Whirlpool Combi Microwave oven

Hi,

Following Samuel Goldwasser's Great(!) Notes on the Troubleshooting and Repair of Microwave Ovens I tested:
* Defective interlock switches or misaligned door. There are four switches, three use only the two terminals to make a contact. The fourth uses all three terminals, to break and make contact. Switch 4 was a bit dodgy so I swapped it with Switch 1:
o Switch 1: 1-2 closed: 0.0 ohm, open: infinite (swapped with Switch 4)
1-4 closed: 0.0 ohm, open: infinite
o Switch 2: 1-2 closed: 0.0 ohm, open: infinite
1-4 closed: 0.0 ohm, open: infinite
o Switch 3: 1-2 closed: 0.0 ohm, open: infinite
1-4 closed: 0.0 ohm, open: infinite
o Switch 4: 1-2 closed: 0.2 ohm, open: infinite (swapped with Switch 1)
1-4 closed: 0.0 ohm, open: infinite
o The door is not visibly misaligned.

* Shorted HV capacitor.
o Tested: ~10M ohm, but not open -> good?

* Shorted HV diode
o Tested: 6.5V -> good.

* Defective HV transformer.
o Primary: 2.5 ohm -> good enough??? (should be 0.2 to 0.5 ohm, 0.2 typical)
o White to frame: ~ 100 ohm -> if this is HV, then OK
o Red to red: ~0 ohm -> if this is filament, then OK

Would the 2.5 ohm of the primary winding definitely be way too high and cause the fuse to blow when I start the cook cycle? I would expect the opposite to be true, however:

After putting everything back together but leaving the transformer primary winding disconnected I switched it on. The conventional oven worked, and heated. When I pressed the microwave button the fan worked and the fuse didn't blow . So I assumed the transformer was faulty. I then reconnected the transformer and tested the microwave function. It worked for three seconds and then blew the fuse, 'confirming'(?) that the transformer was faulty. I replaced the fuse, disconnected the transformer again and tested again. The conventional oven worked again, and when I pressed the microwave button the fan worked, but the fuse didn't blow. 'Confirming'(?) again that the transformer was faulty. Then I put the microwave oven back in the kitchen cabinet and demonstrated that it worked, partially, but the fuse blew...... suggesting again that the switches are (may be) faulty.

Am I overlooking something still?

Regards,

Rouke

 

[Jon Elson]

grbakker@gmail.com wrote:

* Defective HV transformer.
o Primary: 2.5 ohm -> good enough??? (should be 0.2 to 0.5 ohm, 0.2
typical)

Surprising, but a HIGH reistance would not blow the fuse. What I'm
guessing is there is a short in the SECONDARY, and it draws too much current
when energized due to that. Thus, the delayed blow of the fuse.

You should also check the primary to the transformer frame. Must be
infinite resistance. Anything less indicates short to frame.

Jon

 

[Ron D.]

Many microwave issues including the one you describe can also be the magnetron.

I'd go through those tests first. If it lost heating capacity gradually or sometimes would shut off too early, I would definitely suspect the magnetron.

The interlock switches I believe basically work where two allow power to pass and the other is across the line in an open position, so the fuse blows immediately.

 

[Ron D.]

Also 0.2 to 0.5 ohms is VERY difficult to measure without a 4 or 5 wire ohmeter or a 4 terminal measurement. Your test leads and internal meter connections can EASILY have 1 ohm of resistance each.

A way to measure low resistance values is to measure the current through the device and the voltage across it. You can add series resistance and measure the voltage across the known resistor and convert that to a current.

If I'm right, your meter is not capable of measuring 0.2 to 0.5 ohms.

 

On Tuesday, March 10, 2015 at 1:34:03 PM UTC+13, Ron D. wrote:

Many microwave issues including the one you describe can also be the magnetron.

I'd go through those tests first. If it lost heating capacity gradually or sometimes would shut off too early, I would definitely suspect the magnetron.

I have not noticed a gradual loss in heating capacity. The problem with the microwave function happened suddenly after the microwave oven came out of storage after about 2 months and carried back into the kitchen.

> The interlock switches I believe basically work where two allow power to pass and the other is across the line in an open position, so the fuse blows immediately.

And the fuse doesn't blow immediately, after all my searching etc. the microwave function worked for 2 - 3 seconds.

