Microwave oven - replace or repair?

 "Live out your life in truth and justice, tolerant of those who are neither true nor just" - Marcus Aurelius

 

Our daughter loves macaroni and cheese - particularly the Marie Callender's frozen meal type. 

I got an earful recently when the microwave decided that it wasn't going to thaw out and heat a package of mac&cheese.  I was told that it was working, but not very well.  I pulled a spare microwave oven out of the shop for her to use, then tried to decide what to do about the non-working one.

As a guy with a more-than-full-time job, my first instinct was to price out a replacement.  After doing a little research, I found out that it's pretty expensive to replace this particular unit, so that pushed me in the direction of attempting a repair.  The top photo is the front of the microwave oven after I got it out on the operating table.

There are very few things that can go wrong with a microwave, and most of them are simple to test with a meter.  Even so, I didn't quite manage to fix it, as I will explain momentarily.  Below, the internals once the cover is off.

Safety first.  Ensure the microwave oven is unplugged, then discharge the high voltage capacitor.  The transformer in the oven converts 120 volt line power to 2000-3000 volts.  There is a high voltage capacitor and diode that convert the high voltage AC to DC power.  The capacitor can hold a charge for a long time, and you probably don't need to be defibrillated, so discharge the capacitor.  

Below, a test lead grounded to the chassis on one end and connected to an insulated screwdriver on the other end.  Touch the screwdriver to both capacitor leads, then connect each end of the test lead to a capacitor terminal, to completely discharge it.  Capacitors can build up a small charge on their own.

The high voltage section of the microwave oven is pretty simple, and most of it is easy to check.  The secondary (high voltage) side of the transformer - the red and white wires in the above image - can be tested for continuity with an ohmmeter.  It should read nearly zero ohms, because it's just a coil of wire. You can pull the wires off the low voltage side at the same time, and check that winding the same way.  Replace the wires if the primary (low voltage) side of the transformer tests good.

The capacitor can also be tested with an ohmmeter - it should show increasing resistance across the terminals as it builds up a small electrical charge.  It should also show infinite resistance from each terminal to the metal case.

Below:  The transformer at center takes 120 volt power (black white and red leads at left) and outputs 2000-3000 volt power (red/black and white leads).  This goes to the capacitor at the right.

The high voltage diode (inside the pink insulator in front of the capacitor) can be tested with the ohmmeter.  It should show zero ohms in one direction and infinite ohms in the other direction.  

The cavity magnetron (Below) creates the microwaves.  This should show a couple of ohms across the terminals and infinite resistance to the metal case.   These have a ring-shaped magnet.  It should be intact, with no breaks or damage.

That's about it for the high voltage side of things.  Anything wrong with the insulation will probably be obvious - high voltage arcing will be noisy and leave burn marks. 

This leaves the low-voltage section, and several easy-to-fix issues can be found here.  Door safety switches are a common problem.  This particular model has three door switches, and each one has to work for the oven to turn on the high voltage transformer and energize the magnetron. 

Below, the assembly that holds three door switches.  One switch is up top, and two are at the bottom, side by side.  Each is a different type.  You must check each one by putting your meter across the terminals, then opening and closing the oven door.  

The switch below wasn't working consistently.  We occasionally had to open and re-close the oven door to get it to start.  This switch didn't have a clicking sound when the button was pushed and released - it was squishy.
 

I also had infinite resistance in both directions on the high-voltage diode.  An appliance parts store in town carried both items.

Unfortunately the repair parts were "universal parts", and needed some modification.  The door switch terminals (bottom) were too big, so I had to grind them down with a Dremel tool. The new switch has a solid 'clicky' sound when you push and release it.

The diode came with several different connectors, which you attach as needed to fit your microwave.  The added length of the connector left the part of the high voltage end without insulation.

I have some heat-shrink insulation though, so I triple-wrapped it.  You also want to make sure you get the polarity correct, same as the diode that came out.

Besides the door switches, there are some other points of failure.  The 120 volt line fuse, below center.  If this fuse is blown, you won't even have a display or oven light, let alone be able to heat things.

The oven over-temperature switch.  This will prevent 120 volt power from reaching the high voltage transformer.

