Microwave Oven PCB circuit is blown up

Discussion in 'Technical Repair' started by M Samim, Sep 24, 2016.

  1. M Samim

    Thread Starter New Member

    Sep 24, 2016
    6
    1
    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:
    [​IMG]
    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.
    [​IMG]
    Back view of low voltage transformer

    [​IMG]
    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

    [​IMG]
    Top view of PCB

    [​IMG]
    Schematic diagram
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,733
    759
    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.
     
    M Samim and #12 like this.
  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,765
    925
    Two of those electrolytics are bulging and show signs of heating.
    Replace all the can type capacitors whether they check good or not.
     
    #12 likes this.
  4. M Samim

    Thread Starter New Member

    Sep 24, 2016
    6
    1
    @R!f@@
    Thank you for your quick reply.
    I will check and report.

    @Kermit2
    Thanks for the reply and by all capacitors, do you mean all the low voltage transformer capacitors?
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,733
    759
    better to check all the electrolyte's in the supply
     
  6. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
    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.
     
  7. M Samim

    Thread Starter New Member

    Sep 24, 2016
    6
    1
    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.
     
  8. M Samim

    Thread Starter New Member

    Sep 24, 2016
    6
    1
    Problem solved. D1 and D4 were shorted. No problem with other components.
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    Is the common connection of D1 and D4 connected to AC return ( neutral )?
     
  10. M Samim

    Thread Starter New Member

    Sep 24, 2016
    6
    1
    D1 is connected to L (Line) and D4 to N (Neutral).
     
  11. wayneh

    Expert

    Sep 9, 2010
    12,086
    3,024
    So it's working again?
     
  12. M Samim

    Thread Starter New Member

    Sep 24, 2016
    6
    1
    Yes it's working again.
     
    wayneh likes this.
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