 

On Tuesday, March 10, 2015 at 1:35:29 PM UTC+13, Ron D. wrote:

Also 0.2 to 0.5 ohms is VERY difficult to measure without a 4 or 5 wire ohmeter or a 4 terminal measurement. Your test leads and internal meter connections can EASILY have 1 ohm of resistance each.

A way to measure low resistance values is to measure the current through the device and the voltage across it. You can add series resistance and measure the voltage across the known resistor and convert that to a current.

If I'm right, your meter is not capable of measuring 0.2 to 0.5 ohms.

Thanks Ron, but would the meter be correct enough to see a difference between 0.5 or 2.5? Or are both my measurements likely unreliable? If I adjust the ohm meter to 0 ohm while shorting the test pins, then a subsequent reading of 2.5 ohm should be in addition to the internal resistance of the leads etc, and therefore reasonably accurate? It's a pretty robust analogue meter.

 

On Tuesday, March 10, 2015 at 7:54:47 AM UTC+13, Jon Elson wrote:

rouke bakker wrote:



* Defective HV transformer.
o Primary: 2.5 ohm -> good enough??? (should be 0.2 to 0.5 ohm, 0.2
typical)
Surprising, but a HIGH reistance would not blow the fuse. What I'm
guessing is there is a short in the SECONDARY, and it draws too much current
when energized due to that. Thus, the delayed blow of the fuse.

You should also check the primary to the transformer frame. Must be
infinite resistance. Anything less indicates short to frame.

Jon

Thanks Jon, there was this typical smell of electric components overheating after the 2 - 3 second running of the microwave function, and I imagined I felt the transformer to be slightly warm to the touch afterwards. But then why would the next fuse also blow with the primary of the transformer (both terminal) disconnected and taped up?

 

[Leif Neland]

Den 10-03-2015, skrev Ron D.:

Many microwave issues including the one you describe can also be the
magnetron.

I'd go through those tests first. If it lost heating capacity gradually or
sometimes would shut off too early, I would definitely suspect the magnetron.

Btw, it is relativily easy to measure the actual heating capacity of a
microwave oven.

Measure the time to heat a glass of water to boiling point.

200g water * 4.184 J/(gK) = 836.8 J/K
285g glass * 0.840 J/(gK) = 239.4 J/K
Total heat capacity: 1076.2 J/K

Temperature rise 100C-15C = 85K
Energy used = 1076.2 J/K * 85K = 91477 J
Time: 2min 24 sec = 144 sec
Power = 91477 J / 144 sec = 802.4 J/s = 802.4 Watt

This is in SI-units.

If your measurements are based on the phalanges of ancient english
royalty, your constants may differ.

--
Je suis Charlie

 

On Wednesday, March 11, 2015 at 12:35:56 AM UTC+13, Leif Neland wrote:

Den 10-03-2015, skrev Ron D.:
Many microwave issues including the one you describe can also be the
magnetron.

I'd go through those tests first. If it lost heating capacity gradually or
sometimes would shut off too early, I would definitely suspect the magnetron.

Btw, it is relativily easy to measure the actual heating capacity of a
microwave oven.

Measure the time to heat a glass of water to boiling point.

200g water * 4.184 J/(gK) = 836.8 J/K
285g glass * 0.840 J/(gK) = 239.4 J/K
Total heat capacity: 1076.2 J/K

Temperature rise 100C-15C = 85K
Energy used = 1076.2 J/K * 85K = 91477 J
Time: 2min 24 sec = 144 sec
Power = 91477 J / 144 sec = 802.4 J/s = 802.4 Watt

This is in SI-units.

If your measurements are based on the phalanges of ancient english
royalty, your constants may differ.

--
Je suis Charlie

Thanks,

I might do that when it works again....

 

[Cydrome Leader]

grbakker@gmail.com wrote:

Hi,

Following Samuel Goldwasser's Great(!) Notes on the Troubleshooting and Repair of Microwave Ovens I tested:
* Defective interlock switches or misaligned door. There are four switches, three use only the two terminals to make a contact. The fourth uses all three terminals, to break and make contact. Switch 4 was a bit dodgy so I swapped it with Switch 1:
o Switch 1: 1-2 closed: 0.0 ohm, open: infinite (swapped with Switch 4)
1-4 closed: 0.0 ohm, open: infinite
o Switch 2: 1-2 closed: 0.0 ohm, open: infinite
1-4 closed: 0.0 ohm, open: infinite
o Switch 3: 1-2 closed: 0.0 ohm, open: infinite
1-4 closed: 0.0 ohm, open: infinite
o Switch 4: 1-2 closed: 0.2 ohm, open: infinite (swapped with Switch 1)
1-4 closed: 0.0 ohm, open: infinite
o The door is not visibly misaligned.

