SNES Mainboard Repair No Sound [Serial: UP17372657]

Discussion in 'Repair, Restoration, Conservation and Preservation' started by Shane McRetro, Sep 2, 2016.

  1. Shane McRetro

    Shane McRetro Blast Processed Since 199X

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    IMG_0969.JPG

    Three years ago... I came across a SNES 1CHIP with no sound. It had a physical hole blown through the S-MIX chip. That unit has since been given away long ago. The board in question was a SNSP-CPU-1CHIP-01. Today I have finally righted that wrong.

    To get sound working I bypassed S-MIX by tapping directly into the 6379A (U6). To do this I tied pin 1 (right audio) of U6 to pin 12 of the multi-AV out and pin 4 (left audio) of U6 to pin 11 of the multi-AV out.

    However, to my surprise, the audio seems loud and clear! Any suggestions on what circuitry should be added? An amp of some sort? Caps? Given that I've just bypassed S-MIX altogether, surely there's something about to go wrong, right? Nothing I do should ever work this easily... any advice welcomed! :)

    Also while we are talking SNES and SFC, has anyone with a 1CHIP came across this issue personally or known of someone who has? Out of the five SNES units that have been through my hands, 3 have been 1CHIP and two of them had a no sound issue that wasn't capacitor related! Curious to say the least.
     
  2. l_oliveira

    l_oliveira Officer at Arms

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    S-MIX is there exactly as a mixing chip. There are audio inputs on the cartridge connector and EXT port at the bottom.

    Edit: Btw, in the case of NTSC machines, what cause the S-MIX chip to blow is inverted polarity on the power supply. PAL units have AC input just like the NES did. But NTSC ones follow the Famicom/ Super Famicom design. lol

    Same as how people blow SEGA/MEGA-CDs by connecting improper power supplies.
     
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  3. Shane McRetro

    Shane McRetro Blast Processed Since 199X

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    Wonderful! So there's no real disadvantage doing it this way or the other? S-MIX is purely a mixer? No amp? Good to hear! :)

    I wonder who managed to blow up my old S-MIX on the PAL hardware... must have taken someone with a lot of determination and skill at destroying things :) Thanks for your help as always! :D
     
  4. borti4938

    borti4938 Spirited Member

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    hey,

    I made a pinout for the S-MIX a few weeks ago. Here is the part which is point of interest:

    • Pin 3: Audio right in
    • Pin 4: Ext. Audio right in
    • Pin 5: Cartslot Audio right in
    • Pin 7: Audio right out
    • Pin 8: Audio left out
    • Pin 10: Cartslot Audio left in
    • Pin 11: Ext. Audio left in
    • Pin 12: Audio left in
    If I were you I would simply remove the S-MIX and connect either...
    • Pad 3 with Pad 7 and Pad 12 with Pad 8, or...
    • leave your wires as they are
    I would prefer the first option. But with your choice you could also remove C12 and C13!

    Please remind that the DAC is probably not desinged to drive a quite long wire like the cable you connect to your MultiAV. The DAC might be destroyed over the a certain time of that load.

    If you have the time you might want to experiment with a sound mixing circuit build as like as in the 3Chip-SNES (see picture attached). there are some passive components around the S-MIX. It makes sense to 'reuse' them if possible. (some PCB scans
     

    Attached Files:

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  5. Shane McRetro

    Shane McRetro Blast Processed Since 199X

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    Thank you for replying, that is magnificent!
    It's too bad the forums can't handle me liking a post more than once! :cool:
    While I don't have any time at the moment, I'll add it onto the growing to do list to tinker about with.
    Thanks again! :)
     
  6. Bad_Ad84

    Bad_Ad84 Keyboard Error: Press F1 to Continue

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    Can you also post the full pin out? as it can go in my notes.

    Thanks!
     
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  7. borti4938

    borti4938 Spirited Member

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    I'll look for it and will let you know.
     
  8. borti4938

    borti4938 Spirited Member

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    It took me a while but here is the complete pinout :)
    • Pin 1: 12V (C62 to GND)
      (For PAL-Systems: after rectifier and over DA13 (see note) and C59 to Vin, which could be AC)
    • Pin 2: connected to C16 +; ?Preset for right Audio?
    • Pin 3: Audio right in
    • Pin 4: Ext. Audio right in
    • Pin 5: Cartslot Audio right in
    • Pin 6: GND
    • Pin 7: Audio right out
    • Pin 8: Audio left out
    • Pin 9: Vcc (+5V)
    • Pin 10: Cartslot Audio left in
    • Pin 11: Ext. Audio left in
    • Pin 12: Audio left in
    • Pin 13: connected over R21 to S-APU Pin 91
    • Pin 14: connected to C17 +; ?Preset for left Audio?
    Note for DA13: this is interesting in the sense that the other side of DA13 is connected over R28 to the MultiAV pin 3 ;)
     
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