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Thread: Mega CD audio issues

  1. #1

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    Default Mega CD audio issues

    Although I registered quite some time ago, I think this is my first time posting here.
    So first and foremost, hi, folks, and thanks for all the good info you provide!

    I have a problem with my JPN Mega CD 1, and I'm not sure how to proceed in order to solve it. I couldn't think of a better place to ask, so please bear with me.

    Allow me to start at the beginning (because the root of the problem might lie anywhere).
    Out of nostalgia, I got myself a MD a couple of years ago; as I grew up with a model 1 (PAL), I always kind of wanted to own a model 2. So that's what I got.
    I modded it for S-Video, and everything went fine, so I was happy. But I could be happier: now I needed to get the rest of the MD stuff I had always wanted as a kid, but never could have.

    So I bought a 32X and a Mega CD (by the way, I live in Japan so everything is JPN). I was dead set on getting a model 1 Mega CD, because it's the one that captured my imagination when I was younger, I really like its design when coupled with a model 1 MD. I knew it was supposed to be less reliable than the MCD2, but I didn't care. Besides, at that point, I had done a bit of reading so I knew about the differences in sound quality between revisions of the MD. So I was already thinking about getting a model 1 MD anyways, and the cupboard look was too good to pass.
    I found a cheap auction for a model 1 MCD. The unit was labelled as "not working" because the CD wouldn't spin, but again, I had done some reading, so I took my chances and bought it.
    I got kind of lucky: the seller slapped in a VA4 model 1 MD in the box for free, with the MegaCD. Apparently, he didn't think it worked. Well, turns out it did.
    I S-Video modded the VA4, everything (apparently) went well.
    Then I changed the lens on the MCD, and partially recapped the interface board because some of the surface-mount capacitors had clearly leaked.

    Fired up the thing: it worked.
    But here's the first catch: only with the MD2.
    With the MD1, the sound was very low.
    In order to get stereo, I was going the following route: "MD1 3.5 mm headphone jack -> MCD Mix input -> MCD RCA output".
    Out of curiosity, I tried plugging in a pair of cans directly into the headphone out of the MD1. Lo and behold: not only was the sound low, but there was also a huge deal of buzzing between gradations 0 and 5, which disappeared between 6 and 10.
    "Ground loop?", I wondered, knowing no better. So I tried toying around with the the GND wire, but to no avail.
    Thinking it might be a capacitor problem, I decided to do a full recap. But since everything was working fine with the MD2, I thought to myself, "I'll get to it when I have time".

    Then I got busy; then it didn't seem all that important anymore.

    Fast forward about a year. That's today. I get the urge to fix the thing. So I open it up, re-cap he entire board, testing all the caps along the way: all were giving perfect readings. Replaced every single one anyways.
    Alas! Of course: the headphone buzzing was still present.
    After a bit more playing around, I ended up finding the cause: it was the stereo-out RCA plugs I had added when I did the S-Video mod (grabbing L/R out directly on the chip). I honestly don't know what went wrong. Caps were fine, continuity was fine, soldering job was alright...
    Anyways, a bit of digging on the Internet seemingly made it clear I wasn't the first one who encountered this issue, so I opted to repurpose the plugs as double mono-outs taken directly from the DIN-8, because I didn't want to waste anymore time on the issue. Immediately, the buzzing disappeared.
    "Since I can basically use the headphone jack for stereo, that's going to be fine by me!", I thought...
    The sound issues were solved, at long last!

    I went to get my MCD, plugged everything and turned the thing on. CD drive began seeking, it found the data it wanted, the game launched... At that point, no crackling sound, no buzzing… I had great hopes.
    Until I heard no "SEGA" voice when the logo appeared.
    Something was fishy.
    And then, when music from the CD started playing, everything went south.
    All I could hear was "KRSSSSSHHHHHH". A very loud, ear-piercing "KRSSSSSSHHHHHHHHHH". Besides that, no sound at all was audible.

    I tried plugging in my MD2 instead of the MD1.
    KRSSSSHHHHH. But with some(what) audible music in the background.

    Plugged the MD1 back in, did some tests. Here are my results:
    - CD games sound goes KRSSSSSHHHHH, whatever the game. The noise is slightly less torturing (but still brutal) when line out is taken directly from the MD1's 3.5mm jack. MD DIN-8 line out and MCD RCA line out are just atrocious.
    And there's no "SEGA" singing voice on the splash screen when launching a game (-> no MD sound?).
    - There is a slight but noticeable difference between MD1 and MD2. On the MD2, "KRSSSSSHHHH" is about the same as 3.5mm jack line-out MD1: while piercingly loud, the music is somewhat audible. Furthermore, the "SEGA" voice is audible on the splash screen.
    - Audio CDs go KRSSSSHHHHH on both consoles, with some varying degrees of music audibility (same as above: MD2=MD1 3.5mm jack = ear pain, but still more bearable than MD1 DIN-8 and MCD RCA out).
    (Edit: a precision: the "KRSSSSSH" noise is not a background noise. It only occurs when the MCD plays a sound. If I pause a game like Final Fight CD, for example, it disappears. But as soon as I get out of pause and the music starts playing again, it comes back).

