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(Topic ID: 276921)

SB-300 odd issue..

By Coyote

46 days ago

Topic Stats

  • 4 posts
  • 2 Pinsiders participating
  • Latest reply 42 days ago by Coyote
  • No one calls this topic a favorite


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    #1 46 days ago

    Hey guys -
    Have an SB-300 with an odd issue that I haven't seen any troubleshooting tips/guides for, and thought I would get ideas here.

    My Q2 signal (the voice that is modulated through the volume stepper) was completely dead. I replaced U17 since that was my prime suspect, and was able to get ... interesting results.

    Specifically, that second voice now will seemingly randomly be at 100% volume, or be completely dead. It does not seem to relate to any specific sounds that the game wants to play.

    So I'm torn between a bad volume stepper (or driving circuitry) or the 6840 timer's bad?

    #2 45 days ago

    Funny. I have just been replairing an SB-300 and read up on the schematics. The U13 outputs is prepare ramp-up or ramp-down amplitude. If that is messed up, here or upstream, it can cause intermediate max or zero output. If these line are held active, the U12 counter masks the clock signal and holds the set value (at 0 or f).

    A passive Q3 clock signal, or damanged U12 count logic, will cause the same result.

    #3 45 days ago

    Interesting, thanks for that. Looking at the schematics now and the datasheet for U12, I may need to check some of the traces - those chips had been replaced by sockets, and it's possible that in the process of that, a trace got cut.

    Thanks- That gives me starting point!

    #4 42 days ago

    Quick update!

    Replaced U13, and I have both voices!
    It sounded, though, like the second voice now was stuck at 100%. So on a whim, I put a new U12 in. Plug it in, and the second voice is dead. Odd. It was new. Anyways, put the original back in, and.. still no second voice.

    Okay, that's odd.

    U12's socket is an old one, and I thought that it could be it. I test it, by pressing on U12, see if anything changes. The voice cuts in and out. I'm holding the board up by the 6840. Start feeling around there, and *facepalm*.

    I never finished soldering in C28. It was loose. I fix that, and voila. Second voice is back!

    Now I'm not sure if it is stuck at 100% volume or not. I admit, it's been so long since my sound board's been working completely that I honestly can't remember how it *should* be sounding.

    In either case, I'm pleased the biggest hurdle is over.

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