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> Total Long Shot - Megadrive Repair
evildragon
Posted: August 23, 2017 10:44 pm
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This is a total long shot. I have the schematics to the thing, but idk, I don't think anyone will be able to help me even based on this.

Plain and simple, a Sega Megadrive should be able to run 16-bit (68k) titles, and 8-bit (Z80) titles. It's backwards compatible with the Mastersystem games (and technically the SG-1000, but you wouldn't be able to see it, due to the video modes not being in the video processor).

I have an odd case here where one of the Megadrives I own, refuses to boot 8-bit software. It acts like no game cartridge is inserted, black raster. The Z80 bus is dead, there's no activity whatsoever, and the RAM seems empty. But here's the kicker, if I run 16-bit software that utilize the Z80 and its RAM, it works just fine. So the Z80 side is functional and alive, but it will not work to boot the console to Z80 exclusive (Mastersystem) games.

I've run out of ideas. I've replaced both CPU's, and all of the RAM, and I still couldn't get it to boot. I even tried recapping the set, nothing helped.

This is the schematics to the system:
http://gamesx.com/wiki/lib/exe/fetch.php?m...ock_diagram.png
http://gamesx.com/wiki/lib/exe/fetch.php?m...d_and_video.png
http://gamesx.com/wiki/lib/exe/fetch.php?m...ustom_chips.png

I know it's compatible with the 8-bit software, as it used to work on it. One day it just quit running the software. There is ONE pin on the cartridge port that signifies if the 8-bit adapter is inserted, to put the system into compatibility mode---it does make continuity to the chip that it's supposed to be leading to, so that's not the issue. (Pin B32 shorts to B30--this is how you tell the system to enter 8-bit mode).
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Sch3mat1c
Posted: August 24, 2017 06:12 pm
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Not going to check the schems right now, but -- does the 8-bit bus (data and address, and control signals) go to the cart? Everything checks out?

Tim


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evildragon
Posted: August 24, 2017 06:54 pm
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Seems to be, because 16-bit cartridges work, and they connect to the same cartridge port and pins..

I'm almost suspecting IC4 and IC5 are the culprit, which seems to be the glue logic.
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