimeter could tell I had 0.5 ohms on the primary winding and exactly 129 ohms on the secondary. (My microwave's tech sheet specifies 129 ohms.) The filament winding resistance was zero and the windings to ground were both zero.

When the microwave is running, there's a small buzzing noise. It's not the internal cooling fan. It could be the transformer or the magnetron, but it's a slight noise. The magnetron seems to build up a static charge when it's running, so I'm reluctant to get my ear as close as it would need to be to tell the difference.

My tech sheet has a switch chart for the door switches. All three checked infinite resistance when they were supposed to be and zero resistance when they were supposed to be.

All the thermostats/temperature cutouts look OK... nothing charred or melted this time. I didn't put a multimeter on each & every one of them, but I could.

I've attached scans of the schematic, circuits, & parts. Same as last time.

Should I go through the "divide & conquer method" on this page (http://appliantology...conquer-method/ ), or is it something else?

Thanks for reading through this. What's left? What am I missing?
Attached File  Kenmore Samsung 401-80089700 microwave circuits.pdf   136.33KB   4 downloadsAttached File  Kenmore Samsung 401-80089700 microwave parts.pdf   186.5KB   3 downloadsAttached File  Kenmore Samsung 401-80089700 microwave schematic.pdf   149.08KB   5 downloads
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#2 RegUS_PatOff

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Posted 02 June 2012 - 07:23 PM

... The capacitor seems good ... not have been able to see the "Momentarily indicates several ohms and then gradually returns to infinite".

1) try your digital meter on a "higher" OHMs scale, or find someone with an analog meter
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#3 Nords

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Posted 03 June 2012 - 09:38 PM

1) try your digital meter on a "higher" OHMs scale, or find someone with an analog meter

Thanks, I'll try that.

I'd hoped that it would be a lot easier to find the broken component. Other than the control board, I'm stumped. Any other ideas to check while I'm taking another look at the capacitor?
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#4 Nords

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Posted 20 June 2012 - 09:38 PM

Back for a fresh look.

I set my digital multimeter on its max resistance scale and saw the capacitor "Momentarily indicates several ohms and then gradually returns to infinite". So I think the capacitor is good.

I re-checked my readings on the transformer, magnetron, and rectifier. All of those passed, too, and they all look fine. No telltale scorches. I pulled the magnetron out of the chassis and the seal looks intact around the emitter. No corrosion or burn marks.

The door latches appear to have very little play in them. If I move the door a millimeter or two, they click and deactivate the power circuit. When I shut the door again, I'm able to restart the microwave. Everything seems to operate just fine (including the lamp, the turntable, and the fan). It just isn't heating water.

At this point it seems as if I'm down into the weeds of "defective power relay or primary control board". I don't seem to have any way to test those, either, and there's no blatant melted capacitors or scorch marks either. If I start replacing power relays or the PCB then I'd just be randomly searching with my credit card.

What's the next step? Are there any other troubleshooting procedures that I could try, or should I just surrender this appliance to a local repair center for a specialist with advanced tools?
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#5 RegUS_PatOff

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Posted 20 June 2012 - 10:33 PM

How's the (4) Thermostats ?
# 6
# 19
# 43
# 82

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#6 Nords

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Posted 21 June 2012 - 12:11 AM

Well, our other microwave oven of this model had a slagged cavity TCO (#6). That one took me a long time to find, but once I found it there was no doubt that it was the problem.

But this microwave's thermostats all look fine. They're all clean and the wiring connectors fit tight. The cavity TCO (#6), the magnetron TCO (#19), and the bottom TCO (#43? #82?) with the black wires all indicate zero resistance-- and on my circuit diagram it looks like they're normally that way. The hood TCO (#82? #43?) with the red wires is infinite resistance, and that's the way it's normally supposed to be too.
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#7 Budget Appliance Repair

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Posted 21 June 2012 - 06:51 AM

Sounds to me like a bad mag tube, they can test OK electrically and still be bad.
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#8 RussTech

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Posted 21 June 2012 - 02:56 PM

I'd go with a bad maggie also

#9 Nords

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Posted 21 June 2012 - 03:11 PM

That's the hard part of troubleshooting without symptoms-- if I buy a new magnetron and there's a bad relay on the control board, then I pretty much end up with two magnetrons and still have a broken microwave.

When I pulled the magnetron out of the chassis to check its seal, the metal wall behind the magnetron (the outside of the interior wall of the oven) had little flecks of metal on it. Tiny little shards, smaller than the head of a pin, maybe a dozen or so. They were sticking to the wall, and they had little scorch marks around them as if they'd been hot when they landed, but they brushed right off. The magnetron case & cooling fins also had some of those little metal flecks on it, as if they'd become magnetized and stuck to the magnetron. But there's no telling if something melted and spluttered on the exterior of the magnetron and the oven wall, or if that was just the mess left by the Samsung robot on the assembly line.

