Mega Drive / Genesis - RGB caps and resistors revisited

[MtothaJ]

This is the most commonly used Mega Drive / Genesis RGB cable schematic...:

megamstr.png

... while this is the official Sony video encoder chip schematic:

CXA1145_Outputs.jpg

Key differences of the Sony chip schematics:
- 470uf caps and not 220uf caps used on RGB lines
- 470uf on Csync or 100uf on composite vs nothing on sync in the first diagram
- first cap then resistor and not the other way around

Any thoughts / ideas on these differences?
I will want to try out the 'official' version since still not 100% satisified with my existing 75ohm / 220uf setup - getting no interference but at times the screen displays shifts to displaying too much red - its as if either the signal is still too strong and the cap gets overloaded (?) Perhaps a higher capacity one in the correct order will resolve this.

[TmEE]

Resistors are the only ones that are really required as most TVs already have the caps in them. Any cap wil work as long as its higher than ~100µF, lower values will cause ghosting like effect on long stretches of same color. The higher the capacitance the better.
Resistor/capacitor order does not matter.

[evildragon]

Resistors are the only ones that are really required as most TVs already have the caps in them. Any cap wil work as long as its higher than ~100µF, lower values will cause ghosting like effect on long stretches of same color. The higher the capacitance the better.
Resistor/capacitor order does not matter.

PVM's generally need them. Mine sure as hell did.

[MaxWar]

I might add that if you want to use RGB I recommend wiring Csync to pin 20 and not Composite.

[MtothaJ]

I might add that if you want to use RGB I recommend wiring Csync to pin 20 and not Composite.

I am using Csync, but didn't notice that its any better than composite for the sync. Both work fine.
I think the problem I am having is that the monitor is very sensitive and the MD signal still a bit too strong. WIll get the proper caps and experiment with different resistors. Many arcade boards require resistors even of e.g. 500 ohms so I might try to damoen the signal by beefing up those 75ohm resistors and see how that goes.
That said with the caps its almost perfect - e.g. a few days ago I played through the whole of Streets of Rage 1, then halfway through World of Illusion and a short blast of Sonic 1 and Pitfighter - no problem whatsoever. The problem I am now having appears very sporadically but it is really getting on my nerves - its almost perfect yet that 'almost' is pissing me off

[evildragon]

I would honestly just up the capacitor.. My PVM for example, without caps on CSync, I got this:



This is why I state PVM's REALLY need these caps and are sensitive to it. So, by all means, try a larger cap. Or, maybe the cap you have on red is defective, if it's only red that's doing it. I don't think it's the resistor. 75 ohms is all it needs to be correct.

[t0nito]

I never understood why the hell the Genesis/Mega Drive didn't have the caps and resistors built in the system instead of having to add them inside the scart plug.

[MtothaJ]

I would honestly just up the capacitor.. My PVM for example, without caps on CSync, I got this:
This is why I state PVM's REALLY need these caps and are sensitive to it. So, by all means, try a larger cap. Or, maybe the cap you have on red is defective, if it's only red that's doing it. I don't think it's the resistor. 75 ohms is all it needs to be correct.

Thanks, I guess thats a valid troubleshooting suggestion concerning the cap on red.
That said, before I open the cable again I will make sure that I have all the required bits and pieces (correct caps, a bunch of different resistors etc) and see how it goes - will post an update offcourse when I get around to it + have the time to do testing.

[MtothaJ]

I never understood why the hell the Genesis/Mega Drive didn't have the caps and resistors built in the system instead of having to add them inside the scart plug.

Some cheepskate cost cutting measure probably - they might have figured that not a whole lot of people will be using RGB so lets omit the hardware from the console and save ourselves $0.01 in production costs per machine.

[MtothaJ]

Well, just to give the update as promised, being a good Sega-16 soldier I went ahead and bought the needed parts and modded my cable once more as per the OEM spec. The result is that basically there is no difference - it seems that every so often a strong signal burst comes out of the Mega Drive which overwhelms the cap / resistor combo and causes the screen to turn to a different / more redish colour tone. After a while it seems that things come back to normal and the standard colour hue returns.
Not really sure whether placing any additional resistors would really solve the problem since that would really just darken both the initial picture and the subsequent brightness burst giving no net result. As for the caps all I can say is that they are definitely needed for a PVM / BVM setup they seem to smooth out / stabilize the signal – just seems that no enough in my case. One more thing I can try is to buy some big ass caps which are man enough to stand up to any increased strength signals- not sure if this makes sense in technical terms but I am figuring that this is what is happening.
Overall really pissed off with this situation – if this persists then I will think of switching that BVM for the Sony 21” CRT TV with Scart RGB I have in storage to see how that goes. It kind of seems that TV’s are less sensitive to the monitors to this kind of stuff. The Mega Drive played just fine connected to a Samsung LCD TV via SCART RGB – and that was with nothing else but resistors on the RGB lines.

[evildragon]

What's on CSync right now? I might have missed this.. I wonder what would be causing this. On PVM's, Csync is also very important.

[MaxWar]

Does the weird color shifts happen at random or at specific places?
Is there an off chance that using some pull down resistors before the caps would help? Funny thing is how it does not seem needed normally for other people.
Not sure what the ideal value would be, could try with about 500 ohm I guess. If it causes problems then go higher.

The Idea is that when an output is connected directly to a cap like this, its kind of like DC pressure can build up and saturates the cap and it causes glitches.
I know it happened before you added the caps, but if your PVM actually has them right on its inputs, it would amount to the same thing.

[MtothaJ]

What's on CSync right now? I might have missed this.. I wonder what would be causing this. On PVM's, Csync is also very important.

At the moment I have a 470uf cap and a 75ohm resistor as per the specification for the encoder, also I have rewired the round plug that goes into the Mega Drive so that sync comes from Csync and not Composite as per the standard cable specification.
HOWEVER I have read that sometimes in the MD1 the csync is wrongly connected inside the console - that it is connected to the Csync IN instead of Csync out on the encoder - check this out: http://www.sega-16.com/forum/archive.../t-19347.html?

AppoloBoy wrote:
One problem with the model 1 Genesis/MD is that the sync signal on the AV out is actually wired up to the sync input on the CXA1145. I had a problem with my Japanese Mega Drive through RGB on my Sony PVM where the screen would roll if there was a flash of white. I was able to fix the issue by wiring up the sync pin to the sync output on the CXA and all was well after that.

This would seem to kind of correlate with my problem. No other option but to disassemble the MEga Drive for the n'th time and see what the situation is like in my machine in this regard .

[evildragon]

Hmm, never had that issue on mine, but it may be what's causing it on yours.

[MtothaJ]

Hmm, never had that issue on mine, but it may be what's causing it on yours.

AppoloBoy was right - the csync on the AV out was linked to csync in instead of sync out on the encoder - to quote the AVGN - WHAT WERE THEY THINKING!
I quickly put this right - I know its not exactly a professional job hacking the AV connector up like that but frankly I don't give a f*ck - got a whole lot of other stuff to do this evening and I don't have time to spare:

P1010773.JPG

P1010772.JPG

Putting this piece of junk back together now - lets hope it fires up