SNES Mainboard Repair No Sound [Serial: UP17372657]

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.

 

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.

 

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!

 

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

 

Thank you for replying, that is magnificent!
It's too bad the forums can't handle me liking a post more than once!
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!

 

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

Thanks!

 

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

 

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

 

I have somewhat related problem, though it was U6 itself that presumably partially died.
I have crackling digital noise only from the right channel, so I've lifted U6 (DAC) pins 1 (R_Anlg) and 4 (L_Anlg) to verify that is was not from the S-Mix, and Indeed the noise comes from the U6 or worse, from S-APU.
Audio from other sources into the S-Mix like SD2SNES external audio (MSU1) sounds fine.

I have removed the U6 and ordered borti's replacement board from OSH Park and the components.
I really hope that it was the small DAC chip and not the S-APU.
Waiting for my order,, fingers crossed.

 

borti's DAC mod works great, but unfortunately the noise is still there, so my S-APU is dying.