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Old 02-16-2015, 04:55 PM   #1
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Default Inverter or Magnatron - which one is bad

Maybe someone here is able to answer this, or at least improve my odds of choosing the right part one 1st.

This is not a low end "normal" microwave with the big transformer and capacitor. The wife took it to a repair shop 1st. They say discontinued and no parts. That's fine for them, but there are new parts available on ebay for me. Equivalent replacement is in the $500+ range, so I would prefer to fix it for $100 or so.

My magnatron looks ok, physically. But, I do show about 160ohms from either terminal to the frame. Terminal to terminal is ok at about .8 ohms. Frame to terminal should be a lot higher, I believe.

However, I suspect my inverter is not putting out any voltage. I am not sure of the actual voltage I should see. When I check it with my DVM(600v max), it shows zero. I have an electric fence checker which reads to 5,000V, and it does not show any indication of voltage going into the magnatron. I believe that the inverter puts out a DC voltage in the range of 2,000VDC. Not sure if that follows the definition of an "inverter" though. Any reason the electric fence checker would not read the voltage? Any other way to check besides a wet finger?

I definitely have 120v going to the inverter board, and there are no errors shown on the display, just no heat generated.


I can get the inverter OR the magnatron for about $100 each. I don't really want to buy them both, if only one is bad. I think that my magnatron is bad, and the inverter board is good(considering no errors) But how could I double check the inverter board voltage?

Thanks

Last edited by acro; 02-16-2015 at 05:01 PM.
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Old 02-16-2015, 05:09 PM   #2
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The magnetron is not technically an "electronic" device in the common sense, it's a "cross field" device that all by itself is actually very simple; just a machined chunk of copper with cavities and an electrode that heats up. That makes it difficult to be "bad", about the only thing that can happen to it is the electrode can burn out, and that is uncommon. If I had to take a chance not knowing, I'd go for the power supply.
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Old 02-16-2015, 05:15 PM   #3
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But the fact that it shows 160ohms to the frame? Everyone says that should be very high resistance or open. Both those are the transformer/capacitor types too.

I did not find any blown fuses either. I would think I would likely find a blown fuse with the low resistance.
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Old 02-16-2015, 05:35 PM   #4
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I have no idea, but maybe one of these guys knows the answer

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Old 02-16-2015, 05:48 PM   #5
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We fix a lot of microwaves here at the prison and trust me magnetrons go bad. You should see an open reading from either terminal to the frame. I hate to be obvious and ask though, did you discharge the capacitor and disconnect the wiries from the magnetron terminals? I wouldn't ask but I have seen it.
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Old 02-16-2015, 07:17 PM   #6
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There is not a large capacitor to discharge in this one. But i did unhook the wires before I tested the magnatron.
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Old 02-16-2015, 09:36 PM   #7
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Posting the make and model number would be helpful. You might also be able to find a service manual online and there are appliance forums where they could help give a better diagnosis too.

A thought: when you tested the inverter board, did you have the thermal cutout connected? Most magnetrons I have seen have a thermal cutout on them...round, flat black device about the diameter of a nickle with two wires 180 deg apart.

Last edited by guest; 02-16-2015 at 09:38 PM.
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Old 02-16-2015, 09:55 PM   #8
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Quote:
Originally Posted by acro View Post
But the fact that it shows 160ohms to the frame? Everyone says that should be very high resistance or open. Both those are the transformer/capacitor types too.

I did not find any blown fuses either. I would think I would likely find a blown fuse with the low resistance.
Some microwave ovens will switch in a resistor across the high power circuit to ground it out as another safety feature,
it is usually done via the door switchs.

So where are you measuring this 160 ohms and under what conditions ?
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Old 02-17-2015, 09:08 AM   #9
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It is a Whirlpool GH6178XPB-0. 1100W inverter type.
I had searched previously for a repair manual, but came up empty. Installation and operation only.

Thermal cutout is connected and good.
There is no change in the resistance related to the status of the door switch.

Inverter board has 120v supply, which is good, and another small 3 pin connector, which is likely what controls the power level. When in operation, the inverter board does make a slight noise like part of it is cycling on and off. I can make the interface display show an error by disconnecting any of the connectors on the inverter board. However, I do not think I tried running it with one of the magnatron wires disconnected. However, I am skeptical that the control board could read the magnatron status through the inverter board.



This morning, I was actually getting about 138 ohms from the magnatron terminal to the frame - same meter, same range setting, though ambient temp was lower. Tested with the supply wires to the magnatron disconnected. Actually, connected or not, the resistance was the same.

