• ** REMEMBER! **The microwave can still shock you even unplugged!!

    ALWAYS discharge the high-voltage capacitor first if you even think your hands will come close to any HIGH VOLTAGE components.

    Jeff mentions this: Anything in the high voltage ( magnetron, capacitor, diode, wires to and from ):
    ...Use a metal ( not the shiny chrome type ) screw driver with a insulated handle to short across ( touch both at the same time ) the terminals of the high voltage capacitor to discharge it.

    From Jeff's site: http://www.applianceaid.com/component-testing.php

    Jake
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FIXED KitchenAid Superba KEMC308KSS0 Microwave oven acting like we're having repeated powerline brown-outs, but we're not.

pauli_wrench

Premium Member
Joined
Dec 10, 2021
Messages
11
Location
New York
Model Number
KEMC308KSS0
Brand
KitchenAid
Age
More than 10 years
Just started happening. Microwave turns on & all works properly... LED panel & controls. Heats food well. BUT... a few days ago it started acting "funny". While running, all of a sudden you will hear it like it's starting to powerdown briefly and the bulb inside will dim, but then within a second it returns to normal. On a full power 4 minute cycle, it can happen several different times randomly and intermittently, or none at all. LED panel does NOT flicker. NO unusual noises. NO error codes. Also, none of the lights in the house flicker, so it is NOT a power company issue, like a real brown out. Unit was built in 2002. Had the magnetron & control panel replaced about 6 years ago.

My question... is it a failing/shorting magnetron, something in the control panel, a failing door switch or something failing in the microwave's power supply? Has anyone seen this happen in KitchenAid Microwaves? Is it a common thing? Is there a TSB on it?

I realize the oven is old, but replacement is NOT an option as it is the only combo unit where the microwave door opens downward, and not from the side. My wife LOVES this feature. Trust me...we've looked.

Thank you for any help.
 

rickgburton

Appliance Tech - Moderator
Staff member
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Messages
44,788
Location
Murray, Utah 84107, United States
I've seen that happen when the mag tube starts wearing out. Put it on a low power setting and listen for the tube to turn on and off see if it happens then.
 

pauli_wrench

Premium Member
Joined
Dec 10, 2021
Messages
11
Location
New York
I've seen that happen when the mag tube starts wearing out. Put it on a low power setting and listen for the tube to turn on and off see if it happens then.
Thanks! Will give it a try, see what happens and let you know.
Just one quick add'l query... While it does almost sound like the magnetron is going on/off/on/off, like it was on a lower power setting (although we only ever really use it on high power, and that is where this problem is happening), the kicker difference here is that the microwave light dims to almost being off before going back to normal; something that doesn't happen when the tube is at a lower setting. Someone elsewhere mentioned that it could be the big 1.05 mfd capacitor starting to fail. Do you agree/disagree/possible? Thanks again for the prompt response!
 

rickgburton

Appliance Tech - Moderator
Staff member
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Messages
44,788
Location
Murray, Utah 84107, United States
It sounds like the mag tube but it could be any one of the high side components. Low voltage to the transformer will cause that power draw. Here's how to check the high side components. Unplug the machine and drain the cap:
C-HV Testing.jpg

C-HV-Capacitor Test.jpg
C-HV-Diode Test.jpg
 

pauli_wrench

Premium Member
Joined
Dec 10, 2021
Messages
11
Location
New York
OK... did what you said in your first post, "Put it on a low power setting and listen for the tube to turn on and off see if it happens then."
So I put the power on "5" and heated a cup of water. I heard the magnetron cycle properly (on/off/on/off) as it should, but it was ONLY during the "on" part of the cycle that the microwave light and LED panel would dim and it sounded like the unit was about to stall briefly and then it would suddenly recover. During the "off" part of the magnetron cycle, everything looks normal (no dimming, no stalling).

