Mega Drive / Genesis - RGB caps and resistors revisited

[ApolloBoy]

kool kitty89 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.
Oh, cool, that's a really simple fix then (just cutting 1 trace and soldering 1 patch wire from the encoder to the AV port) . . . and then you'd just need to pull the composite video pin from the SCART cable (on the MD AV connector end) unless said cable already uses only c-cync.

The part before "Oh, cool" is actually my post.

[MtothaJ]

The part before "Oh, cool" is actually my post.

Sorry for confusion - edited my posts to give righfull credit for this insight.

[MtothaJ]

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.

The colour shift happens on bright backgrounds / visuals. I actually put the console back together - it fired up fine, but in reality my sync mod based on AppoloBoy's advise made no difference - in Sonic 1 just after Green hill zone act 1 special stage when I got the emerald and the bright white summary screen appeared the colours shiffted and remained shiffted until about 20 seconds after act 2 started.
I guess that what you are suggesting with the pull down resistors might be a good move to try - time to get hacking on that poor cable again

EDIT: tried the following resistors before the 470uf cap + 75ohm resistor combo on the RGB lines:

- 440ohm - way too dark, no visual enjoyment whatsover
- 220ohm - better than the above, but still too dark e.g. Streets of Rage first stage instead of having vibrant neon like colours was just dull, it helped if I turned the contrast up to max on the BVM monitor but can't see myself going down that road for the long haul
- 180ohm - testing now - brightness seems acceptable, we will just need to see whether any colour shift glitches occur.

[MtothaJ]

EDIT: tried the following resistors before the 470uf cap + 75ohm resistor combo on the RGB lines:

- 440ohm - way too dark, no visual enjoyment whatsover
- 220ohm - better than the above, but still too dark e.g. Streets of Rage first stage instead of having vibrant neon like colours was just dull, it helped if I turned the contrast up to max on the BVM monitor but can't see myself going down that road for the long haul
- 180ohm - testing now - brightness seems acceptable, we will just need to see whether any colour shift glitches occur.

Update - unfortunately colour glitches still occur. I have narrowed down the capacitors to be responsible for the colour shift - with the caps removed there is no colour glitch but from time to time the horizontal interference type liens appear (as per description in the earlier thread: http://www.sega-16.com/forum/showthr...VM-BVM-monitor)
So it seems that the caps help with the inerference / to smooth things over but get over saturated - perhaps much bigger caps are the answer? THe onyl question is why does this hae to be this difficult / problematic...

THe only real things to try I can think of next are:
- build a small kind of project board where I could try different caps / resistors
- fit pots instead of resistors (say 500ohm) for RGB lines
- get a generic scart cable, chop one end off and wire it directly to the encoder - would eliminate any possible problems with the AV out - although these are unlikely since I have another Genesis and symptoms are much the same as the first one.

[evildragon]

I wonder at this point, if something is up with the monitor? Cause with my PVM wired up the way I originally stated, I have zero issues at all. So this all makes me so curious as to what's going on now.

[MtothaJ]

I wonder at this point, if something is up with the monitor? Cause with my PVM wired up the way I originally stated, I have zero issues at all. So this all makes me so curious as to what's going on now.

The thing is that Supergun / Neo Geo works without any problems, same for a DVD player.
Maybe something is up with the voltage regulator / PSU on the Genesis?

[evildragon]

It's possible, but I doubt it.

I think at this point, this is something an oscilloscope would be good to have..

[MtothaJ]

It's possible, but I doubt it.

I think at this point, this is something an oscilloscope would be good to have..

I think that its a bit over my head - I'm just an amateur. THe problem is that e.g. with the caps you can play for an hour or too without any glitches.
Then e.g. when playing Desert Strike I noticed that the desert started to got progressively more orange in colour up to becoming almost red - pausing and unpausing the game (brings up a darker colour status screen) made the colours go back to the way they should be.
At the moment my setup is 180ohm resistors on RGB and the 470uf cap / 75ohm resistor combo on csync - no colour glitches however this kind of setup got me the interference I was describing.

Just out of interest what would happen if I would take the RGB signal from the RGB IN rather than RGB OUT pins on the encoder? I figure this signal is a less amplified one...

[evildragon]

Well 180ohms on the RGB lines are dimming the picture too much. That's a lot.

Are you near Orlando Florida? I'm interested in checking this out actually. If too far, I may be still interested in letting you borrow my own cable, to see how it operates.

I'm sorry if I missed it, but is the input to your BVM the same DB-25 pin connector my PVM is using?

[MtothaJ]

Well 180ohms on the RGB lines are dimming the picture too much. That's a lot.

Are you near Orlando Florida? I'm interested in checking this out actually. If too far, I may be still interested in letting you borrow my own cable, to see how it operates.

I'm sorry if I missed it, but is the input to your BVM the same DB-25 pin connector my PVM is using?

Really appreciate you offer of help, problem is I am in the EU and not the US. The input to the BVM is actually BNC connectors to a female scart socket and to that I just plug in any Scart devices (actually also have a Hama scart switcher as well but for the purposes of troubleshooting I disconnect that).
THe one setup which I am presently thinking of reverting back to 75ohm resistors and putting some big ass caps which would be 'immune' to this oversaturation problem.
The other thing which would be worthwhile is to probably see how a Mega Drive 2 behaves when connected via RGB - as mentioned I have another MD but this is also a Mega Drive 1 HDG / no TMSS model so this is not really ideal as there may be a compatibility issue with the actual motherboard design / hardware type and I am just replacing like for like.

[MtothaJ]

Just dropped a pretty penny on an official french Sega 'Adapteur R.V.B.' - basically the offical Sega RGB cable for Mega Drive 1:

md scart cable.jpg

Lets hope it will be worth it.

[evildragon]

French? They run on SECAM, that may not even be RGB. :/ I wish this was mentioned first.

[MtothaJ]

The TV standard is not relevant since we are talking about RGB connection.
French Mega Drive actually does not output anything other then RGB - no RF lead socket / no composite.
It uses csync instead of composite for sync but the cable can eb modded to enable both composite and csync:
http://www.segakore.fr/articles/mod_...rgb_fr_md.html

[evildragon]

It was my understanding that the French had a special cable with a built in SECAM modulator that was not RGB.

[MaxWar]

The colour shift happens on bright backgrounds / visuals. I actually put the console back together - it fired up fine, but in reality my sync mod based on AppoloBoy's advise made no difference - in Sonic 1 just after Green hill zone act 1 special stage when I got the emerald and the bright white summary screen appeared the colours shiffted and remained shiffted until about 20 seconds after act 2 started.
I guess that what you are suggesting with the pull down resistors might be a good move to try - time to get hacking on that poor cable again

EDIT: tried the following resistors before the 470uf cap + 75ohm resistor combo on the RGB lines:

- 440ohm - way too dark, no visual enjoyment whatsover
- 220ohm - better than the above, but still too dark e.g. Streets of Rage first stage instead of having vibrant neon like colours was just dull, it helped if I turned the contrast up to max on the BVM monitor but can't see myself going down that road for the long haul
- 180ohm - testing now - brightness seems acceptable, we will just need to see whether any colour shift glitches occur.

You did not try with pull down resistors though. Pull down means that one end of the resistor connects to ground.

Try using a pull down before the Cap on your output ( In the cable )
Edit : Since the shift occurs on bright colors background specifically and that removing the caps stoped the issue, I am fairly confident that pull downs might solve your issue. Start with around 500 ohm on each color lines and see if it works.