Modding Megadrive to 60Hz + NTSC colour

[t0nito]

Hi! I'd like to contribute by showing you how I've done this mod.

This mod not only allows you to switch regions AND 50Hz/60Hz with only one switch, it will allow you to choose between PAL colour or true NTSC colour compatible with any capture card, multisystem, or even American TVs.

So here goes:

First I've done the 1 switch region mod following this tutorial:
http://mdpal60.net/wiki/megadrive/regionmod/start

Now here's what I've done differently for the other tutorials.
Every single one tells you to bridge pin 1 to pin 7 on the CXA1645 video encoder chip to force to PAL mode and add a 4.43Mhz crystal to make it a PAL 60 signal.

But since I have 3 NTSC compatible TVs and I wanted the console to output proper NTSC instead, I installed a 3.579 MHz crystal to the Mega Drive to allow NTSC colour on 60Hz mode.

I used a SPDT (single pole double throw) switch to choose from the original signal (Mega Drive's own 4.43 PAL and the added 3.58 MHz crystal.

Unlike the other mods you DO NOT BRIDGE PIN 7 TO GROUND on the encoder chip, we wan't it to be able to switch between PAL and NTSC when selecting 50 or 60 Hz.

The result is true PAL on 50Hz and true NTSC on 60Hz.

Here's the schematic:



I wanted to add the switches in a nice and discrete place, so here how I did it:



And here is the result:




Contrary to what other people say, the crystal not only does not interfere with RGB it eliminates the jailbars completly!

RGB 50Hz original setting (crystal turned off, original 4.43 clock):



RGB 60Hz crystal turned on:



Completely noise free clear picture, pretty neat huh?

Sorry for the long post!

[Kamahl]

Pretty cool!

[MaxWar]

Hmm, very interesting. Would this possibly indicate that the jailbars on the genesis are interferences caused by the clock divider???

Then again I have eliminated my rgb jailbars by rewiring the RGB cable to use sync instead of composite as sync.

[t0nito]

Yeah, I read that it is in fact the chroma carrier that interferes with the signal, some people even cut the trace with the clock signal at the source so it won't "bleed" in the blue trace that is right next to it.

All I know is that when I have it in 60Hz with the crystal turned on, when I switch the mega drive there is very faint diagonal and moving bars that clear up completely after the mega has warmed up (capacitors going bad?).

When I have it on 50Hz with the crystal on the bars are always there but very very faint.



When I have it on 50 or 60Hz with the original signal ugly bars show up...

60Hz original clock (3.54MHz no colour on composite)



Curious thing though, with the original signal whether it's 50 or 60Hz, all bars are stationary. When the 3.58 MHz crystal is on, those faint diagonal bars on 50Hz are moving.

[floob]

I have a PAL MD modded to support 60Hz as well, but I find when playing some games (particulalrly JPN Desert Strike) in 60Hz there is a lot of judder/stutter like this:
http://www.videogameperfection.com/2...es-to-stutter/

Is this a typical issue? Am I better of getting a native JPN MD?