So, as an electrical engineer & ham radio operator myself (I have experience & knowledge in this stuff, high voltage & all), I will pull the unit sometime this week and perform the diagnostics you recommend. I'll let you know what happens.

Thank You again for helping me keep my beloved old appliance running. You're giving my wife hope and making her very happy!
 

rickgburton

Appliance Tech - Moderator
Staff member
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Messages
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You can check the power output of the mag tube as long as the oven cavity is clean and line voltage at the outlet is 120 VAC. Use a measuring cup and fill it with 16 oz of tap water. Use a thermometer to stir the water until the temperature stabilizes. Place the cup in the center of the oven and turn it on HIGH for 60 seconds. Stir the water with the thermometer and record the maximum temperature reached then subtract the cold temperature. There should be a 20° -38° rise in temperature.
 

pauli_wrench

Premium Member
Joined
Dec 10, 2021
Messages
11
Location
New York
WOW! That's awesome. Never knew that. Just so happens I have a digital IR/probe thermometer. Will give it a try. Thank you for the additional info!
 

pauli_wrench

Premium Member
Joined
Dec 10, 2021
Messages
11
Location
New York
Update...

Did the test with heating the water as specified. Worked OK. Passed test.

So, as recommended by someone else, I replaced the big 1.15mfd cap connected to the Toshiba magnetron and checked the adjacent diode... (OK). Still having same problem. When microwave runs, all the lighting (inside the microwave oven AND the below oven (I manually turned it on to watch without the bottom oven actually being on) and led panel lights intermittently dim and the magnetron sounds like it's about to stall out, then everything recovers and runs normally till the next "brown-out". Happening randomly... can happen several times in a row for a few second each, or may only happen once over a period of 3-4 min run time.

So I'm thinking of checking voltages coming off the two transformers: High Voltage Transformer Part # WP4375286 and Control Transformer Part # WP9760589. I don't think it's the lighting transformer (Transformer Part # WP9760588), as if this transformer were bad, only the lights would dim and nothing else.

It's also NOT a bad connection as I've pulled on the various discrete component and PCB connectors and CANNOT reproduce the problem.

And, finally, the microwave continues to heat food and liquids well. The last time the magnetron went and had to be replaced 9 years ago, nothing in the microwave was heating.

So this is where I am. That is why I want to check voltages and test points as the unit runs to see if there are voltage fluctuations that follow the intermittent symptoms of the dimming of the unit. This is why I am looking for the unit's Service Manual. I understand electronics, components and can read wiring diagrams and schematics.

Otherwise, I'm just going to have to wait until the microwave goes completely "caput" and then diagnose what component is then dead.
 

Jake

Appliance Tech - Admin
Staff member
Joined
Aug 24, 2004
Messages
132,593
Location
Redmond, Oregon
@rickgburton has been sick for the majority of January and I don't know when he will return, hopefully soon!

I have no training or experience working on Microwaves at all.

I posted the service manual in your other thread:

Jake
 

pauli_wrench

Premium Member
Joined
Dec 10, 2021
Messages
11
Location
New York
RESOLVED!!! Turns out that upon further inspection, two of the wires in the molex connector on the high side (220V from the wall), were singed/charred and they MELTED the connector and the electrical contacts were touching intermittently. Must have been a bad capacitor that was causing a LOT more current to be drawn than should have been. So after changing the microwave capacitor, I replaced the melted Molex connector, made all new connections and voila... it WORKS!!!!
Just glad it didn't catch fire! And don't know why the circuit breaker didn't trip for such a current draw.
 

Jake

Appliance Tech - Admin
Staff member
Joined
Aug 24, 2004
Messages
132,593
Location
Redmond, Oregon
Excellent, glad to hear your back in business.(y)

When Rick returns he will be happy you said that too!

Thanks!

Here's the microwave capacitor for your model, in case others need it too:
Capacitor WP4375020


Note to others: ALWAYS discharge that capacitor FIRST:

Jake
 
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