Megadrive RGB Scart

[djlooka]

Hi everyone,
I recently purchased a PAL-G (probably means German?) MegaDrive-1, which I modded with the 2 switches (JPN/ENG and 50/60Hz) mod.
Now I would like to add a SCART female connector on the back of it, instead of buying a rare/useless 8-din plug/cable.

I have tried to follow 2 tutorials without success.
They were meant to build an 8-din -> SCART cable, so I thought I could adapt them, simply connecting the wires from the pins on the backside of the mainboard to the SCART female.

The first one I tried was this: http://members.optushome.com.au/eviltim/gamescart/megamstr.png (with stereo audio coming from the front instead of mono).

If I plugged the scart on my tv I saw a very distorted picture, moving across the screen, but the image was coloured (even in 60Hz).
The weird thing was that this signal was "overwriting" every other channel!
Even if it was plugged on AV1, I could see the same signal on AV2 and also every other channel, including 0 (zero), which is the channel I use for the RF (antenna) plug of the MD.
So I tried to connect both cables, RF and SCART. On the AV1/2 nothing changed, but amazingly on the 0 I had a perfect image and sound (don't know if stereo or mono), with colour both @ 50 and 60Hz.
If you look at the schematic, you will notice that the composite pin on the 8-din plug goes to the C-sync of the SCART, which is IMHO weird.
My thought is that the signal on the SCART was lacking Sync, which was somehow "added" by the RF signal when they both were connected.

After some googling I found another schematic for the same purpose (8-din -> SCART), but this one (http://i16.photobucket.com/albums/b14/moosmann/electronic/md1rgb.png and http://i16.photobucket.com/albums/b14/moosmann/electronic/din-mdng.png) was quite different from the previous one.
First of all, now the composite pin is not connected and the Sync is correctly connected to the SCART Sync pin.
Then, the site was German, and I thought it would better fit my German MD
Unfortunately this schematic gave me worst results. No signal, neither on the RF, neither on SCART. Totally black screen.

If we compare the 2 schematics, the main differences are:

• the values of the caps/resistors - no clue here which is better
• 1 has many gorunded pins on the SCART, 2 only 4 - 1 is better?
• 1 wires composite to sync, 2 wires sync to sync - 1 doesn't make sense to me, but partially seems to work

Another thing I noticed is that my MD has a cap (56uF I think) soldered on the backside of the board, connected to the pin under the 7th pin of the 8-din slot, which isn't present in any of the pics available in the tutorials (maybe because it's PAL-G?).

I am now going to re-wire everything back to the 1st tutorial, trying to get the sync signal from somewhere on the mainboard.
I also tried to use the 11th leg of the Sony CXA1145P video encoder without success - see here: http://www.mmmonkey.co.uk/console/sega/md-sync.htm and here: http://nfggames.com/forum2/index.php?topic=3387.0 .

My English isn't very good so please ask for further detail if you don't understand what I meant.

Any help will be very much appreciated,
Luca - desperatenerd.wordpress.com

[NFG]

PAL G seems to actually be specific to Syria and Monaco.

[albino_vulpix]

If I plugged the scart on my tv I saw a very distorted picture, moving across the screen, but the image was coloured (even in 60Hz).
The weird thing was that this signal was "overwriting" every other channel!

This is normal. It's to do with how TV implement RGB SCART. It will only display properly on the correct channel.

On the AV1/2 nothing changed, but amazingly on the 0 I had a perfect image and sound (don't know if stereo or mono), with colour both @ 50 and 60Hz.

Looks like that's your SCART channel. To test the sound, put a Sonic game (you have a Mega Drive, you must have one) in and grab a trail of rings. The chime should alternate between the left and right speakers.

If you look at the schematic, you will notice that the composite pin on the 8-din plug goes to the C-sync of the SCART, which is IMHO weird.
My thought is that the signal on the SCART was lacking Sync, which was somehow "added" by the RF signal when they both were connected.

The SCART standard specifies pin 20 as composite video, which is where it gets its sync for RGB. As for getting sync from the RF, I suppose it could happen :p

After some googling I found another schematic for the same purpose (8-din -> SCART), but this one (http://i16.photobucket.com/albums/b14/moosmann/electronic/md1rgb.png and http://i16.photobucket.com/albums/b14/moosmann/electronic/din-mdng.png) was quite different from the previous one.
First of all, now the composite pin is not connected and the Sync is correctly connected to the SCART Sync pin.
Then, the site was German, and I thought it would better fit my German MD
Unfortunately this schematic gave me worst results. No signal, neither on the RF, neither on SCART. Totally black screen.

The Mega Drive's composite sync can't be used as it is. It's slightly out of spec. More importantly, the model 1 Mega Drives connect the composite sync output to the wrong pin of the video IC. The IC's composite sync IN is connected to the Mega Drive's OUTput. You can fix this, but for a SCART application, just use composite video.

If we compare the 2 schematics, the main differences are:

• the values of the caps/resistors - no clue here which is better
• 1 has many gorunded pins on the SCART, 2 only 4 - 1 is better?
• 1 wires composite to sync, 2 wires sync to sync - 1 doesn't make sense to me, but partially seems to work

Follow viletim's diagram: http://members.optusnet.com.au/eviltim/gamescart/gamescart.htm It has 220uF capacitors and 75ohm resistors on the RGB lines. As for grounding, technically you only need one, but it's a good idea to connect as many as you can.

Another thing I noticed is that my MD has a cap (56uF I think) soldered on the backside of the board, connected to the pin under the 7th pin of the 8-din slot, which isn't present in any of the pics available in the tutorials (maybe because it's PAL-G?).

