• ** 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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No heat. Turntable doesn't turn. Kenmore Countertop microwave and pizza oven, Model 721.66993800

oldandslow

Premium Member
Joined
Apr 16, 2020
Messages
4
Location
Kansas City, MO
Model Number
721.66993800
Brand
Sears Kenmore
Age
6-10 years
Hi Everyone - Thanks in advance for your help. My Kenmore Countertop microwave and pizza oven, Model 721.66993800, recently stopped heating and the turntable stopped turning. Everything else *seems* to work: Lights, touch panel, fan, and the pizza drawer heating element. I can set the cook time and it counts down, but doesn't heat the food. Over the last few weeks we also noticed it would occasionally stop after a few seconds, but would restart by pushing the Start button.

I have opened the case, discharged the capacitor, and tested many of the components. I do not see any evidence of burned components (but I can't see everything)

I'm pretty confident in my tests of these:
  • Fuse (nothing would work otherwise)
  • 3 door switches (I've diagnosed and replaced one 3 years ago)
  • Turntable Motor - removed from Microwave, attached a lamp cord, and plugged into wall outlet - Rotates slowly, as expected.
  • Also confirmed there's nothing interfering with the motor's shaft where it enters the bottom of the microwave

I'm less confident of these tests (I'm competent mechanically, but a novice at electronics)
  • Diode (using a 9v battery and a multi-meter)
  • Capacitor
  • Thermostat
  • Magnatron

Haven't figured out how to test
  • Transformer
If it were just not heating I'd suspect the diode, magnatron, and/or transformer. But neither of those should affect the operation of the turn-table, right? Does that point to the circuit board? And if so, is it possible to diagnose the circuit board (with a basic multi-meter and limited expertise)?

Just trying to narrow it down and buy the right parts. Thanks again!
 

rickgburton

Appliance Tech - Moderator
Staff member
Joined
Jul 11, 2006
Messages
43,263
Location
Murray, Utah 84107, United States
Start the microwave and check for 120 VAC on the primary side of the transformer. If the voltage is correct the problem will be one of the high voltage components. If the voltage is 0V, the HV components are most likely good. Check the door switches by opening and closing the door. I'll see if I can locate a wiring diagram. If you have yours can you post a copy here?
 

oldandslow

Premium Member
Joined
Apr 16, 2020
Messages
4
Location
Kansas City, MO
Thanks Rick! Appreciate the quick response!
Wiring diagram attached (sorry, just an iPhone pic, but hopefully it's readable)

Also attached pics of the tranformer:
How do I "check for 120VAC on the primary side"? The transformer has two wire connections on it. Will one be the primary? And what should I touch the other probe too? Chassis ground?
And (sorry, a lot of questions), will my basic little multimeter handle 120V? it's a Radio Shack 22-820 (Instructions show up to 500V, but I've never tried A/C current with it before - just being careful!) https://images.pinside.com/a/89/ba/a89ba94ee8d65a641065bb9c5dd67c3786564e4a.pdf
 

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rickgburton

Appliance Tech - Moderator
Staff member
Joined
Jul 11, 2006
Messages
43,263
Location
Murray, Utah 84107, United States
How do I "check for 120VAC on the primary side"?
The primary transformer windings needs 120 VAC to transform into 4000 VDC. However, don't worry about taking any voltage readings for now. The problem is probably going to be the primary interlock switch. When closed the L1 circuit is complete to the TT motor and the primary side of the transformer. Remove power from the machine and remove the control panel. From there you can check the door switches for continuity.

lg-micro-switch-primary-3b73362f-ap4437957_01_m.jpg
 

oldandslow

Premium Member
Joined
Apr 16, 2020
Messages
4
Location
Kansas City, MO
THANK YOU! You were right! I've made a small donation to the site :)

But I still have questions. I checked all three door switches multiple times, using the continuity setting on the multi-meter.
Each time, for each switch I pulled the wires off the switches, then checked

The upper and lower switches are "Normally Open" (NO).
With the door shut they both registered continuity (the meter made a sound, same as if I touched the probes together)
With the door open, no continuity.
All as expected (right?)

The middle switch is "Normally Closed" (NC)
With the door shut, no continuity.
With the door open, continuity.
Again, as expected (right?)

Next I bypassed the upper and lower NO switches, one at at time, with a jumper wire.
With the bottom switch bypassed, IT WORKED! Turntable + heat!

I replaced that switch with a new, spare NO switch leftover from the last time I fixed this microwave.

But.... the old switch STILL registered continuity when the button is pressed (and it clicks smartly).

So it solved the problem and I'm thrilled! But why does the old switch *seem* to be OK (continuity) but doesn't work in the microwave?? (obviously difficult to diagnose from a distance, but curious and want to learn). The only noticeable difference appears to be some heat damage on the plastic near the COM connection (see pics)

Thanks again Rick!
 

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rickgburton

Appliance Tech - Moderator
Staff member
Joined
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Messages
43,263
Location
Murray, Utah 84107, United States
but doesn't work in the microwave??
When the door is closed the latch is not pressing the button on the switch hard enough to close it. That happens when they wear out.
 

oldandslow

Premium Member
Joined
Apr 16, 2020
Messages
4
Location
Kansas City, MO
When the door is closed the latch is not pressing the button on the switch hard enough to close it. That happens when they wear out.
Thanks again Rick - It sure *looked* like the lever was pressing the switch fully closed, but clearly not. Regardless, it meant a $5 fix instead of ordering (and wasting time installing) hundred+ $$$ of magnetrons, circuit panels, and transformers...which wouldn't have solved the problem!

(y)(y)(y)
 
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