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Can anyone explain these megadrive mods to me?

Started by phreak97, June 26, 2011, 12:07:39 pm

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phreak97

So I just got a japanese megadrive which had been premodded to be fixed as pal50.. expensive way to play local games but whatever..

it seems to have four areas modded. two I recognise.

one is the jumpers obviously, the other is the crystal for colour, which was done much more simply than I've seen before (see photos),
and two more..

one is a cut trace with an added resistor and wire link on the 68000, and the other is a resistor to ground on the av output (composite video line I think).

anyone know what these are about?






l_oliveira

The Japanese console is NTSC-J which means it has a little more bright on the video output levels which cause problems with the sync level when there is too much white on the screen, with out of norm TV sets and RF modulators (mostly the RF modulators, yes)

The jumper on pin 16 is a fix for a board design errata and exists on all MD boards of that revision. The 220 ohm resistor on the 68000 pin #AS(pin 6) is there to fix an issue with the SEGA chipset overloading that line, causing freezes.   (These two mods are errata fix and were done at the factory)

Also got fixed later on with newer chip revisions. That console has an Motorola 68000, right ?

ApolloBoy

Quote from: l_oliveira on June 28, 2011, 08:46:08 am
The Japanese console is NTSC-J which means it has a little more bright on the video output levels which cause problems with the sync level when there is too much white on the screen, with out of norm TV sets and RF modulators (mostly the RF modulators, yes)

Do you know what resistor value would be good in that case? My Japanese MD has the same issue with my Sony PVM through RGB and I can't play certain games because of the sync issue.

l_oliveira

Quote from: ApolloBoy on September 12, 2011, 05:18:00 am
Do you know what resistor value would be good in that case? My Japanese MD has the same issue with my Sony PVM through RGB and I can't play certain games because of the sync issue.


Your issue have nothing to do with this.

Your monitor is overloading the sync line, causing the CXA1145 chip to go bananas, killing it's RGB amplifier. The fix is mod the sync pin to use the CXA1145 internal amp instead of going straight to the VDP sync pin.

Please refer to this thread:
http://nfggames.com/forum2/index.php?topic=3387.0

ApolloBoy

Quote from: l_oliveira on September 21, 2011, 12:10:35 am
Your issue have nothing to do with this.

Your monitor is overloading the sync line, causing the CXA1145 chip to go bananas, killing it's RGB amplifier. The fix is mod the sync pin to use the CXA1145 internal amp instead of going straight to the VDP sync pin.

I finally got around to doing the mod you linked to. I'm getting a totally stable picture now whenever the screen whites out, thanks a lot! I'm still surprised that Sega would make such a huge mistake like that.

l_oliveira

Regarding the 220 ohm resistor on /AS pin of the 68000 we (me and Tmee) figured out the the resistor is required when the CPU is MC68000 (Motorola) and the VDP chip is the FH1001 (315-5313A) and the original version of the chip YM-7101 (315-5313) works fine without that resistor in a Motorola CPU.

It has to do with DMA transfer errors on the newer chip. If your MD has a Hitachi CPU and the FH1001 VDP it won't need board changes.