My PAL unit has one too. It won't make a difference to anything.


Any help will be very much appreciated,
Luca - desperatenerd.wordpress.com

[djlooka]

PAL G seems to actually be specific to Syria and Monaco.

I live in Italy and looking at your list, it seems to be one of the countries with PAL B/G.

[djlooka]

you have a Mega Drive, you must have one

I have Sonic 2 and 3  I'll try as soon as I re-solder everything back.

The SCART standard specifies pin 20 as composite video, which is where it gets its sync for RGB. As for getting sync from the RF, I suppose it could happen :p

Ok, so you confirm that I have to connect a "sync" signal to the 20th pin on the SCART.

Follow viletim's diagram: http://members.optusnet.com.au/eviltim/gamescart/gamescart.htm It has 220uF capacitors and 75ohm resistors on the RGB lines. As for grounding, technically you only need one, but it's a good idea to connect as many as you can.

Pins 4, 5, 9, 13, 17, 18 and 21 will be grounded and the R, G and B lines will have both caps and resistors.

The Mega Drive's composite sync can't be used as it is. It's slightly out of spec.

If I use the CSync signal directly, it won't work (fuzzy picture and so on).
I forgot to mention that I had already tried to put a 220uF cap on the CSync (alone and with a 75ohm resistor), without any change in the picture.

My goal is to have a decent rgb signal on the SCART, so I don't want to have to keep the RF cable plugged for the sync.

the model 1 Mega Drives connect the composite sync output to the wrong pin of the video IC. The IC's composite sync IN is connected to the Mega Drive's OUTput. You can fix this, but for a SCART application, just use composite video.

Not sure of what you meant here.
Where can I get a correct composite signal, if the one coming to the 8-din is IN instead of OUT?
About the fix you mentioned, does it involve the use of the 11th leg of the Sony CXA1145P and the cut of the trace somewhere between this leg and the pin on the 8-din?
I mean: supposing that the signal coming from the 11th leg is OUT, what if I totally bypass the trace, and directly (or with caps/resistors) connect the 11th leg to the 20th pin on the SCART? Won't it be the same?
I am asking because I already tried it without success, so I am wrong, but don't know why.

Thank you,
Luca - desperatenerd.wordpress.com

[MKL]

Composite video is on pin 20 when the scart socket is an input but it's on pin 19 when it's an output (same difference for audio pins). The scart on the back of your Megadrive is an output so you need to wire composite (or sync) to pin 19, not 20!

[djlooka]

Composite video is on pin 20 when the scart socket is an input but it's on pin 19 when it's an output (same difference for audio pins). The scart on the back of your Megadrive is an output so you need to wire composite (or sync) to pin 19, not 20!

Hmm... I understand your point, which seems to be confirmed by this (http://pinouts.ws/scart-pinout.html).
And that also seems to be a very good reason for my previous attempts to fail!
Don't be offended, but as I have very few spare time now for the modding, I'd like to hear a "second opinion" before I start soldering again

Thank you very much!!

[Midori]

Second opinion: Just do it.

:-)

Don't you have a multi-meter? Use it in that case to verify that pin 20 in one of your scart cable ends is connected to pin 19 on the other end.

[djlooka]

Hahaha ok 
As I said before, I have no time this week, so any "further"  opinion... Just kidding
Thanks

[djlooka]

Here's the diagram which seems to be the right one, done following your suggestions.
[The SCART on the right is a female connector, seen from the backside]
http://djlooka.altervista.org/Diagram.jpg
Hope this is finally correct. If it isn't , feel free to insult me  

P.S.:
Very bad news...  
Before uploading the diagram I desoldered everything because I wanted to try a game, but *BLAM* totally black screen!
I screwed up something and fried my MD  
The power led is lit, but I get no signal on the RF. The mainboard is now untouched, the only things that are wired up are the switches for 50/60 and JPN/ENG...
Is there anything I might try to bring it back to life?

[djlooka]

Still wondering about what could have killed my MD...
During my various attempts I had erroneously soldered the +5V on the 16th pin of the SCART, instead of the 8th, which I now think is correct.
Anyway, could this have brought my MD to heaven? I mean, with the wrong pinout as said before, I could even play for a while.
Of my attempts, the killer seems to have been this: http://i16.photobucket.com/albums/b14/moosmann/electronic/md1rgb.png.
My guess is: wrong +5V wiring and 75 Ohm instead of 180 Ohm between pin 8 and 16 might have done the trick.

Anyway I'm still interested on the diagram, so that if I can somehow resurrect the MD (or eventually buy another one) I can finally perform this mod.

Thanks

[Midori]

Strange that your machine was fried, and sad of course.

But it should have nothing to do with 5V going to either 8 or 16.  That doesn't matter, at all. The only thing that would be damaged from that would be your TV, not the MD.(although that is a pretty sad TV if it can't take that.)

The TV has two sensors. One connected to pin 8, it "listens" for a voltage between 0 and 12. If it is around 5V then it switches the TV to the AV channel and in widescreen mode, it it is around 12V it switches to AV in 4:3 aspect ratio mode.

Pin 16 is the RGB switch, the resistor is there because it doesn't require the voltage to detect RGB to be 5V. It is good with less, so the resistor lowers the voltage. But there is nothing dangerous mostly with sending 5V directly. As I said, it would be the TV it would be dangerous for. Not the MD.

[djlooka]

After quite a while, I put my hands on the MD again.
The SCART is now connected, and everything looks in its right place.
But the MD is still dead. The power LED doesn't even light up.
Any suggestions? Where should I look for faults?
The caps look fine, none of them seems to be fried.