Hi all,

I really hope you guys can help resolve this mystery. Thanks in advance for your consideration.

Recently, my microwave/oven combination (Etna CM751ZT/E01) stopped working. As the display wouldn't even turn on, I opened it up to check for any faulty fuses or thermal cutoffs.


I did the following:

• Tested the slow blow fuse (250V 15A) at the noise filter for continuity
  
• Tested the other fuse between the condensator and the high voltage transformer for continuity
  
• Tested the 3 thermal cutoffs for continuity
  
  
• Tested the doorlock switches for continuity
• Followed the power supply circuit to the main control board

As all fuses, thermal cutoffs and switches checked out and the main control board receives power, I went on to check the main control board. I checked:

1. The different capacitors for continuity
2. I tested all relais with an external power source

And then I found something strange: the IC switcher (Power integrations TNY276PN) had a small crack in its housing.


I ordered a new one, replaced it and: it worked! The display turned on and I could navigate through the touchscreen interface. However...

When I start any program, the fans start up as if it begins operation. Unfortunately, after about 3 seconds, the machine cuts the power and immediately starts up again. It seems like the moment when power is supposed to be supplied to a main component (heater, magnetron etc), it resets the entire thing. There is no short circuit of any kind since the fuses stay intact and the ground fault circuit breaker also doesnt flip out.

To consider:

1. Both the microwave function as the oven function trigger the same response (although in the oven function it takes slightly longer to trigger the reset)
2. Temperature or time settings dont affect the response
3. The new IC I put in is also a Power Integrations TNY276PN, however the rest of the code on the IC is different. I assumed these are indicators for production date and location and have nothing to do with the funcionality of the IC, am I correct?
4. I read online that the magnetron could leak radiation which leads the control board to quit. So, I disconnected the magnetron and started an oven program. Unfortunately, the same reset occured. Therefore, I ruled out the magnetron to be the problem.

I am really hoping for any new insights/strategies to test out. I am not ready to give up when I have the feeling I am close to fixing it.

 

Power Integrations | TNY276PN - Datasheet PDF & Tech Specs (datasheets.com)
TNY276PN + Resistance 22 Ohm, 3 Watts - Repair Kit AEG, Jacuzzi, Bauknecht | eBay
Have you checked the 22 ohm resistor? Yours could be a different value.
Please keep us informed with your progress.
Good luck...........

 

Here is a circuit diagram for this switchmode chip from a datasheet, see if yours is similar ..are there components on the track side?