GE built in Microwave / Oven - Microwave does not heat and oven works fine

Microwave Repair
1

Brand: GE
Model Number: JT3800SH3SS
Main Symptom: Microwave not heating
What happens & when: place item in MW enter settings time / power and start, light goes on platter turns fan runs, no heating.

Error Code (if any): No error codes displayed
Parts or tests already tried: replaced magnatron, replaced power control board, replaced relay board, fuse is good, only things left are transformer and master board for both oven and MW. I tested transformer from a utube video and seems like it should be good. Have a hard time replacing master board since oven is working fine. Which should I replace next?
Photos / video link:

2

There should be a microswitch (like the one in the link) on door latch at the right and it will have a brown wire with a red stripe on one terminal and a white wire on the other. Does this switch have continuity when the door is closed? WB24X10047 Door Interlock Switch | Genuine Ge OEM In Stock

3

Yes the door sensors both have continuity.

Seems I missed noting that I tested the capacitor and it was good.

4

If there is not a hum from the transformer when the microwave is in operation, the most likely cause of failure is the relay control if there is not a damaged wire. WB27X32625 Relay Board | Genuine Ge OEM In Stock

5

I thought so too, that is why I replaced the relay board.

Wires all look good too.

6

could the temperature sensor on the Magnatron go bad and cause this?

7

Since the turntable is operating, the thermostats aren’t the problem. The parts that could stop heating while still allowing the turntable to run include the wiring, primary interlock microswitch, relay control, transformer, capacitor, diode, and the magnetron itself. When you start the microwave, does it sound like the magnetron is trying to power up, or is it quiet except for the fan and turntable motor? That difference can help narrow down which component has failed.

8

There is no sound of the magnetron trying to start.

9

With the microwave shell removed and the microswitches accessed, the upper monitor microswitch on the right side can be tested for voltage. With a multimeter set to VAC, is there 120 Volts AC when the microwave is started and testing from the red wire to the white wire with the multimeter?

10

Sorry it too a couple days, wife needed the oven.

I show no voltage at the top switch on the right side with or without the MW running.

11

If you test from the red wire to the metal frame of the microwave, is there 120 Volts AC? If there is, either the white wire is damaged between it and the relay control, or the relay control is faulty.

12

Question, this relay control is on the relay board?

I’ll check the voltage next chance to pull from wall.

13

Correct. The relay control and the relay board are one and the same.

14

There is 120 vac from the red wire.

Re-checked the micro-switch, with door open there is continuity with the door closed there is none.

15

This particular microswitch is a safety that should be open when the door is closed. This was a way to identify that there is indeed 120 Volts AC getting through the other microswitch. Since it is, only one of two things can be happening since there was not 120 Volts AC when the microwave was in operation. Either the white wire has a break in it somewhere between the transformer and the relay control, or the relay control is faulty.

16

I replaced the relay board already, I’ll be looking at the wire next.

17

Looking at the wiring diagram, can the humidity sensor cause this?

18

The humidity sensor is used to adjust time when using an auto cook cycle like a popcorn selection. It should not prevent the unit from heating. The thermistor on this model is used specifically for the convection heater and the heating element, not for the magnetron.

19

Thank you for the info. Just checked continuity on the white wire which is good.

I think that means my test of the transformer was not done properly.

Do you have testing instructions for the transformer? It looks like that is next in line?

Thanks

20

If there was not 120 Volts AC when testing from the red wire to the white wire at the switch while the microwave was in operation, it means something is stopping the transformer from having a complete circuit to operate. The resistance checks for the transformer are as follows: The primary windings should have .5 Ohms of resistance when testing from the red wired terminal to the white wired terminal. The secondary winding should measure 87 Ohms when tested from the built-in red wire to chassis ground. The magnetron harness should measure .2 Ohms.