Help with Sega Master System 2 RGB mod.

Hello everyone.


I'm stuck. I have been performing a RGB scart mod to my SMS2. The guide I am using is here. It's pretty much a guide that is dotted about everywhere except the connector is a SCART bus. So far on my LCD TV I have got a rolling image I can see a garbled "SEGA" logo on the screen when I switch the machine on. On my CRT TV I can't get any picture, although it seems the TV is picking up some sort of signal because the AV input logo on the screen shifts when the system is switched on. I think it's a sync issue but I'm not certain.

Has anyone else successfully performed one of these mods that can help me?

Take it easy,

Neil

 

I'd double check your wiring first, it sounds like it may not be getting sync or your TV isn't properly switching over to RGB.

 

I'm pretty certain my wiring is ok, but I will recheck everything tomorrow.

It's really strange, I must be doing something wrong because I tried this same mod on another system that had a Fujitsu MB3514 encoder and I had exactly the same problem. I bought another master system, assuming it was the encoders fault

 

Most older consoles are low resolution. Even with RGB the TV still needs to support their graphics mode.

Have you tried RGB from a Megadrive or other similar consoles?

 

Hey Lum, yeah I have had my Megadrive running through RGB SCART on the same 2 TVs I'm testing this on, so that's probably not an issue.

 

Ok this is what I've got.

I checked my wiring through with the multimeter, no shorts and all good connections.

I removed the 100 Ohm resistor from the Blanking/RGB switching and subsituted it with a 1K potentiometer so I could try different resistances. At the bottom of the pots range I got a clean, stable composite signal. Turning the pot switched the composite signal over to the RGB, which is still rolling at different rates, depending on how far the pot is turned.

 

In addition to SCART pins 4, 17, and 21, you could try grounding SCART pins 5, 9, 13, 14 and 18. Try connecting them all up together inside the SCART plug, so that basically every single ground point is then connected up to the console's ground (Pin 24 on the Sony IC).
I think you could get away with using the metal casing of the consoles RF modulator as a ground point, although I don't know this for certain.

The 75ohm resistors probably aren't required, so I'd remove (or bridge over) them. Also make sure the SCART lead you're using has no additional resistors and/ or capacitors inside the plug.

 

Ooof. I'm getting close to getting this right. I copied the schematic under "Sega Master System/Mega Drive" from this website on how to make your own SCART leads except rather than putting this in a lead I put it in the circuit inside the Master System. It's essentially the same circuit as all the other guides execpt the composite comes straight out of the chip to pin 20 of the scart lead. Also, for blanking instead of a 100 Ohm resistor from the chip to pin 16, the guide recommends the blanking goes straight to pin 8 and is then bridged to pin 16 via a 180 Ohm resistor. I don't have a 180ohm resistor I only have the choice of 150 or 200 ohm here. At the moment, the picture is most definitely there, but there is still tearing and rolling - I'll get a pic for you at some point.

oldgamingfart - I did tie all the grounds on the scart together, just to eliminate that possible issue, I have them connected to pin 24 on the CXA1145. I will give removing the 75ohms a shot at some point when I have time. I have checked the SCART for any resistors or caps, it's all clear. Cheers for the advice!

 

Make sure you connected the ground well????

 

Yes it's recommended you use pin 8 and 16, as some TV's can be a bit fussy when Pin 8 is not used. I think it's probably still a sync issue so a direct composite connection (minus the in-line cap and resistor), is worth a shot.

 

Ok, I pretty much exhausted all I can think of, here are the results of the various things I've tried. All this time the +5v has been going straight to pin 8 and then bridged via a 200 resistor to pin 16 on the scart connector.

Attempt 1 - Composite/Sync straight to pin 20, RGB - 75ohm res + 220uf caps - Over bright colours, reds look pink, yellow looks cream etc, slightly flickering RGB picture

Attempt 2 - Composite/Sync straight to pin 20, RGB - 75ohm res - Stable RGB picture, over bright colours

Attempt 3 - Composite/Sync - 75ohm res, RGB - 75ohm res + 220uf caps - normal composite picture, no RGB

Attempt 4 - Composite/Sync - 75ohm res + 220uf cap, RGB - 75ohm res + 220uf caps - very flickery, almost indistinguishable picture

Attempt 5 - Composite/Sync - 75ohm res + 220uf cap, RGB - 75ohm res - dark, stable composite picture.

