Microwave Oven PCB circuit is blown up

Thread Starter

M Samim

Joined Sep 24, 2016
Dear experts

As of a sudden our microwave oven stopped powering on. I removed the cover, checked the fuses, thermal cutouts and switches, all were fine but there was no power feeding into high voltage transformer. I removed the PCB board and noticed that the circuit was blown out in two places as shown in picture below:

At points 1 and 2 electricity is being fed into low voltage transformer which is located at the other side of the board. I soldered some wires for reconnecting the lost circuit and placed the PCB to check how it performs. As I plugged in the power cord to the main, the circuit blown again at the back of low voltage transformer PCB.

Back view of low voltage transformer

Front view of low voltage transformer

The red line is where the circuit is blown up between two components. Now I am not sure where is the problem, is it due to a bad diode or a malfunctioning relay or something else? I really appreciate any help in this regard.

Below are some pictures for more clarification on PCB

Top view of PCB

Schematic diagram


Joined Apr 2, 2009
Welcome to AAC

You need a DMM at diode range for the test.

1. You need to check D1 to D4 for short.
2. L1 and L2 for open circuit.
3. C1 and C2 for leaky capacitance.
4. U1 for short circuit.

PS.. you cannot check U1 for short since you are not familiar with it. You need to check for short between pin5 and pin8.
If not short then most of the time it is OK but better to replace it.

I have fixed dozens of these.
Most of the time Diode, inductor and IC are blown due to leaky capacitors.


Joined Feb 5, 2010
Two of those electrolytics are bulging and show signs of heating.
Replace all the can type capacitors whether they check good or not.


Joined Nov 30, 2010
Ooh! Melty!
Power line POP.
It can be done, but you must find the short first.
All that board does is fire relays, so you're in good hands with the people already involved in this Thread.
and yes, I can see a bulgy capacitor.

Thread Starter

M Samim

Joined Sep 24, 2016
The electrolytic are not bulged. They may look this way in the photos, but inspecting them they look fine from out side. I will check them to see if they are faulty.