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Thread: I'm at a loss repairing this Turbo Duo with no audio with both Hu Cards & CDs

  1. #1
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    toploaderleo's Avatar
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    Default I'm at a loss repairing this Turbo Duo with no audio with both Hu Cards & CDs

    I'm working on this TurboDuo. I've recapped about a half dozen in the past & a few I had to repair broken traces, one I even had to replace the 4558 OpAmp.
    When I got this one it had audio for a second & then it would completely cut out, with both Hu Cards & CDs, through the headphone jack it plays the audio very low. If you turn it off & on again it has no audio at all but if you let it sit it gets the audio back for a second again then cuts out. When I opened it up I saw a few caps had leaked but not that bad, except C322, which had corroded so bad it fell clean off the moment I touched it. Usually when I recap these I test them as I replace caps to see if I get an improvement or the problem gets worse so I know what went wrong where. This case it never got any better. Iím using the same caps Iíve always used so quality of caps shouldnít be the issue. So far Iíve:
    -Replaced all the caps.
    -Pulled most of them back out, test them & check adjacent traces for breaks, no breaks all caps are good
    -Replaced the 4558 at IC 506 & 507
    -Replaced the Volume Balance IC M51131L at IC 505
    -Replaced the ceramic resistors above C322. I tested it without the resistors & I got audio for a little longer before it cut out. I was applying pressure unintentionally to the board & the moment I let up the audio cut out so I donít know if it was the pressure that did it or the missing resistors. When I replaced the resistors it went back to the same issue. I havenít been able to replicate this with pressure a second time.
    -I re-flowed IC 515 to see if that would do anything, nothing.
    Iím at a loss at the moment what to try next.

  2. #2
    celerystalker is a poindexter celerystalker's Avatar
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    The missing resistors and pressure both would indicate a broken trace making renewed contact either by the pressure or heat from the unrestricted current. Dollars to donuts you've got a very small cut somewhere, and the reason you get a moment of sound when turning on is due to the spike in voltage to the circuit causing a brief bridge.

  3. #3
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    Damn, I've checked everywhere capacitor for broken traces, guess it could one I haven't checked yet.

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    ServBot (Level 11) jb143's Avatar
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    You need yourself a signal tracer. It's just a simple device that lets you probe the board and listen to your signal to see where you loose it at. You can do the same thing with any powered and amplified sound system like a stereo or old computer speakers and just a couple of other parts.

    Here's a link to the first I found...
    http://musicfromouterspace.com/analo...naltracer.html

    You connect the ground to a ground point on the PCB, and use the other wire to probe the audio circuit. If you find the signal, check everywhere else that should be electrically connected to that pad. If you loose your signal then you've found your broken trace and a simple jumper should do the trick.
    "Game programmers are generally lazy individuals. That's right. It's true. Don't let anyone tell you otherwise. Since the dawn of computer games, game programmers have looked for shortcuts to coolness." Kurt Arnlund - Game programmer for Activision, Accolade...

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    Found it. I have a digital multi-meter that has a signal tracer setting. I spent 2 days looking for this break & couldn't find it because it was on the bottom of the board.
    Someone on a different page recommended using a black light on it, some PCB are florescent & the areas damaged by the capacitors will not reflect the light. When I used it the areas I knew were badly damaged showed it, like around C322, but I also noticed a deal of damaged in the area between C613, C323, C306, & C615. I had checked this area before & couldn't find any broken traces but while I was inspecting this area I noticed light wasn't shining through one of the PTH & when I flipped the board I noticed that R324 (the only component connected to that PTH, was a bit discolored, not enough to notice at a glance but enough for me to check if it had continuity with the opposite side & it didn't. The leakage had damaged only the area inside the PTH, leaving both the top & bottom intact & with good continuity to their abject components but disconnected them from the underside of the board. I poked out the collected corrosion out of the PTH, exposed the trace on both sides & jumped through the PTH with a bit of bus bar & it works perfectly now.
    Thank you all for your help.

  6. #6
    celerystalker is a poindexter celerystalker's Avatar
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    Congrats! Nice fix!

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