Sega Nomad crackling sound from speaker (not simple cap fix)

Discussion in 'Repair, Restoration, Conservation and Preservation' started by segasonicfan, Jul 3, 2016.

  1. segasonicfan

    segasonicfan Robust Member

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    Hey all,
    so I bought a Sega Nomad off of ebay boxed and was really excited to have a complete system finally (I have a working one from some years ago that is system only). However, it arrived with a broken AC plug (which I fixed) and, what is seemingly a big problem, some crackling sound output from the speaker / headphones (as well as a LOT of background distortion noise at high volume). Have a listen here to see what I mean:

    www.mediafire.com/download/au151aq68q7lh6h/New+Nomad+from+Ebay.m4a

    For reference, here is my old Nomad:
    http://www.mediafire.com/download/cpk8k0hhsxx6fvl/Old+Nomad.m4a

    This is the title screen from Sonic 3. I tried swapping the speakers cause sometimes crumbs and stuff can get in there. That didn't work. Then I tried changing the 2x 100uf caps, the 47uf cap, and the 22uf cap (all electrolytics near the Sega 315-5684) but that didn't help either. I also tried swapping the back halves since they separate from the ribbon cable to see if the issue was with the Sega ASIC. Nope.

    So my feeling now is it must be either a) some weird power supply regulation issue (input does measure +5v on the 315-5684 mixing amp) or b) some SMD ceramic caps are bad around there or c) something I haven't thought of yet.

    The problem also occurs with headphones :/ blegh so bummed. Any help greatly appreciated!

    -Segasonicfan
     
  2. Bad_Ad84

    Bad_Ad84 The Tick

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    Volume pot issues?

    Have you tried giving the pot a good turn back and forward like 20-30 times and see if it improves?
     
  3. segasonicfan

    segasonicfan Robust Member

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    I did try that. I can look into it more by shorting the input and outputs of the pot. But I really don't think it is that at all because I get a bunch of white noise/crackling sound in the background all the time and the distortion is only on certain effects, like the audio is clipping, not just the whole audio output. It's hard to explain...it just doesn't seem like it is a pot issue at all. It also doesn't make noise as I turn the pot at all. But I will double check that again, it's a good thing to rule out. Thanks for the speedy reply!

    -Segasonicfan
     
  4. Bad_Ad84

    Bad_Ad84 The Tick

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    Id rule it out, it certainly might not be that - I am not there to hear it etc.

    Have you confirmed all voltages are present on any opamps etc?
    I have heard very similar sounds out of amigas when the negative voltage supply is missing.

    Ive never worked on a nomad, so dont assume everything I say applies - it may not. Just trying to help steer you to solving it.
     
  5. segasonicfan

    segasonicfan Robust Member

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    I checked the main power rail (there is only one op amp, its a custom chip with multiple op amps inside actually) and it shows a steady +5v. The nomad doesn't use negative voltages at all.

    -Segasonicfan
     
  6. segasonicfan

    segasonicfan Robust Member

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    So I figured it all out. There were a number of things going on:

    1) The caps by the 315-5684 were bad (common problem) 2x 100uf (audio output) + 47uf (power coupling cap)
    2) The speaker itself was damaged internally and needed to be replaced (maybe punctured somehow?) I have not seen this before.
    3) Both caps at the input (25v 100uf I believe + 6.3v 820uf) needed to be replaced.

    Replacing the 2 caps at the power input *greatly* reduces the white noise / static / crap at the audio output. It's kind of amazing actually what an improvement it makes. The DC-DC circuitry must be highly sensitive to noise and needs extra attention to regulation of the 7-9v DC in.

    This problem was tricky because the crackling in the broken speaker mimicked the crackling white noise from the bad caps. This had me chasing my tail believe the problems must be relating when in fact it was two very separate issues.

    In unrelated issues, this Nomad also has a slightly washed out display compared to my other one. It seems the cathode tube is showing its age and no longer makes a perfect white but is getting slightly pink. The Nomad logo on the plastic on the front is also off-white, getting faded with age or possibly sun damage. Ah, all our consoles are senior citizens now!

    -Segasonicfan
     
  7. radiokid

    radiokid Rising Member

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    Was yours anything like mine?

     

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