26 Apr 2017

Sega Megadrive 2 (repair)



I've got this faulty unit from a member of a NZ forum (who got it from an other member I guess).
After a quick test I obtained only a black screen, no sound. The first obvious reason can be a dirty cartridge port or some bent pins on the port.
So I cleaned it carefully and checked the pins with no success. Well, I didn't expect much since both previous owners must have tried this before too.
From my experience Megadrives are rock solid unless you use the wrong AC adapter.
So I inspected the board carefully and found a trace going to the video encoder has already been repaired but nothing more.
It was time to go deeper. I dug out my logic probe and quickly found address line A17 was floating on the main CPU. Normally all address lines are tied to VCC via pull-up resistors so there was 3 possibilities :
- either a trace is cut somewhere
- or the pull-up resistor is bad
- or the CPU is faulty and drags the signal somewhere between Vcc and ground and my logic probe detects it as "floating" (not a valid state)
It was option 1. I was about to patch it with a piece of kynar wire when I saw a very small but clean cut in the concerned trace near the cartridge port. Some solder was enough to fix it:


Time for a test and... Bingo!


I really wonder what has caused this cut???

2 comments:

  1. Hi, I have the exact same issue with a Mega Drive II. Replaced the cart slot with no change. What should I be seeing on the logic probe when testing? Do you have any schematics showing what should be shown for each pin..?

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    Replies
    1. Hi,
      basic rule is when testing a signal with a logic probe it should indicate either a low state (green led on mine), a high state (red led on mine) or a pulsing state (yellow led on mine). If not then signal is "floating".
      Schematics can be found online on SegaRetro website for instance:
      https://segaretro.org/images/a/af/Sega_Service_Manual_-_Genesis_II_-_Mega_Drive_II_%28PAL%29_-_001_-_June_1993.pdf

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