    And here comes another catch:
    - pure cartridge passthrough works flawlessly.
    I even get perfect stereo with the MD1, connecting the headphone plug to the MCD's "mix" input, and using the MCD RCA as line out.

    Here's catch no. 3 (and 4):
    - a few months ago, everything worked (apparently) flawlessly with MD2.
    - a few months ago, MCD sound was very low but audible on the MD1. There was no "KRSSSHHHH" whatsoever.

    So something must have happened between then and now?
    My first guess would be: "a capacitor died, perhaps around the audio-out area. Do a full re-cap." That's what I'll do somewhere down the line, but... I'm going to be a bit busy in the upcoming months so if I could avoid a procedure that'll take me half a day for potentially no results, I'd be really glad.
    Something I noticed: some rust has apparently begun to appear on the anti-static plate that shields the CD drive. Could that be related? (seems unlikely, but... Is it something I should be worried about at all?)
    Did some component (beside capacitors) go belly-up?
    If that's the case, what would be the likely culprits?

    Thanks in advance for your input!

    Edit: holly cr*p, sorry for the wall of text!

    Edit (the following day):
    So I went back in and looked a little more carefully.
    3 caps turned out completely busted near the voltage regulator. I replaced them and then cleaned up corrosion on surrounding components. There's definite improvement: splash-screen sound is back, music is back. But noise is still present, so I guess I'll have to perform a full re-cap and see how it turns out.
    Too bad I ran out of 10uf caps when I re-capped the MD; I'm going to need quite a few of those...
    Last edited by oare; 07-18-2015 at 10:23 AM.

  2. #2
    Hedgehog-in-Training Hedgehog-in-Training toto99303's Avatar
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    Yep, I bet full re-cap will do the job

  3. #3
    Road Rasher l_oliveira's Avatar
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    Recap isn't a silver bullet. Recap doesn't solve all problems.

    Now, the MEGA-CD has two sound sources:

    RICOH sampler chip (used by the BIOS opening screen to play it's music)
    CD Drive (CD DA tracks)

    It's possible that when you disassembled the unit you damaged one of the two ribbon cables and because of that the sound is bad. A PCM stream reproduced through hardware with a fault can sound like telephone modem noise (hiss, or noise similar to a TV without antenna).

    When you have the issue? When you try to play some CD track or when it tries to make a sound through the sampler chip?
    I'd like to share some wisdom:
    Quote Originally Posted by TmEE
    I'm too lazy to be unstoppable.
    you can have shame, as long as you don't feel it

  4. #4

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    Quote Originally Posted by l_oliveira View Post
    RICOH sampler chip (used by the BIOS opening screen to play it's music)
    CD Drive (CD DA tracks)

    It's possible that when you disassembled the unit you damaged one of the two ribbon cables and because of that the sound is bad. A PCM stream reproduced through hardware with a fault can sound like telephone modem noise (hiss, or noise similar to a TV without antenna).

    When you have the issue? When you try to play some CD track or when it tries to make a sound through the sampler chip?
    Thanks for the input.
    I actually re-opened the unit a few days ago to perform the full-recap; once finished, CD sound was back loud and clear for about 10 minutes.
    Unfortunately, when I turned the Mega CD off and back on again, sound gradually got scratchier, and quickly ended up disappearing entirely.
    I then noticed I had missed one of the caps (hidden below a small daughter board connected upside-down to the main board).
    Changing that last cap brought me back to square one: scratchy CD sound. The only sounds I get correctly are in-game splash-screen sounds, and sounds like the jumping sound effect in Sonic CD (which I guess are handled by the Mega Drive and passed-through to the Mega CD?).

    I'll test the ribbons for continuity, see if I actually damaged one. Now that I think of it, even during the brief while during which the CD sound was back, I don't recall hearing the Mega CD BIOS music when booting the unit up.


    Edit:
    Quick follow-up.
    Tested continuity on the ribbon cables, both were OK.
    I did however find a few clogged holes in which the plating had apparently been eaten away by corrosion.
    Re-establishing a proper connection brought back CD sound, but still no luck with the sampler chip side of things.
    Last edited by oare; 09-07-2015 at 06:07 AM.

  5. #5
    Hedgehog-in-Training Hedgehog-in-Training Eidolon's Avatar
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    Quote Originally Posted by oare View Post
    Re-establishing a proper connection brought back CD sound, but still no luck with the sampler chip side of things.
    In my case, having had a similar problem, the problem could be solved by reflowing the solder connections of IC3, IC13 and IC14.

    This strange PCM sound symptom seems to happen when one of the RAM ICs (13/14) has a bad connection to the PCM chip (IC3).

    Some PCM music plays well, if the PCM samples are so small as to fit in one RAM IC only.
    The garbling only starts when both RAM chips are utilized.
    Last edited by Eidolon; 03-24-2017 at 05:58 AM.

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