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Thread: Sega CD systems repair thread

  1. #796
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    I have a Japanese Mega CD 1 that doesn't power on. Surprisingly the mainboard looked brand new with absolutely no capacitor leakage. The same couldn't be said about the sub board so I recapped it and still had no power. The original fuse checks fine. TR4 looked fine but I replaced it anyway. Still nothing.

    The OP loses me when it says to check the 7805 since there isn't one. There is a NEC 2405 which is basically the same thing but I only read very low voltages there. It's on the opposite side of the board from the fuse and TR4 and I can't seem to trace how power is even supposed to get there. Since the thing has no On/Off switch I have to assume some signal from the console triggers TR4 or TR3 or something to turn on the 2405 so it isn't always pumping 5v into everything. That's where I'm stuck but the OP kinda skips that part.

    I wish I has a known-good sub board to try just to see if I'm barking up the wrong tree here.

  2. #797
    Hedgehog-in-Training Hedgehog-in-TrainingWildside Expert majinga's Avatar
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    Follow the input voltage.
    Check if you have the input voltage on TR4. And if the base of TR4 is correctly driven when you turn on the MD. If so check the input on the voltage regulator. If all of this are ok, check the output on the voltage regulator.
    As a general rule, never replace something without a clue. Ok for electrolytics, but for other stuff, replace them only if you have a reason to do it.

  3. #798
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    Quote Originally Posted by majinga View Post
    Follow the input voltage.
    Check if you have the input voltage on TR4. And if the base of TR4 is correctly driven when you turn on the MD. If so check the input on the voltage regulator. If all of this are ok, check the output on the voltage regulator.
    As a general rule, never replace something without a clue. Ok for electrolytics, but for other stuff, replace them only if you have a reason to do it.
    Thanks, but it wasn’t exactly for “no reason.” …unless “no power” wasn’t a good enough reason. Public sentiment is that the component is often to blame for exactly that issue.

    I got it from Japan in this condition and it did not appear to have any prior service attempted (thankfully). When I saw how great the mainboard looked I thought this was going to be easy! Now it’s been a couple years. Probing the power board with the Genesis attached is proving difficult but hopefully I’ll find a way to resume soon.

    I’ll probe around those components for voltage when I can dig it out of storage where it has been since the tornado last year. Thanks!

  4. #799
    Video Game Modder Outrunner segasonicfan's Avatar
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    Quote Originally Posted by WeStillPlay View Post
    I have a Japanese Mega CD 1 that doesn't power on. Surprisingly the mainboard looked brand new with absolutely no capacitor leakage. The same couldn't be said about the sub board so I recapped it and still had no power. The original fuse checks fine. TR4 looked fine but I replaced it anyway. Still nothing.

    The OP loses me when it says to check the 7805 since there isn't one. There is a NEC 2405 which is basically the same thing but I only read very low voltages there. It's on the opposite side of the board from the fuse and TR4 and I can't seem to trace how power is even supposed to get there. Since the thing has no On/Off switch I have to assume some signal from the console triggers TR4 or TR3 or something to turn on the 2405 so it isn't always pumping 5v into everything. That's where I'm stuck but the OP kinda skips that part.

    I wish I has a known-good sub board to try just to see if I'm barking up the wrong tree here.
    I have a JP MCD 1 as well, might be able to check later aince I know there are some slight hardware differences. I assume youve looked over schems online? Thats a golden resource to start with...

    Edit: found the power section for you in the service manual. The "sub board" as they call it (see attached).

    So your problem is *very likely* D1, TR3 or TR4 or IC6. TR3 and TR4 essentially work as switches, the base of TR3 is driven high when your Genesis powers up (since it receives VCC voltage from the Genesis) which drives the base of TR4 low, sending the 9V MCD power supply onto IC6 (7805 regulator). D1 works as reverse polarity protection.

    So I would (assuming FB1 is not broken / open):
    0. DiSCONNECT 21Pin cable from sub board(!) before testing power unscrew sub board and test outside of case.
    1. check if D1 is shorted. replace as needed
    2. check if any pins from TR3, TR4, or IC6 are < 500 ohms (within the pins of their respective packages). Replace as needed.
    3. No shorts? apply 9V power ONLY. see if voltage appears on fuse side of TR4
    4. Yes? ok connect 21P and turn on Genesis. See if VCC is supplied to TR3 base. R37. Alternatively, add 5V to R37 (and ground to board ofc) from bench supply. Limit current from supply to <10mah and leave 21P disconnected.
    5. check IC6 output /J3. if not 5v but input has >7v, it needs to be replaced.

