Microwave Repair

My microwave decided to stop working last week.  Everything seemed normal but it didn’t warm anything up.  It is a GE Space Saver model JVM1631.


I’ve never worked on microwaves before so I did some research on the internet and found out they’re pretty simple devices.  The Sci.Electronics.Repair FAQ has some excellent information in it and I highly suggest you check it out if you’re repairing any appliances.  Youtube also had some good videos on the topic.  I couldn’t find a schematic for my microwave but was able to find one for a JVM1450 which seems pretty similar.  I didn’t want to repost the entire picture but did hack out the important bits out which I’ve posted in the picture below.  The top is the plug that goes into the wall and receives 120VAC.  The two gaps on either side is where I’ve cut stuff out of the diagram.  In the gap, there are thermal cutouts, switches, fan motors, the control panel, etc.  All of this stuff can be ruled out though with a simple test that I’ll cover below.  The High Voltage transformer (marked H.V. Trans) bumps the line voltage up to around ~2000V.  The capacitor (marked H.V. Capacitor) stores energy during one half of  the 60Hz cycle and dumps it during the other half.   This is controlled by the High Voltage diode (marked H.V. Diode).  When the capacitor discharges, its voltage is combined with the transformers to send ~4000V-5000V to the magnetron.  The magnetron uses these high voltage pulses to generate microwaves and cook your food.   I edited the resistance and capacitance values to what I’d measured.

schematicSafety time.  Microwaves contain high voltages that will ruin your day quickly.  Be careful if you decide to work on yours.

I don’t show it in the schematic above but microwaves also contain a fuse.  The way the fuse is wired in mine, if it blew, the microwave wouldn’t do anything.  This indicates that the fuse wasn’t the problem on my microwave but I showed it here anyway.



I mentioned above that I’d cut part of the pieces out in my schematic above.  I can do this because the functions of all the thermal cut outs and switches are to keep line voltage from reaching the transformer.  If I run the microwave and check the voltage going to the transformer I can quickly rule out everything upstream of the plug.  Shown below is the side of my microwave.  The silver box in the middle top of the picture is the magnetron.  Below it is the transformer.  The black and blue/white wires supply 120V to the transformer.  To the top left, out of the picture is where the capacitor and diode are.



To see if voltage is present at the transformer, disconnect the plug and test for voltage when the microwave is running.   With the transformer disconnected, there is no high voltage being generated.  If you have line voltage here everything above it in the schematic is good.  I had 120VAC here as I should.



The next item to test is the transformer.  Transformers are simple loops of wire that are near each other.  This means you can test each loop for continuity.  If you have continuity through all the loops the transformer is probably good.  My transformer has three loops.  The loop that hooks up to the 120VAC is called the primary.  The  other two loops are secondary loops. On the schematic, the primary loop is on top and the two secondaries are on the bottom of the symbol.  The secondary winding on the left provides a low voltage to the magnetron all the time while the one on the right, combined with the capacitor, pulses the magnetron with high voltage.  With the transformer disconnected each of the loops can be checked fro continuity with a DMM.  I did this to my transformer and had continuity through each loop.  It appears the transformer is good.  Transformers are more complicated in practice and you can read more here.

The next item to test is the capacitor.  My DMM has a capacitance mode that allows me to test it.  The capacitor can shock you with high voltage if charged.  Be sure it is discharged before trying to measure it.  Disconnect the wires connected to it and test.  My DMM showed a value close to what the capacitor’s label said indicating it too was good.



The last item I can test is the high voltage diode.  Diodes are a one way valve for electricity.  So, they pass current in one direction but not in the other.  High voltage diodes are slightly different from regular diodes.  They have a higher “cut-in” voltage.  This is the voltage that is required before the diode will pass current.  As such, a regular DMM cannot provide enough voltage to “turn-on” the diode. If you try to test it with your DMM, the diode while block current in both directions leading you to falsely think it is bad.  The diode is hiding under the capacitor and must be removed to test.



To test the high voltage diode requires more voltage.  A simple 9V battery can provide this voltage.  To test the diode, create a circuit loop that has a battery, resistor, DMM, and the diode in it.  The resistor adds some resistance to the circuit to reduce the current.  Set the DMM to measure voltage and then connect everything.  When the diode is functioning with current in the correct direction, the meter will read the voltage minus the “cut-in” voltage.  Shown below is the test on the diode when current is flowing in the correct direction.  If you look at the diode carefully, you’ll see a little arrow on it that indicates the direction current flows through it.



If I reverse the direction of the diode in the circuit loop, it should not pass current.  You can see below that this is what happens.  So, this indicates that the diode is good.



This only leaves the magnetron, which I cannot really test.  I found some instructions saying you could test both prongs in the connector for continuity and then check each prong of the connector against the casing for continuity.  It should have continuity between the two prongs but net between a prong and case.  I did this and the magnetron appeared good, but I wondered about the validity of this test.  The inside of the magnetron is evacuated (under vacuum) and if that were no longer true it would still pass the test above but not work.  To really test the magnetron correctly, requires specialized equipment I don’t own.  After thinking about it some and talking with my father, we decided it had to be the magnetron.  Everything else tested fine and I couldn’t really test it correctly.  So, my dad ordered a new magnetron and it arrived here yesterday.  In the picture below is the new (left) and old (right) magnetrons.  Note the little black thing on the front, a thermal cut out, needs to be switched as the new one doesn’t come with one.



To remove the old magnetron requires removing four nuts and lowering the shelf the transformer sits on.  After this the magnetron can just barely be removed.  Then you swap the new one back in place and connect everything back up.  To see if it worked I grabbed a cup of water and put it in the microwave for a minute.  As you can see the microwave is working once more!  Hurray!



That’s the end of the section on microwave repair.  Now it is time to dig into the magnetron.  I’d noticed that there were a couple of large magnets in it and I was also curious what was inside of it.  Wikipedia has an entry on magnetrons that shows how they work and what the insides look like.  I decided to check it out for myself though.  The first thing to do is remove the metal casing off the top of it.



I thought I’d also see what was on the bottom.  They’d crimped the cap on pretty well but the grinder took care of it.  Nothing interesting here.  I clipped the two wired that allowed me to separate the magnetron from the metal casing. With it apart I could take off the two magnets (pictured at the bottom of this post).



Back to the top of the Magnetron.  Hmm I wonder what is in here?



That looks like the cavities they were talking about on Wikipedia.



A little work with a hacksaw shows the cavities.



Anyways, here’s the two magnets I was after.  They’re stacked on a 3/4″ dowel.



That concludes my post on microwave repair.  I’m happy to say I was never shocked while working on it and now have a couple more magnets stuck to my toolbox.

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