• ** 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

Whirlpool Microwave Buzzing Loudly

pwrsrge220

Premium Member
Joined
Aug 4, 2011
Messages
18
Location
Baltimore
Model Number
MT4155SPS-2
Brand
Whirlpool
Age
1-5 years
My Whirlpool MT4155SPS-2 microwave oven has recently started to buzz loudly when operated. The sound is getting louder and more frequent. Initially it happened now and then, now it happens pretty much any time you operate the device, and it's getting louder. It's typically worst at the start, then recedes somewhat as things get going. Sometimes it seems to hesitate and just buzz at the start, sounding like it's going to go high order, at which time I shut it down and try again. So far it still heats the food sufficiently.

Not sure if I've got a simple resonance issue with loose fasteners, or it's a magnetron issue or similar. I haven't trouble-shot m-waves before, so am looking for some initial guidance. I have not had the unit apart yet for the simple reason I didn't have tamper-proof Torx bits. Just got some and ready to attack, just need a battle plan!

Thanks.
 

pwrsrge220

Premium Member
Joined
Aug 4, 2011
Messages
18
Location
Baltimore
Rick, it heats AND buzzes. Might just be my perception, but it doesn't seem to be quite as effective in heating as in the past. But yes, up to now, it still does heat whatever is put in.
 

rickgburton

Appliance Tech - Moderator
Staff member
Joined
Jul 11, 2006
Messages
35,120
Location
Murray, Utah 84107, United States
Take the cover off and see if you can isolate the sound. Split the system into two sections, low side and high side. Take the two primary wires off the transformer. They are the two wires coming from the control and supplying the transformer with 120 VAC and are usually the smaller wires. Start the microwave and see if the noise still exists. If it does then the noise is most likely a vibration somewhere like the fan motor, stirrer motor, door hinge, or anything loose. If the noise doesn't exist when the primary supply voltage is disconnected, the noise is going to be on the high side. The high side is the transformer, magnetron tube, HV capacitor, or diode. Before I walk you through any more testing, let me say, YOU'RE DEALING WITH UP TO 4000 VDC AND THE CAPACITOR WILL HOLD A CHARGE EVEN AFTER THE MICROWAVE IS UNPLUGGED. Each time you unplug the microwave now, short the two terminals on the capacitor together before removing any wires. It's more of a process of elimination now because the microwave is working. With the microwave unplugged, reconnect the primary voltage wires and short the capacitor terminals together. Remove the two wires from the magnetron tube and tape the ends to be sure they don't touch anything. Plug it back in and see if the noise exists. Unplug the microwave, short the capacitor and remove the wires from the high voltage side of the transformer. One will be going to the mag tube and one to the HV capacitor Tape them off so they don't touch anything and try again. This is not a perfect test for the transformer because if it is the transformer making the noise, it may not make the noise without a load. Let's see what you get before I keep going.
 

pwrsrge220

Premium Member
Joined
Aug 4, 2011
Messages
18
Location
Baltimore
Alright Rick, now that sounds like a battle plan! I particularly appreciate the caution, as I'm aware there are significant voltages present in these appliances even unplugged, but didn't know how to minimize the danger.

I'll try to fit this into the other fun activities ahead of me this weekend,:3: and will let you know how it goes.

Thanks much!
 

pwrsrge220

Premium Member
Joined
Aug 4, 2011
Messages
18
Location
Baltimore
OK Rick, I opened up the unit and followed your trouble-shooting steps. Looks like it's a "low side" issue. The sound was still there when the wires were removed from the front end of the xfrmer. Sounded local to the fan, so I disconnected power to the fan, and the noise was not present. Also noticed that when it makes the noise I described at the start "like it's going to go high order", the fan was not turning, and when the fan started to turn the noise changed character and was not as bad.

So, can I replace just the fan motor, or do I have to get the whole fan subassy?


BTW, I noticed that Whirlpool included a set of schematics and notes in a plastic sleeve under the cover; nice, I wasn't expecting that. :) Not being an elec-tech type though, much of it is lost on me....:(

Thanks again for the excellent assistance with this!
 

pwrsrge220

Premium Member
Joined
Aug 4, 2011
Messages
18
Location
Baltimore
Rick, I got the motor and installed it. Checked operation, all OK. Compared to the way it's been behaving, you can hardly tell it's running now it's so quiet! :)

Now, my question is, what's the failure mode of these motors? It's such a solid chunk of stuff, I don't see how it could have been vibrating so badly. Screws are tight, etc. There is some axial play in the shaft assembly, but no different than what's in the replacement item. So what went wrong?

Thanks again for all the help!
 

rickgburton

Appliance Tech - Moderator
Staff member
Joined
Jul 11, 2006
Messages
35,120
Location
Murray, Utah 84107, United States
I have no idea. I leave those kind of problems to the engineers that design them. I'm glad you got it fixed. Good job!
 
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