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Panasonic NN-SD776S microwave Start button stopped working. I know, the first thought is a bad door switch because "everyone knows a microwave should not turn on if the door is open." I am experienced with electronics, but even the timer won't start at all. Funny thing is, I had this exact model do this exact thing just over 2 years ago - troubleshot all the hardware until I was lost, finally bought a new one from Costco (June 4/08). And now 2 years later this one craps out - a 2 year timer on the keypad? As others have said, all buttons work except the Start button - even on the simple timer function. Any help?

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  • aaron541 Aug 24, 2010

    I have a hard time believing that the power supply can cause everything to work except the Start function - I can set the time, set other functions, but even the simple kitchen timer won't work - seems like the same circuit that would run the clock just fine would run the kitchen timer. Anyway, is there any place to find a schematic for this microwave?

  • aaron541 Aug 24, 2010

    Also, how did this discussion end up in Vacuums? I'm pretty sure I selected small appliances when creating this.

  • aaron541 Aug 24, 2010

    Just for fun, and while I was messing around, I recalled that this unit has a child lock function - looking in the manual to confirm, pressing the Start button 3 consecutive times enables the child lock, pressing the Stop/Clear button 3 times disables the child lock. The Start never beeps, does not enable the child lock, and generally does nothing. haha

    2 exactly the same units, purchased 2 years apart, fail with the exact same problem after almost the same time in service. Panasonic. Ugh.

  • aaron541 Aug 24, 2010

    Quote from second last sentence of original post "...all buttons work except the Start button...". As for the model number, I'm embarrassed to say my dislexia burned me again - should be NN-SD677S - that shows up on the web. I apologize for the error.

    Boards - keypad connects via 9-conductor flex to display board (part #S-7A1AU), which connects via 17-conductor flex to the power supply and rotary encoder (for setting values) board (part #S-7A1DU). There are test points labeled with voltages, but I'm not sure what kind of tolerances the boards need (ie. TP shows 5v and I measure ~4.779).

    Truth be told I just had time to open the unit up tonight. Now that I have these front boards out I see what looks like water stains in a few areas - one being around the main flex connector of the display board (that connects to the power supply board). I can also see evidence that one transistor of the display board has been very warm - Q180, likely the power driver for the LCD so I expect it should get warm.

    Not sure where the moisture came from - the boards are behind the keypad matrix and front escutcheon. Don't think the cleaning lady dunked the whole thing in the sink. I will bring home some alcohol and acid brushes tomorrow and clean this up as a start.

    Looks like the keypad circuit feeds directly into the IC on the display board - Panasonic MN101C54CHX.

    I'll start with the cleaning, easy part, and go from there.

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Hello Aaron,

Since your microwave is controlled by logic, and the start button / timer portion is not working, I would start troubleshooting the power supply.

Get a look at the integrated circuits that are on the circuit board, and find their datasheets. From the datasheet, you can find the operating voltage. Then see what the power supply is putting out, and if it is different, find out why.

Posted on Aug 19, 2010

  • 1 more comment 
  • Sheldon Dedek Aug 24, 2010

    You indicated that the "timer wont start at all" and Start button stopped working. You didn't say that all the other buttons were working.

    Given new data, try taking a look at the scan lines of the keyboard matrix.

    I am unable to find any information, including at Panasonic web site (owners manual), for your microwave. Can you confirm that the model# is accurate? Panasonic doesn't list it in the extensive series of microwaves.

    For a schematic, the PCB part number, if present, may be very helpful in perhaps locating a partial, or whole, schematic as part of patent process. Do you have the patent # as well, is there a separate number on the PCB?

  • aaron541 Aug 24, 2010

    Quote from second last sentence of original post "...all buttons work except the Start button...". As for the model number, I'm embarrassed to say my dislexia burned me again - should be NN-SD677S - that shows up on the web. I apologize for the error.

    Boards - keypad connects via 9-conductor flex to display board (part #S-7A1AU), which connects via 17-conductor flex to the power supply and rotary encoder (for setting values) board (part #S-7A1DU). There are test points labeled with voltages, but I'm not sure what kind of tolerances the boards need (ie. TP shows 5v and I measure ~4.779).

    Truth be told I just had time to open the unit up tonight. Now that I have these front boards out I see what looks like water stains in a few areas - one being around the main flex connector of the display board (that connects to the power supply board). I can also see evidence that one transistor of the display board has been very warm - Q180, likely the power driver for the LCD so I expect it should get warm.

    Not sure where the moisture came from - the boards are behind the keypad matrix and front escutcheon. Don't think the cleaning lady dunked the whole thing in the sink. I will bring home some alcohol and acid brushes tomorrow and clean this up as a start.

    Looks like the keypad circuit feeds directly into the IC on the display board - Panasonic MN101C54CHX.

    I'll start with the cleaning, easy part, and go from there.

  • Sheldon Dedek Aug 24, 2010

    Your "water traces" could actually be flux residue. It is possible that water vapor from the oven cavity has condensed on the ribbon cable, but from your description, the cable goes -up- to the display board.

    In the manual, a demo mode is mentioned. This probably doesn't apply, but it is activated by pressing clock 3 times to turn on / off. Description indicates that power is removed from the oven for demonstration purposes. Not sure if it would disable the start button (probably not, since timer might be demonstrated).

    On the ribbon cable from key matrix, find the corresponding pins on the back side of the connector, or TP, or even via if they are used. You should be able to create a map / truth table of which line(s) change state when a button is pressed. It is possible that digital data is being sent as well. Given that there is a rotary knob, I would expect that Gray Code is being used on at least 3 of the lines, to indicate direction and increment / decrement, and a 4th line for setting clock. A function / truth table needs to be developed to understand how the key matrix is communicating to the control board.

    The power supply is giving you 4.77vdc off a 5v power supply. That is still within range of 5% (4.75min), and 10% (4.5min). Power supply isn't likely to be the issue (is there AC ripple on it?) since 5% is above the tolerance of most TTL active devices.

    The transistor that gets warm likely 1) drives the backlight for the display or 2) is a current source / sink. Since there are not any display problems, this is probably (just) within design tolerances regarding heat dissipation.

    No information, so far, has been found for the board part numbers. partstore.com lists individual components available for this microwave. Service manuals are also available for purchase, no luck on finding free to public though.


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