Sega Mega Drive PSU Exploded

Discussion in 'The Projects Forum' started by meshif, Jul 27, 2013.

  1. meshif

    Thread Starter New Member

    Jan 16, 2011
    3
    0
    Hey guys. Just wanted to make sure I'm reading this PCB properly before I build it and blow up myself or my Mega. I know I can (and would probably be better off) buy a new wall wart but where's the fun in that. Below are some pictures of the original PCB and what I think is the schematic for it. Can someone take a quick look and confirm? TIA


    [​IMG]
    Solder Side

    [​IMG]
    Component Side

    [​IMG]
    Connection from resistor to mains

    [​IMG]
    Schematic
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,127
    3,048
    Your schematic isn't right. Look up a standard wall wart with full wave bridge rectifier.

    You should check the transformer with an ohm meter to be sure it hasn't gone shorted or open. That roasted diode could have melted some of the transformer winding insulation.
     
  3. meshif

    Thread Starter New Member

    Jan 16, 2011
    3
    0
    I checked voltage in the output barrel before I cracked it open and was getting 1-2vdc. Also, the primary and secondary have some resistance in them. That said, is this schematic closer?

    [​IMG]
     
  4. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    The two circuits are the same (except that the neutral line is connected to the center tap for 220V power feed).
     
  5. Relayer

    New Member

    Jan 1, 2013
    18
    1
    Hmm, I would purchase a Walwart PSU that is regulated.
    Your fried supply definitely wouldn't be regulated.
    Is your PSU an original Megadrive type?
    If so, I'm surprised at Sega for the cheapness of their supplies.
    Regards,
    Relayer :D:D:D
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
    Looks dangerous to me, Your DC out is referenced directly to the mains live side via the resistor?
    Also as already has been pointed out, a section of your primary is shorted out!
    Max.
     
  7. meshif

    Thread Starter New Member

    Jan 16, 2011
    3
    0
    I've spent about an hour looking at this PCB and schematics for full wave rectifiers and I can't figure out what I am doing wrong (other than drawing the cap backwards...d'oh). Well, looks like this one is just going in the trash then. Thanks for the help though

    @Relayer: Yes, this is the original Sega MK-2103 adapter that came with the Mega Drive. I was expecting more from it too.

    @MaxHeadroom: Yea, that red wire goes right from the resistor to mains. That's another thing that didn't seem too safe/proper to me.
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,127
    3,048
    Just curious if anyone knows why that high-ohms resistor is even there? I'll bet it could be removed with almost no effect. RF noise reduction? Why did they put that in there? I've seen hundreds of wall warts and not one had that.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    It's a grounding resistor. It goes from the AC mains neutral to the bridge rectifier ground, and is a very high value 8.2 Meg ohms. Normally they are more common when the AC mains has a 3rd (ground) pin.

    Its job is to keep the DC -ve wire at roughly ground potential, so there won't be a static voltage etc built up in the PSU before it's plug is touched to the socket of the appliance (or touched by a human).

    It can't be connected to the AC mains live wire, that would be a terrible mistake. My guess is that it's really attached to the neutral (even though they used a red wire).
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,127
    3,048
    That makes so much more sense. It's hard to tell from the picture but the PS prongs don't look polarized.
     
  11. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,362
    The prongs look polarized. The one prong (neutral) appears to be wider than the other one.

    So what happens when you replaced the bad rectifier?
     
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