Hi All,

Whirlpool microwave / oven combo - cabinet mounted. Few weeks ago, microwave stopped heating. Display, timer, lights, turntable works fine.

I have seen several videos, studied the circuit diagram carefully and did troubleshooting for all the parts - switches, diode, capacitor, magnetron, fuse etc. and they all seem to be okay.

1) HV Transformer (when removed) is testing fine on it own. Less than 1 ohm across the terminals and no shorting with the body. No visual signs of problems or smell.

2) HV Transformer (terminals removed from transformer), I can see 120v coming in primary input terminals. I think this means circuit board and switches are fine.

3) When HV transformer is connected, I don't see 120 v input coming in to terminals. This might mean transformer is shorted (based on 2 above). However, I am not sure if this can be caused by something else (because transformer is testing fine on its own as I mentioned above).

At this point, I am not sure what to check next. I have spent 2-3 weeks on it and would greatly appreciate your help.

Thanks in advance.

 

Relay that powers the transformer primary?
Max.

 

Hi Max - What's the best way to test relay? Do I have to remove from the circuit board? Thanks.

 

Hi Max - What's the best way to test relay? Do I have to remove from the circuit board? Thanks.

Because you would need to test the relay with 120V on the contacts and switch a fairly high power load to make sure it is OK it would probably be better just to replace this relay.

 

Most microwave ovens theres at least one Microswitch on the door interlock, that needs to be closed circuit when door is shut. If it doesn't then the unit wont operate the relay. Check that switch.

 

Most microwave ovens theres at least one Microswitch on the door interlock, that needs to be closed circuit when door is shut. If it doesn't then the unit wont operate the relay. Check that switch.

As TS says everything else works including the turntable motor the interlock must be OK.

 

Make and model of the microwave?
Posting a link to the circuit schematic would be helpful.

One of the most frequent failures of MWO is the door interlock switch. There are numerous interlocks and over-temperature switches. All these need to be checked. Everything else might still function except the HV supply.

 

As TS says everything else works including the turntable motor the interlock must be OK.

As @debe noted some M.W. activate more than one micro switch when closing the door.
Just serviced one such type.
Max.

 

As @debe noted some M.W. activate more than one micro switch when closing the door.
Just serviced one such type.
Max.

+1. Just serviced a MWO that had three micro switches when the door is closed. One switch in particular needs to be open when the door is closed. The switch creates a short across the power supply when the door is open. (There is also a relay before the power supply that is open when the door is open.)

 

Most microwave ovens have circuit diagram on the inside case, has yours? If not make/ model searching will help...

 

Thanks for everyone's responses. Please see circuit diagrams attached. I spent lot of time on other components including all three switches, fuses, capacitor, magnetron, diode. They all pass the test (on their own).

Here is what I am confused about:

- If I disconnect HV transformer and measure RED and BLUE input primary terminals - I see 120 v. This happens even when microwave is not running. I thought relay will activate it only when microwave is on, but not sure.
- The reason I think switches, fuses and circuit before HV transformer is fine is because I see 120 v on RED/BLUE (as above).
- As soon as I connect transformer, I see 0v on RED and BLUE input terminals.
- Voltage around M-Fan (BU and GY terminals in the diagram below) is 120 v when microwave is ON, 0v when off. I am assuming this is because of relay P27-2, P27-4 (and is working as expected - I think).

 

When the microwave is on,are you getting 120V across terminals p26_2 and the interlock switch2 BU/W (high voltage transformer feed)?

 

When the microwave is on,are you getting 120V across terminals p26_2 and the interlock switch2 BU/W (high voltage transformer feed)?

No, I don't see 120 v between P26-2 (R) and BU/W on the interlock switch. However, I see 120v between P26-1 (BK) and BU/W on interlock switch.

Also, I see 120v between P27-2 (BLU) and BLU on HV transformer. Also, interlock switch shows connectivity fine.

Does that mean it's a relay problem? Are P26-1 (BK) and P26-2 (R) supposed to be connected when microwave is on?

 

I tried to short P26-1 (BK) and P26-2 (R) to see if that was the problem. But, I might have caused another issue. Now, off button on keypad doesn't work.

Appreciate your help.

 

Okay, so ignore my post about keypad issue. I think I fixed it by reinserting the ribbon. So, it's back to the original problem (microwave not heating).

To dodgydave's original question, here is what I see.

No, I don't see 120 v between P26-2 (R) and BU/W on the interlock switch. However, I see 120v between P26-1 (BK) and BU/W on interlock switch.

Also, I see 120v between P27-2 (BLU) and BLU on HV transformer. Also, interlock switch shows connectivity fine.

Does that mean it's a relay problem? Are P26-1 (BK) and P26-2 (R) supposed to be connected when microwave is on?

 

Okay, so ignore my post about keypad issue. I think I fixed it by reinserting the ribbon. So, it's back to the original problem (microwave not heating).

To dodgydave's original question, here is what I see.

No, I don't see 120 v between P26-2 (R) and BU/W on the interlock switch. However, I see 120v between P26-1 (BK) and BU/W on interlock switch.

Also, I see 120v between P27-2 (BLU) and BLU on HV transformer. Also, interlock switch shows connectivity fine.

Does that mean it's a relay problem? Are P26-1 (BK) and P26-2 (R) supposed to be connected when microwave is on?

Yes it's possible the relay is not giving out 120V on terminals p26_2 (R), you could put the magnetron feed onto Terminal P27-2 BU, ( magnrtron fan and turntable)..to prove it..

 

Yes it's possible the relay is not giving out 120V on terminals p26_2 (R), you could put the magnetron feed onto Terminal P27-2 BU, ( magnrtron fan and turntable)..to prove it..

I removed R and BU connectors from HV transformer. Now I can see 120 v between R & BU connectors/pins as well as 120v between P26-2 (R) and BU/W on the interlock switch. When I connect the pins back to transformer terminals I don't see 120 v. Does this indicate HV transformer issue?

 

I removed R and BU connectors from HV transformer. Now I can see 120 v between R & BU connectors/pins as well as 120v between P26-2 (R) and BU/W on the interlock switch. When I connect the pins back to transformer terminals I don't see 120 v. Does this indicate HV transformer issue?

No, not necessarily. It indicates a high impedance in the 120 VAC path. Rather than the transformer, try connecting a lamp (25W would be good) to those terminals. Does it light up?

 

I connected LED bulb (LED - 10w, 60w equivalent) to connectors and I see it flickering (when microwave is in on or off mode).

Thoughts?

 

Just to clarify - I meant to say light flickers constantly (no difference between microwave on or off state).

I think I could be close, but would really really appreciate if someone can help with getting to the end and provides steps from here to determine which part might be faulty.

Thanks in advance.