* Shorted HV capacitor.
o Tested: ~10M ohm, but not open -> good?

* Shorted HV diode
o Tested: 6.5V -> good.

* Defective HV transformer.
o Primary: 2.5 ohm -> good enough??? (should be 0.2 to 0.5 ohm, 0.2 typical)
o White to frame: ~ 100 ohm -> if this is HV, then OK
o Red to red: ~0 ohm -> if this is filament, then OK

Would the 2.5 ohm of the primary winding definitely be way too high and cause the fuse to blow when I start the cook cycle? I would expect the opposite to be true, however:

After putting everything back together but leaving the transformer primary winding disconnected I switched it on. The conventional oven worked, and heated. When I pressed the microwave button the fan worked and the fuse didn't blow . So I assumed the transformer was faulty. I then reconnected the transformer and tested the microwave function. It worked for three seconds and then blew the fuse, 'confirming'(?) that the transformer was faulty. I replaced the fuse, disconnected the transformer again and tested again. The conventional oven worked again, and when I pressed the microwave button the fan worked, but the fuse didn't blow. 'Confirming'(?) again that the transformer was faulty. Then I put the microwave oven back in the kitchen cabinet and demonstrated that it worked, partially, but the fuse blew...... suggesting again that the switches are (may be) faulty.

Am I overlooking something still?

I'd be wary of the cap or diode. Can you power it up again with just the magnetron disconnnected to
see what happens? If it doesn't blow a fuse your magnetron is what is bad.

 

On Wednesday, March 11, 2015 at 8:46:18 AM UTC+13, Cydrome Leader wrote:

Rouke wrote:
Hi,

Following Samuel Goldwasser's Great(!) Notes on the Troubleshooting and Repair of Microwave Ovens I tested:
* Defective interlock switches or misaligned door. There are four switches, three use only the two terminals to make a contact. The fourth uses all three terminals, to break and make contact. Switch 4 was a bit dodgy so I swapped it with Switch 1:
o Switch 1: 1-2 closed: 0.0 ohm, open: infinite (swapped with Switch 4)
1-4 closed: 0.0 ohm, open: infinite
o Switch 2: 1-2 closed: 0.0 ohm, open: infinite
1-4 closed: 0.0 ohm, open: infinite
o Switch 3: 1-2 closed: 0.0 ohm, open: infinite
1-4 closed: 0.0 ohm, open: infinite
o Switch 4: 1-2 closed: 0.2 ohm, open: infinite (swapped with Switch 1)
1-4 closed: 0.0 ohm, open: infinite
o The door is not visibly misaligned.

* Shorted HV capacitor.
o Tested: ~10M ohm, but not open -> good?

* Shorted HV diode
o Tested: 6.5V -> good.

* Defective HV transformer.
o Primary: 2.5 ohm -> good enough??? (should be 0.2 to 0.5 ohm, 0..2 typical)
o White to frame: ~ 100 ohm -> if this is HV, then OK
o Red to red: ~0 ohm -> if this is filament, then OK

Would the 2.5 ohm of the primary winding definitely be way too high and cause the fuse to blow when I start the cook cycle? I would expect the opposite to be true, however:

After putting everything back together but leaving the transformer primary winding disconnected I switched it on. The conventional oven worked, and heated. When I pressed the microwave button the fan worked and the fuse didn't blow . So I assumed the transformer was faulty. I then reconnected the transformer and tested the microwave function. It worked for three seconds and then blew the fuse, 'confirming'(?) that the transformer was faulty. I replaced the fuse, disconnected the transformer again and tested again. The conventional oven worked again, and when I pressed the microwave button the fan worked, but the fuse didn't blow. 'Confirming'(?) again that the transformer was faulty. Then I put the microwave oven back in the kitchen cabinet and demonstrated that it worked, partially, but the fuse blew...... suggesting again that the switches are (may be) faulty.

Am I overlooking something still?

I'd be wary of the cap or diode. Can you power it up again with just the magnetron disconnnected to
see what happens? If it doesn't blow a fuse your magnetron is what is bad..

The fuse blew with the transformer disconnected, but not immediately. If it blew immediately I would have thought the door switches were at fault, but now I'm not sure.