Same "problem" with the control board-- it looks clean. No obvious blown capacitors or melted relays or scorched circuit paths.

Between the price for a new control board and a new magnetron, it's probably pretty close to the price for a new microwave.

Of course I could always pull the other microwave out of the tenant's house and swap parts between the two microwaves until I get the symptoms to move. Maybe I'd try that if the tenants were out of the house for a few days' vacation, but otherwise I think that'd be asking a lot from them.
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#10 RegUS_PatOff

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Posted 21 June 2012 - 03:14 PM

Is there any "high voltage" when discharging the Cap ?
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#11 Chat_in_FL

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Posted 22 June 2012 - 04:51 AM

That's the hard part of troubleshooting without symptoms-- if I buy a new magnetron and there's a bad relay on the control board, then I pretty much end up with two magnetrons and still have a broken microwave.


Check voltage on the input/primary side of the high-voltage transformer. If it has voltage when heating, then your relay on the board is good.
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#12 Nords

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Posted 22 June 2012 - 02:11 PM

Is there any "high voltage" when discharging the Cap ?

Check voltage on the input/primary side of the high-voltage transformer. If it has voltage when heating, then your relay on the board is good.

Oh, boy. Energized troubleshooting. Haven't tried any of that yet, and I've even waited out the capacitor for 30 minutes before grounding it. I can see why these techniques don't show up in the manufacturer's flowcharts.

Looks like I should see AC voltage to ground at the wire coming out of the high-voltage transformer's fuse and going to the high-voltage transformer. Looks like I should see AC voltage coming out of the wire from the primary interlock switch, but I already know the interlock's working because fan motor runs. And if the capacitor has a charge on it, then I guess that also tells me the control board and the transformer and the rectifier are doing their jobs.

Which would make me a lot more confident about buying a new magnetron.

Let me go take the readings.
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#13 Nords

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Posted 22 June 2012 - 05:10 PM

Thanks for the advice! That was educational and I think I made a lot of progress. I'm learning far more about microwave oven troubleshooting than I probably should know.

I get nothing when I try to discharge the capacitor. When I run the microwave at its full power setting for a couple minutes, and then stick my biggest slot-head screwdriver into the capacitor's positive terminal, and then ground the screwdriver blade to the chassis-- there's nothing. No sparks, no noise, nothing.

I'd stick a multimeter on the capacitor's positive terminal to see if it's charging, but my cheap Radio Shack electronic multimeter is only rated for 500 VDC. I don't think I can move fast enough to avoid a mishap.

But I don't think it's the capacitor's problem. When I plug in the microwave and check the AC line voltage to the transformer's primary terminal (the black wire coming through the primary interlock switch), it reads 120 VAC (so I know my multimeter is working). Before I started the microwave, I checked the voltage at the other two terminals-- the blue wire coming out of the high voltage transformer fuse read 8 VAC and the red wire going into the transformer's primary read 4 VAC. When I programmed the microwave to cook for a minute and pushed the "Start" button, both of those voltages went to 2 VAC as soon as I pushed the button. On my multimeter's 200 VAC scale (the lowest scale it offers), that's probably the same as zero.

So I think the transformer is not getting any voltage from the primary control board. I measured 120 VAC going into the noise filter and 120 VAC coming out to the control board, and both 20-amp fuses have continuity.

The PCB is part #58 on the diagram, part number RAS-SM7MGV-04. The Repair Clinic website linked from this forum doesn't list it with the microwave but it's under that part number as "Unknown part type, Item #1612512, OEM, $52." Our local appliance parts store can't order it from Sears or from Samsung. To my surprise, Sears sells the control board for only $35. Even more surprising, shipping to Hawaii is less than $30. I think you've shown me enough troubleshooting that I'm willing to make that bet.

I tried to look up the magnetron, but it's "not available" on Repair Clinic's site. Sears' website won't even tell me the price. The shipping weight & size specs look right, but I have to call their toll-free technician to try to put the order through. So if the magnetron is bad then I might not be able to replace it.

Did I misinterpret anything? Anything else I should look at before I order the part?

Thanks again for all the help. I appreciate having a better idea of the problem.
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#14 Nords

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Posted 24 June 2012 - 03:28 PM

Ordered the control board today. We'll see if it really gets here in a week.
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#15 Nords

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Posted 05 July 2012 - 09:26 PM

I put in the new PCB, but the symptoms have not changed.

I'm going to go through the live voltage checks again to see if there was more than one problem, but otherwise I think I've just created a spare-parts locker for the other microwave of this model.
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