It seems that when ever I use capacitors I get flickering and dropout.

I can't think of anything I havn't done. My head is going to explode!

 

Sounds like your Sync is weak / noisy. (there are MANY circuits to clean / fix this)
Also if the color is excessively bright - try a stronger resistor value on the RGB lines only.
You probably just need to tune the RGB resistor values - if you had an Oscilloscope this would be a cinch.

 

Are the resistors really 75Ohm (4-stripes: violet green black gold / 5-stripes: violet green black gold brown) and not 7.5, or some other eye-misleading value? But yeah the correct approach is #2

R -» 75Ohm -» +Cap- -» SCART_R
G -» 75Ohm -» +Cap- -» SCART_G
B -» 75Ohm -» +Cap- -» SCART_B

Composite video -» SCART_V //OR// Composite Sync -» 1kOhm -» +Cap- -» SCART_V (pin 20)
Do any of the above, but not both at the same time

Ground «- SCART_Grounds and chassis

5V -» 100 upto 200Ohm resistor -» SCART_FastBlank (pin 16)

5V -» SCART_AVswitch (pin 8) (may turn on 16:9 squished mode) (not necessary)

 

I think you're right Melchior. No matter what I do with the composite/sync line on my breadboard, I cannot get my screen to stop still - if I pull out the comp/sync, the screen goes blank so it must be doing something, just not what it is supposed to. It's incredible, a lot of people on the net seem to have pretty good success with this mod and I've borked it - TWICE!

I fiddled around with the RGB lines a bit, and from what I can see of alex kidd - very primary colours - I've got the RGB about right-ish... maybe. Unfortunately, I don't have the luxury of an Oscilloscope.

 

Attempt 2 - Composite/Sync straight to pin 20, RGB - 75ohm res - Stable RGB picture, over bright colours

If this attempt looks good except for the colors being too bright, you might increasing the value of the resistor. I've seen people use 330ohm with success in N64 RGB mods and while an n64 and SMS2 are different beasts it might attenuate the signal properly. You can try stringing 2 75ohm resistors to see if it improves things at all before you drop the cash on more resistors.

 

Hey everybody, I thought I'd report back. I am pretty much shot for ideas now and I just can't get a stable picture. I have attached some pics to look at so you can get an idea of what I'm seeing - good and bad. The best results I have gotten so far have been set up like this:

Composite/Sync - 75ohm res, 220uf cap
R - 68 or 75ohm res
G - 68 or 75ohm res
B - 68 or 75ohm res
Fast blanking - straight to pin 8 on scart, bridged to pin 16 via 150ohm res.

I'm next to quitting and sticking with s-video, which would be a shame because the RGB signal I have seen looks amazing, also I don't have s-video on all of my TVs, so it's not as useful.

 

Try pulling sync from pin 11 instead of 20. Be very sure not to pull anything from 12 for it provides power.

You may need to attach a 470uF capacitor, positive side towards the CXA1145 IC, to pin 11 followed by a 75ohm resistor if sync from pin 11 doesn't work on its own. The datasheet calls for it but it may not be necessary.

 

Hey APE - I tried pin 11 without the cap and res the picture rolled violently, when I added the cap first followed by the res it got a little worse. Then I tried switching the cap and res around which landed much the same results as the ones I have posted above in the pics.

 

Cap and a 75 ohm resistor? I'd try it on another display, maybe yours really hates the SMS.

Only other option I can think of is a sync stripper on composite video. Though I'm unsure if it'd be any better.

 

Hey guys I thought I'd report back on this. The good news is that the RGB mod is working but not on the TV I was testing on above - it just took me a while to work out that not all SCART leads are created equal. I've got it all mounted up and finished. Thanks for all the help guys.

It does beg the question - why won't it work on my main TV? My Megadrive 2 is running through an RGB scart lead and that works flawlessly on my main TV. :S

Oh well - I'm off to play some cool 8-bit video games