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Digital piano won't power ON

Discussion in 'Troubleshooting and Repair' started by Alektron, Jan 18, 2018.

  1. Alektron

    Alektron

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    Jan 14, 2018
    View attachment 38707 Hello

    I've got a Technics SX-PR902C digital piano that will not power ON, only the RED LED lights up, when the ON button is pressed, but the display won't.It quit in the middle of a show.

    I did not find a Service Manual online for it yet, I'm still searching. The SX-PR902C and SX-PR602M share the same User Manual. I know it's not he same piano, but I found a Service Manual for the SX-PR602M here:
    https://elektrotanya.com/technics_sx-pr602_sx-pr602m.pdf/download.html

    So far I checked the fuse, it was OK, then the transformer, input 110V AC OK, then I checked the 3 output AC wires the blue, black, blue (see photo). Between each black and blue, DMM measures 38V AC.

    I don't see any fried components, although I noticed these 2 giant caps with the tops bulged (see photo), but I'm not sure if that is bulging or not, maybe that's how they normally look like.



    What should I do next? IMG_0100.JPG IMG_0101.JPG IMG_0102.JPG IMG_0106.JPG IMG_0104.JPG
     
    Last edited: Jan 18, 2018
  2. Alektron

    Alektron

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    Jan 14, 2018
    Some more pics of the Capacitors and the circuit board. FYI, in the lower right corner those 6 black wires on the PCB are the speaker wires. I also checked those 8 diodes they are OK. IMG_0108.JPG IMG_0109.JPG IMG_0114.JPG IMG_0115.JPG IMG_0116.JPG IMG_0117.JPG
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Thanks for the photos.

    Take a close look at the bottom of those large capacitors and tell me if the stuff around the base looks like glue, or something that's leaked out.

    I don't like the look of those capacitors. I'd probably replace them even if the piano hadn't failed.

    I would measure them with an ESR meter, but that's not something I guess you have :)
     
  4. Alektron

    Alektron

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    Jan 14, 2018
    My LCR-T4 ESR is on the way, I won't get it till February, but I have a DMM that does Capacitor testing. Looks like there is a glue underneath those caps for support, other components seem to have it too.
     

    Attached Files:

  5. Alektron

    Alektron

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    Jan 14, 2018
    In the meantime I took some measurements with my DMM at the 14 pin connector, that connects the two boards. (see photo) IMG_0118.JPG
     
  6. kellys_eye

    kellys_eye

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    Jun 25, 2010
    No 5V supply? Can you show the digital board in more detail - there must be a 5V regulator somewhere!

    I note in an earlier pic that there is a backup battery on the digital board - does that measure ok?
     
  7. Alektron

    Alektron

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    Jan 14, 2018
    IMG_0125.JPG IMG_0128.JPG IMG_0129.JPG IMG_0130.JPG IMG_0131.JPG IMG_0132.JPG IMG_0133.JPG
     
  8. Alektron

    Alektron

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    Jan 14, 2018
    I couldn't remove that battery, I had to measure it in circuit with the power OFF. DMM showed 378 mV.
     
  9. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Battery is flat - should read around 3.6V (or maybe 3V), certainly not 'mV'. It needs to be changed - as you;ve discovered it's soldered into place but you can get replacements from the usual suppliers (Digikey etc).
     
    Alektron likes this.
  10. Alektron

    Alektron

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    Jan 14, 2018
    I see. So before we proceed any further I would need to replace that battery? Is this battery used for the internal memory storage after the unit is turned off?

    Thank you Kelly's eye.
     
  11. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Yes, that is it's usual function. You can't discount any sort of 'problem' without first ensuing the battery is ok as the digital part may invoke many other states based on the backed-up parameters it's supposed to retain. Your switch-on problem may be only ONE issue controlled by it.
     
  12. Alektron

    Alektron

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    Jan 14, 2018
    I'll remove that Main board today, and see what type of battery it is, hopefully there is a number on it. I was trying to find it in the service manual that I've got for the SX-PR602m (lower model), hopefully the two digital pianos would have the battery as a common part, but I can't find it inside the document, or I'm just not looking in the right place.
     
  13. Alektron

    Alektron

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    Jan 14, 2018
    " CMOS (complementary metal-oxide-semiconductor) is the term usually used to describe the small amount of memory on a computer motherboard that stores the BIOS settings. " quote from this website.
    https://www.lifewire.com/what-is-cmos-2625826

    Also called Real Time Clock, (RTC battery). Wow, I'm learning something new every day.:)
     
  14. shrtrnd

    shrtrnd

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    Jan 15, 2010
    Batteries can explode when exposed to excessive heat.
    Many batteries have the tabs spot-welded on, and the other end of the tab is soldered to the board.
    Are you trying to desolder the battery at the wrong location?
     
  15. Alektron

    Alektron

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    Jan 14, 2018
    Battery came off leaving one leg behind. Now that leg that was left behind, gave me difficulty, the solder needed a lot of heat in order to melt, I guess this tin has a high melting point. Never encountered anything like it.

    Battery is an: M GC5.5V 1.0F It measured 300 mV on the bench.
    Actually it's a super capacitor.

    IMG_0136.JPG IMG_0137.JPG IMG_0138.JPG IMG_0141.JPG
     
    Last edited: Jan 19, 2018
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    That may well be a multi layer board, and in any case one leg was connected to a fairly large ground plane.

    There's probably nothing special about the solder, the board was just sucking heat from your soldering iron.
     
  17. Alektron

    Alektron

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    Jan 14, 2018
    That's something new, I thought I was doing something wrong. Thank You Steve.
     
  18. 73's de Edd

    73's de Edd

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    Sir Alektron . . . . .

    GREAT SHADES of the very-very last of the late 1980's . . . . . . here we are having some state of the art digital design.

    Battery came off leaving one leg behind.

    DRATS ! I've even got some MORE bad news .

    F.I.O. . . . . . for all you battery hounds . . . in actuality that "battery" is being a 1 farad @5.5 VDC super capacitor for short time retention of the units dynamic and static RAMs.

    With the unit using a linear supply, with a power transformer whose mother was a microwave transformer and dad was an AC line transformer, I wouldn't be worrying too much about the state of those two large MAIN power filter capacitors either.

    73's de Edd
     
    Last edited: Jan 19, 2018
    Alektron likes this.
  19. Alektron

    Alektron

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    Jan 14, 2018
    I realized that after I pulled it, that it was a super Cap, but what do you suggest De Edd?
     
    Last edited: Jan 19, 2018
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Get a new one and replace it. Be VERY sure to get the polarity correct.

    Also I tend to agree with Edd. I didn't spot that it was a linear power supply. There's a far smaller chance of those caps bring bad
     
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