 

 

 So I replaced the door switch and the high voltage diode, and gave her a test.  It worked OK for me on the bench.  

 

I did not test the control relay (below), because that would have been a little tricky.  I would have needed to put 24 volts on the control board from an external source or plug in the microwave for live testing.  I wasn't willing to go there.

 

I put the microwave oven back together and re-installed it - and it didn't work reliably.  It works sometimes, but that's not really good enough.   Below:  The control relay that is probably the issue.

Why would the relay have gone bad?

Most microwave ovens operate at either full power or zero power.  This is similar to the operation of a standard oven - the heating elements are either powered on or off, based on the temperature inside the oven.  In a microwave oven, the magnetron (heater) is switched on and off by a timer.  For example, if you set the power level at 50%, the relay will energize the magnetron for 30 seconds, then turn it off for 30 seconds, allowing time for heat to diffuse through the food before the next heating cycle begins.

The control relay turns the magnetron on and off, and so it gets cycled a lot - particularly when the oven is at a reduced power setting.  Over the years, the contacts have likely worn and are developing burn spots, so the oven is not consistently heating up food. 

You can tell when the relay cuts in and is working though.  The relay clicks, transformer hums, the light dims, and the rotary table inside the oven slows down slightly.  The oven is 1000-1200 watts, so it's a noticeable power draw.  Likewise - you can tell when the relay clicks and nothing changes. 

The relay is an Omron G5G-1A with a 24 volt DC control coil.  Omron no longer makes these, because of course they don't!  Someone in China still does though, so I ordered a couple off Ebay for $10.  The package is supposed to arrive before year's end.  

This component will be a bit more of a pain in the butt to replace.  The relay is soldered to the control board, so the board will have to come out, the relay de-soldered, and the new relay soldered in.  I also ordered three replacement door switches (with the correct terminal size), and I will replace those next time I have the microwave oven apart.

All I can say at this point is that while the project hasn't been too expensive - less than $80 - it has been a minor waste of time so far, with not much progress to show for it.  This is not the time of year to piss away precious time off.

EDIT:

So based on a comment, there is a bit of confusion about what happens in a microwave oven circuit.  I just drew a crude (and probably slightly inaccurate diagram of how electricity runs through a microwave.

At the far left is the 120V plug.  Hot goes up through a fuse, takes a right and goes back down through the 120/24 volt transformer, then back to neutral.  So long as the line fuse is good, your display panel and clock have power at all times.

Hot also goes up to the top, and has to get through three door safety switches that ensure the door is shut.  In case one switch fails shut, you still have two other switches preventing you from getting a face full of microwaves.  The oven cannot be in over-temp, or electricity won't flow past that switch either.  The circuit drops down and the last contact that must be closed to complete the 120 volt circuit is the control relay.  So you push "start" on the microwave.  The 24 volt relay energizes, which closes contacts in the 120 volt circuit (sort of like the solenoid for a car starter).  Once all these switches are shut, we have completed the circuit, and 120 volt electricity flows through the primary side of the 120/2000 volt transformer.

On the secondary side of the transformer, 2000-3000 volt electricity flows to a Diode and Capacitor arrangement that converts the high voltage AC power to high voltage DC power.  This is done because the magnetron (far right, no label) needs high voltage DC power to produce microwaves.  

Anyway, below are most of the normal things that tend to fail: 

• Line fuse - this is usually caused by some other problem that will also need to be figured out
  
• Door safety switches not making contact, even though the door is shut.
  
• Oven over-temperature switch - probably not a common issue unless the oven has overheated.
  
• Control Relay - probably very common
  
• High voltage transformer - primary or secondary side could have an issue.
  
• High voltage capacitor - These fail over time.  Eventually there will be a problem
  
• High voltage diode - Same.
  
• Magnetron - Should be good almost forever, unless the magnet cracks.
  

Keep in mind that a bad fuse will also kill the display, so that's a smoking gun if the display is dead.

Edit 12-19-21:  We've been having a new issue with the microwave, and when I took it apart recently, I noticed an envelope attached inside.  It had this handy wiring diagram - way better than mine!