    Try to test IC6 outout without 21P adapter connected if this doesnt work. Its pretty straightfoward power board so you want to make sure VCC2 isnt shorted on the main board side (you can check this too though).
    btw, there are 3 power voltages:

    M.VCC (~9V power in)
    VCC (5V power in from Genesis / MD vreg)
    VCC2 (MCD 5V reg power)

    Hope that helps.
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  5. #800
    Hedgehog-in-Training Hedgehog-in-Training Eidolon's Avatar
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    I recently bought for (fairly) cheap a troubled Japanese MCD1.
    It has a weird symptom I never saw before.
    Does anyone have an immediate idea what I should put my repair focus on?
    Here's video w/ sound: https://twitter.com/Goetholon/status...11099483226114

  6. #801
    Hedgehog-in-Training Hedgehog-in-TrainingWildside Expert majinga's Avatar
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    That's indeed odd.
    I'm not sure who handle the sound in the BIOS title, if is up to the MD or the MCD.
    If is the MCD to hadle the sound you have something wrong in the audio section.

    Have you tried with another MD? If you play a game, especially one that use the MCD for sounds, the issue still there?
    I will try to clean the contacts on the expansion port too.

    With the MD is a bit tricky some times, because there are many way to get sound from the MD, so it can work well with some games, and give trouble with others.

  7. #802
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    I've got a unique issue I think. I have a Sega CD Model 1 with a JVC drive assembly. I must have caused the issue myself during a full recap I recently did. The drive functions mostly perfectly and the laser reads accurately. The drive only opens or closes when I tell the system to eject or close, so the limit switches as far as that goes are correctly operating. However, it can't tell when I'm pushing on the ejected tray itself and doesn't close from that action. I'm not sure where the internal switch is for this function, but I know it was performing this correctly before I worked on it. I see a lot of guides on the Sony drive, but very little information online about the JVC one. Would someone be able to help me with some info on how this version of the drive detects when the tray is being pressed so I can fix this one last function?

  8. #803
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    I figured out what I did wrong! For reference to any others reading this, remember to put the screw back in the hole near the rear of the sliding tray itself when reassembling. That screw is used as part of the mechanism triggering pressure based tray closing.

  9. #804
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    Hi all

    I'm having trouble with a few japanese MegaCD here where the spindle sits to low, looks like the archimedes screw is worn out, or whatever else mechanism lifts the laser assembly when the disc is inserted.
    The result is the scratching noise when playing a CD, especially prominent when CDRs, these seem to be thinner on the most inner ring thats being held up by the spindle..
    Sometimes the scratching is so bad that it slows the CD down.

    I've read posts about people sticks tape to the part that goes on top of the spindle to lift it higher.
    https://imgur.com/140UoX5

    Then I've found this video, describing how to pull the spindle up, seems a bit rough but might be the longest lasting fix with sideffect/extra stuff, assuming I won't break anything in the process. (enable subtitles and translate to your language in case your french is a bad as mine)
    https://www.youtube.com/watch?v=9xTppzollQg

    I was wondering if there is other things I could do to fix that problem?

  10. #805
    Hedgehog-in-Training Hedgehog-in-TrainingWildside Expert majinga's Avatar
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    Are you sure it scratch on the top?
    Do you see some mark on the top side of the cd?

    Do you checked the bottom side?
    Because, most of the issue are caused by a loosing belt, that can't apply enough force to completely sit in position the CD assembly.

  11. #806
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    Quote Originally Posted by majinga View Post
    Are you sure it scratch on the top?
    Do you see some mark on the top side of the cd?

    Do you checked the bottom side?
    Because, most of the issue are caused by a loosing belt, that can't apply enough force to completely sit in position the CD assembly.
    Thanks.

    The top part doesn't get lifted high enough so it scratches, I'm talking about the part that goes on top of the spindle and the CD, not talking about the CD itself.
    Not uncommon for Sega-/MegaCDs, check the video/picture.

    I've replaced the belt am I'm testing if the CD can move freely while the drive is outside and not plugged in, so I'm manually putting the tray in position and its certainly not the belt, it just doesn't lift high enough.

    Have a few of these units here, was wondering what the consensus is or if there are other suggestions than the one I've linked for this particular problem.

  12. #807
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    Did you re-greased the gears and the others mechanism?
    The original grease can be dried and do not allow a proper movement.

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