microwave no heat

Tonyr1084

Joined Sep 24, 2015
7,998
is RY2 opening and closing? And just because you find zero ohms when it's closed doesn't mean it's conducting the full current needed for the MOT. If door sensors are faulty then you can set a time but when you hit start it won't start or count down. If everything in the system seems to be working correctly then either the relay can measure zero ohms but be bad - or you have a bad wire, plug or connector.

I'll go back to my suggestion of testing whether you have 110VAC on both terminals of the relay with a load attached. Like suggested, disconnect the MOT and rig up a test lamp. If the test lamp goes on and off with the timer control then you may have a bad wire from the relay to the MOT. I can't zero it in any closer than that. Unless your "Cooking Power Level" is set to zero (if possible) or 1. Meaning it will be on 10% of the time.

Stranger things have happened. I was given a TV because it was haunted. You could turn on then 15 minutes it shut itself off. When I got it I checked the sleep timer. It was set for 15 minutes. Check your power level setting.
 
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Tonyr1084

Joined Sep 24, 2015
7,998
three relays inside PCB are all good.
It's the big black relay that controls the MOT. I'm sure you know that. The other two likely control the light or fan. Whichever - there looks to be a MOSFET by the three capacitors near the bottom of the PCB. It will control the fan or the light, whichever isn't controlled by the relays. Nevertheless, the two white relays are not the problem.
 

michael8

Joined Jan 11, 2015
428
Another switch just above the MOT appears as if it would short the power.

That's the last chance safety check -- short the power input to the MOT if the door is open and thus blow the fuse.
 

Thread Starter

shawdou

Joined Feb 8, 2021
14
is RY2 opening and closing? And just because you find zero ohms when it's closed doesn't mean it's conducting the full current needed for the MOT. If door sensors are faulty then you can set a time but when you hit start it won't start or count down. If everything in the system seems to be working correctly then either the relay can measure zero ohms but be bad - or you have a bad wire, plug or connector.

I'll go back to my suggestion of testing whether you have 110VAC on both terminals of the relay with a load attached. Like suggested, disconnect the MOT and rig up a test lamp. If the test lamp goes on and off with the timer control then you may have a bad wire from the relay to the MOT. I can't zero it in any closer than that. Unless your "Cooking Power Level" is set to zero (if possible) or 1. Meaning it will be on 10% of the time.

Stranger things have happened. I was given a TV because it was haunted. It would turn on for 15 minutes then shut itself off. When I got it I checked the sleep timer. It was set for 15 minutes. Check your power level setti
is RY2 opening and closing? And just because you find zero ohms when it's closed doesn't mean it's conducting the full current needed for the MOT. If door sensors are faulty then you can set a time but when you hit start it won't start or count down. If everything in the system seems to be working correctly then either the relay can measure zero ohms but be bad - or you have a bad wire, plug or connector.

I'll go back to my suggestion of testing whether you have 110VAC on both terminals of the relay with a load attached. Like suggested, disconnect the MOT and rig up a test lamp. If the test lamp goes on and off with the timer control then you may have a bad wire from the relay to the MOT. I can't zero it in any closer than that. Unless your "Cooking Power Level" is set to zero (if possible) or 1. Meaning it will be on 10% of the time.

Stranger things have happened. I was given a TV because it was haunted. You could turn on then 15 minutes it shut itself off. When I got it I checked the sleep timer. It was set for 15 minutes. Check your power level setting.
Thanks, I will make a test lamp to check it out.
 

Thread Starter

shawdou

Joined Feb 8, 2021
14
It's the big black relay that controls the MOT. I'm sure you know that. The other two likely control the light or fan. Whichever - there looks to be a MOSFET by the three capacitors near the bottom of the PCB. It will control the fan or the light, whichever isn't controlled by the relays. Nevertheless, the two white relays are not the problem.
Yes, one big black one, two whites. At the first I thought the one white close to big black was bad. I desoldered it and rechecked again. It was good and I soldered it back.
 

Thread Starter

shawdou

Joined Feb 8, 2021
14
Yes, one big black one, two whites. At the first I thought the one white close to big black was bad. I desoldered it and rechecked again. It was good and I soldered it back.
one white is for fan and the other is for turntable closed to the big black.
 

Thread Starter

shawdou

Joined Feb 8, 2021
14
After connecting a lamp to A and B of MOT input and starting microwave, a lamp is on for a very short time and went off. checked normal closed monitor switch. Its ohm resistance goes to zero in both cases, open or closes the door. Sounds like the switch is bad. Then look at two wires go into the switch. they do not look like power wires go into power relay, big black, more like control circuit wires. Is it possible the wires are overloaded and burn the switch?
 

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Tonyr1084

Joined Sep 24, 2015
7,998
Without putting my hands on it I can't say for sure what's going on. I do believe the control board is faulty. At this point you need to check the relay control coil for voltage. The control board should send a constant signal to the relay coil. If it switches on then off again after a short time then it's not the relay, it's the control board.

How much more time do you want to invest in fixing this? At some point it's going to be worth more to replace than to repair.
 

Tonyr1084

Joined Sep 24, 2015
7,998
Remove the relay. Be careful not to damage the pads or traces. Tack a lead in each point where the relay coil is connected and check for voltage across those points. Should be zero. Then turn the oven on for 20 seconds (give or take a few sec's - not important) and while the oven SHOULD be on check for low voltage. Probably around 12 to 24 volts, depending on the coil voltage. That voltage should stay on constantly when the oven is supposed to be cooking. If it's on for a short time then off - you have a bad control board.
 
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