microwave oven no display

Discussion in 'Technical Repair' started by ruleworld, Apr 4, 2008.

  1. ruleworld

    Thread Starter Member

    Jun 9, 2006
    22
    1
    i have a panasonic microwave oven model NN-S551WF. when i power it on there is no display and microwave have no power. i opened the digital programmer control and found that voltages are not correct. the power circuit is almost like the attached file.

    at the 5v point i get 1.14 volt.
    at the 18v point i get 3.84 volt.

    i removed Q1 and ZD1 and checked with meter and they seems to be ok. the transformer seems to be good too, i get 22v ac when i remove D1.

    then why am i getting low voltages at those points? could this be fault with the digital programmer circuit?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    With power applied, but with the load removed, do you see 5.6 volts at the base of the transistor? The zener is supposed to hold that voltage there so the transistor passes 5 volts to the load. If the 18 volts is not present, then disconnect that load and see what happens.

    If you have a meter with a diode check function, it won't hurt to check D1. look at the filter cap to see if it's changed size (grown taller than the label) or has popped the vent. That would indicate a failure in the capacitor.

    If the power supply looks ok with the controller not attached, then the controller is the problem. If things come up with the 18 volt load disconnected, then that is the problem.
     
  3. ruleworld

    Thread Starter Member

    Jun 9, 2006
    22
    1
    how can i remove the load. its connected by a white connector strip(see attach pic). is there any way to remove it?

    D1 is ok. the capacitor has not changed form, seems ok from outside.

    thanks.
     
  4. techroomt

    Senior Member

    May 19, 2004
    198
    1
    you could desolder and lift the emitter leg of Q1 to isolate the load. look elsewhere to break the 18v line. which by the way, i can't see how 18v is attained from half wave rectified 20vac, 14.14v at best if adequately filtered.
     
  5. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Because you have +3.8V instead of +18V, you won't get the +5V.

    If Q1 is not hot, then it's probably not the +5V being pulled down. It wouldn't hurt to lift the emitter of Q1, just to remove/isolate the +5V. If the +18V shows up, then it is on the +5V side.

    If R1 and R2 are not burned or particularly hot, then ZD1 is probably not shorted.

    Although it could be a leaky or reversed C1 (560uF), it too would probably get hot.

    Most likely, there is a bad component or some type of short on the +18V that is pulling it down. Try looking for a hot spot, or a cold solder joint.

    12.6VAC RMS would be ~18Vpk and ~17VDC after rectification, depending on the transformer rating and loading.

    22VAC RMS would be ~31Vpk, you wouldn't get 18VDC unless it is loaded down considerably.
     
  6. ruleworld

    Thread Starter Member

    Jun 9, 2006
    22
    1
    both of you are right, yes its 22vac.

    thx for the great help. now i will try those things.
     
  7. ruleworld

    Thread Starter Member

    Jun 9, 2006
    22
    1
    i removed all the 18v line and disconnected emitter leg of Q1. then i get 5.2v there. when i connect emitter to load then voltage drops to 1v. so i think the digital programmer control has fault. if thats the case then buying a new microwave oven is probably the best option.
     
  8. ruleworld

    Thread Starter Member

    Jun 9, 2006
    22
    1
    i noticed when emitter of Q1 is connected to load it gets so hot within about 10 seconds that i can't touch it anymore. when i remove it from load then it does not get hot. i measure the transistor with meter and it seems to be ok. so does that conclude the dpc is faulty? thanks.
     
  9. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    With the emitter lifted, connect a 100 ohm 1/2W from the lifted emitter to GND. If it has +5V on the emitter, then it's working. The circuit is okay, but the output is effectively shorted. Remove 100 ohm resistor.

    With the emitter connected, see if you can find any other hot components (other than the xstr), do this repeatedly but only for short periods.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Look at the top of C1. In the photo, it looks like the top has a hole blown out. Is that what has happened? (that indicates a bad capacitor)

    When the emitter of Q1 lifted, what voltage do you get on it's collector?

    All that aside, if you were getting 5.2v with no load, and 1v and a hot Q1 with it's emitter connected, you have a problem (shorted component) somewhere along the 5V bus. You could try isolating it further by snipping various 5V bus jumpers, like the one in the lower right of the photo. It might be just a shorted cap or diode somewhere.
     
  11. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    It's possible that with the excessive current draw from the +5V, that the ripple voltage on the cap increased significantly and caused it to blow.
     
  12. Pawan Yadav

    New Member

    Jul 7, 2016
    1
    0
    If it is inveter technology then such problem is normal and there is no solutions!
     
  13. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    Wow !.
    As ur first post you found a 8 year old thread to reply.
    Putting aside the old thread issue, how did you figured inverter ones have no solutions.
    I repair those inverter or not without a problem.
     
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