Microwave

Microwaves are one of the most commonly thrown away appliances, while also being one of the easiest to repair, thanks to the common use of interchangeable parts, as of the writing of this page. This page is for most non-inverter models manufactured from the late 1970s to modern ones. Microwaves contain high voltage, which should be discharged before working within. Caution should be used when testing components while the unit is running. High voltage runs between the transformer and ground; do not handle an unmounted transformer while the microwave is running. Doing so will result in a shock, if you touch a ground. (Don't even ask how I learned). It is always dangerous to run a microwave that is not grounded.

No Power
The most common cause for no power is a blown internal fuse. Test the fuse; if it's blown, replace it. Next, check that all the door switches work, that they are properly aligned, that all their screws are tightened, and that they all engage at the same time. The fuse is typically blown when something causes fewer than all of the switches to trigger, such as misalignment, foreign material in the latches, curious tinkering, etc.

Powers on & runs, but does not heat up food
Test all connections coming into the logic board & out to the transformer

Check all wires that supply power to the transformer & magnetron for loose connections & reconnect, burned wires which need to be replaced & oxidised connections that need to be cleaned.

Test input power of the transformer to ensure 120 volts AC is present then test output voltage to ensure between 1800 - 2800 volts AC is present.

If input voltage is not present, check the relay on the logic board to ensure power is being supplied, it is switching & outputting power to the transformer & replace if it doesn't switch or output power.

If the relay switches & no power it output, it most likely has oxidised connections within & needs to be replaced.

120 volts AC is present but 1800 - 2800 volts AC output is not or is weak
If 120 volts AC input is present & 1800 - 2800 volts AC output is not present, make sure fuse on output(if present) is not blown, if fuse is blown replace it, if the problem persists & blows the fuse again, replace the transformer.

Test the diode running from the transformer to ground & replace if necessary.

Test the high voltage capacitor connected between the transformer & ground to ensure the values are within spec & replace if necessary.

1800 - 2800 volts AC is present & microwave does not heat up food
If 1800 - 2800 volts AC output is present & the magnetron does not run, check connections between the output wires & magnetron for corrosion, test wires for continuity & clean contacts or replace wires if necessary.

1800 - 2800 volts AC output is present & all connections & wires are ok, Replace the magnetron.

Breaker trips as soon as the microwave is plugged in
Check for a short between AC input & ground as well as the condition of the AC cord.

Check the main high voltage capacitor for a short & replace if necessary.

Test the power board for continuity & ensure live, neutral & ground are independent to each other & don't short. If a short is present, replace or repair the power board.

Touch pad does not work
Ensure the ribbon cable is properly connected.

Test for continuity while the button is pressed to ensure it is registering button presses, if button presses are not registered, replace touch pad.

If the touch pad passes the test, clean the connector & repair or replace the logic board if necessary.

Light & fan stays on & platter spins(if your model has one) while the door is closed & microwave is not running
Test door switches for continuity & ensure they work, all connections between the logic board & switches are proper, all screws are tightened, switches are properly aligned & plastic casing holding them in is not cracked.

Platter does not spin
Test the wires between the synchronous motor for 120 volts AC or 12 volts DC depending on the motor.

120 volts AC or 12 volts DC is present but synchronous motor does not spin
Check connections on the synchronous motor for corrosion & replace if necessary.

120 volts AC or 12 volts DC is not present
Test wires connection 120 volts AC or 12 volts DC to synchronous for continuity & replace if necessary. Also test the door switch that switches it on for continuity as the switch is engaged & for power output & replace switch if no output is present.

120 volts AC or 12 volts DC is present but light does not work
Replace the light bulb.

120 volts AC or 12 volts DC is not present & light does not work
test connections to make sure 120 volts AC or 12 volts DC is present. If 120 volts AC or 12 volts DC is not present, test wires for continuity, check connections for corrosion & test the door switch for continuity when engaged & replace switch if continuity test fails.

Wires are burned or connections corroded
Check the connections & conditions of all wires. Wires heating up & burning as well as connections corroding due to exposure to moisture are common issues, if you find burned wires replace them with new ones. Clean corroded connections & re plug wires.

All connections & wires are ok & free of burns, damage & corrosion
Test all door switches for continuity to ensure a connection is opened or closed when the switch is engaged & replace switch(es) if necessary. Also test relays for continuity to ensure a connection is made every time it engages & replace if necessary.

Microwave shocks you when you touch it while it is running
This is caused by the high voltage en route from the transformer to ground, but not actually grounding. On the device side, test the continuity between the chassis and the ground prong of the plug at the end of the power cord. If this test fails, check all connections for the cause, eg, corrosion, cuts, and loose connections; replace the AC cord, if necessary. If these groundings are good, then test the outlet receptacle for proper grounding, including that it is not a "bootleg ground," viz, no ground-to-neutral connection (which is, in fact, illegal).