Last edited by acro; 02-17-2015 at 09:22 AM.
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Old 02-17-2015, 10:54 PM   #10
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I found 9/10 times it was the magnetron or the fuse. If you replace the magenetron we generally ordered/replaced the high voltage diode at the same time
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Old 02-18-2015, 03:24 AM   #11
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Wiring diagram should be in the machine.

Also, never test one of these plugged in. A micro magnetron runs at 3000 volts, and yes unlike a small television the current produced is very lethal.


If this is an inverter oven Id guess bad board.

If this is just a basic run of the mill you have 4 components:

1. Diode (which can be ohmed)
2. transformer (rarely do these fail)
3. capacitor which can fail
4. magnetron which is not unknown to fail. They are nothing more than giant diode, a vintage LED to be precise where light is radio energy instead. The issue is that the filament can break causing a not heat.
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Old 02-18-2015, 03:26 AM   #12
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Quote:
Originally Posted by Kiknads View Post
I found 9/10 times it was the magnetron or the fuse. If you replace the magenetron we generally ordered/replaced the high voltage diode at the same time

If the fuse is blown I always replace all 3 or 4 door switches. The switches are wired so if the primary, secondary or monitor switch fail an interlock switch shorts out the fuse. Unless you know its from something else.
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Old 02-18-2015, 03:56 AM   #13
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Ok, doubting is good. I looked up your model and it is an inverter type.

Parts list (if it helps)

http://www.whirlpool.com/digitalasse...%208205167.pdf

The inverter board:

http://www.repairclinic.com/PartDeta...b5d652026&mr=1

Whirlpool used to have all their stuff (wiring diagrams, tech sheets, service manuals ect) on line for free but they pulled the plug on that 6 years ago. You now have to be an appliance dealer or certified tech to access those

Some of the old stuff is still floating around but none of it looks related to inverter types, closet I could find, 2/3s through is service:

http://adcxns1.whirlpool.com/Service...0Microwave.pdf


If you can post the tech sheet (it will be in the unit) I can guess whats wrong, but my money is on the inverter.

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Old 02-18-2015, 06:04 AM   #14
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Thanks. I will look the unit over for the tech sheet.
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Old 02-18-2015, 06:11 AM   #15
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Quote:
Originally Posted by acro View Post
Thanks. I will look the unit over for the tech sheet.
Sounds good
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Old 02-25-2015, 08:38 PM   #16
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Guess I have been a little sidetracked.

Procedure for checking the inverter is to measure the the input current. It says if it is more than 0.5A the inverter is "probably" OK. And to check the magnatron.

Quote:
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When in operation, the inverter board does make a slight noise like part of it is cycling on and off.
So, this noise coincides with a fluctuation of the amperage draw when attempting to heat something. I see an amperage that cycles from about 5.5A to a little below 1A.
The spec sheet does not mention any cycling.

Then, back to the magnatron. The spec sheet clearly states that the normal resistance is infinite from the filament to the chassis.
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Old 02-25-2015, 09:09 PM   #17
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Correct. Filament to chassis will be infinite, and filament (the two terminals) will read continuity as stated in the tech sheet.
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Old 02-25-2015, 09:33 PM   #18
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Is the cycling of the current "normal"?

Would a bad magnatron cause the cycling of the current draw?
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Old 02-25-2015, 09:40 PM   #19
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Quote:
Originally Posted by acro View Post
Is the cycling of the current "normal"?

Would a bad magnatron cause the cycling of the current draw?
To the first, I'd say no, because one reason microwaves use an inverter is to provide adjustable power levels to the magnetron...unlike non-inverter ovens which literally switch the magnetron on and off in a duty cycle rate to average out the power to the desired percentage. (Which makes them suck at even cooking because the food keeps getting hit with full power, then cools down, then full power, etc.)

To the second, I'd say yes it is cycling on and off to prevent the inverter from burning out. The peak current of 5.5a is way too low for an operating oven, it should be double that or more (depending in the power level selected.)

All this tells me the inverter is most likely ok, the magnetron is toast. I'd change the magnetron.
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Old 02-25-2015, 09:44 PM   #20
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Quote:
Originally Posted by mxslick View Post
To the first, I'd say no, because one reason microwaves use an inverter is to provide adjustable power levels to the magnetron...unlike non-inverter ovens which literally switch the magnetron on and off in a duty cycle rate to average out the power to the desired percentage. (Which makes them suck at even cooking because the food keeps getting hit with full power, then cools down, then full power, etc.)

To the second, I'd say yes it is cycling on and off to prevent the inverter from burning out. The peak current of 5.5a is way too low for an operating oven, it should be double that or more (depending in the power level selected.)

All this tells me the inverter is most likely ok, the magnetron is toast. I'd change the magnetron.

That's pretty much what i thought, but I did not feel like typing that much.

Thanks
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