Mega Drive / Genesis - RGB caps and resistors revisited

[MtothaJ]

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.

Really appreciate this! I think that I actually saw something similar in one of the manuals for the encoder chip. WIll definitetly give this a try.
In the meantime I actually modded the cable again
THis is what I got:
- RGB: 1000uf caps + 75ohm resistors
- csync: 470uf cap + 75 ohm resistor
WIll post update about this setup but in the meantime when I have a moment I will try to do another scart plug as per your diagram.
As for the resistors on the RGB lines - definietly anything above 75ohms kind of makes the visuals duller / less vibrant.

[MaxWar]

Just keep 75 ohm in line with the cap. I think 1000uf is way overkill btw but should not hurt anything beside maybe impact image quality a little but Im not sure either. But im pretty sure it will not cure your color shifts either.

As I said I would start off with a 500 ohm pull down on each color lines Before the capacitor ( this is really important )and work your way down gradually if the issue is still present. But do not go under 75 ohm pull down. You can put one on the sync but I doubt that will be necessary. You would more likely be getting screen stutters instead of color shifts if the sync was out of spec.

Edit: Also, a detail, I'm not sure if putting the 75 ohm resistor before the pull down ( like in the diagram i made ) or after the cap would change anything. I usually put the resistor after the cap, I do not know why i put it first in the diagram, I made this in a hurry during my lunch break.

[MtothaJ]

Just keep 75 ohm in line with the cap. I think 1000uf is way overkill btw but should not hurt anything beside maybe impact image quality a little but Im not sure either. But im pretty sure it will not cure your color shifts either.

As I said I would start off with a 500 ohm pull down on each color lines Before the capacitor ( this is really important )and work your way down gradually if the issue is still present. But do not go under 75 ohm pull down. You can put one on the sync but I doubt that will be necessary. You would more likely be getting screen stutters instead of color shifts if the sync was out of spec.

Edit: Also, a detail, I'm not sure if putting the 75 ohm resistor before the pull down ( like in the diagram i made ) or after the cap would change anything. I usually put the resistor after the cap, I do not know why i put it first in the diagram, I made this in a hurry during my lunch break.

THanks again - I actually found the page of the manual for a CXA1145 clone chip and it does indeed feature pull downs:

pull down.jpg

In this configuration its 75ohms after the cap / resistor combo but I would imagine that in both cases the acheived result will be the same. I will give this a try - will probably put all these parts on a seperate board / box or maybe inside the genesis, becasue its getting too big for the SCART enclosure (especially with the 1000uf caps).
On the subject of the the new 1000uf configuration, I played a few hours yesterday on various games, also played around with the 240p test suite via the Everdrive - tested variosu options eg. bright white screen etc. - so far so good, didn't get any colour shifts or interference, but will keep you guys updated.\
As can be seen there is much more to properly wiring these cables then initially meets the eye.

[MaxWar]

Notice however that in this case the pull downs are 75 ohms and are placed after the caps not before.

They are there for a totally different reason. This is a test setup to check for output levels and I think the 75 ohm pull downs are there simply to simulate the standard 75 ohm input impedance of television sets. They would not be there in an actual application setup, only for testing purpose.

[tomaitheous]

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.

Are you running 75ohm impedance cables for each R/G/B element, to match the 75ohm pull down? It's there to remove ghosting/ringing effect (transmission line theory).

I remember reading that some RGB monitors required 150 ohm pulldown (and I assume matching cables) and some that required non at all.

[MtothaJ]

Notice however that in this case the pull downs are 75 ohms and are placed after the caps not before.

They are there for a totally different reason. This is a test setup to check for output levels and I think the 75 ohm pull downs are there simply to simulate the standard 75 ohm input impedance of television sets. They would not be there in an actual application setup, only for testing purpose.

I have found that if the 240p test suite bright screen is left on for a suitably long period of time the screen will turn a pinky hue.
I guess this is a stress test condition thing since with the new caps it hasn't happened yet in game.
I will try the pull down thin and in the meantime await for the offical Sega cable - I wonder how thats going to turn out.

[evildragon]

http://atariage.com/forums/uploads/m...1248492376.jpg

This states that the Sony PVM's 25-pin RGB input must be 75 ohms terminated.

[MtothaJ]

http://atariage.com/forums/uploads/m...1248492376.jpg

This states that the Sony PVM's 25-pin RGB input must be 75 ohms terminated.

The ones with the BNC connections are automatically terminated @ 75ohms if the video is not being looped to another monitor.

[MtothaJ]

Just keep 75 ohm in line with the cap. I think 1000uf is way overkill btw but should not hurt anything beside maybe impact image quality a little but Im not sure either. But im pretty sure it will not cure your color shifts either.

As I said I would start off with a 500 ohm pull down on each color lines Before the capacitor ( this is really important )and work your way down gradually if the issue is still present. But do not go under 75 ohm pull down. You can put one on the sync but I doubt that will be necessary. You would more likely be getting screen stutters instead of color shifts if the sync was out of spec.

Just to check with regard to the pull down resistor values - what is the relationship between value of resistor and transmitted current - is it that the higher the pull down resistor the lower the current transmitted to ground hence the higher the current going to the monitor? WHat are the consequences of starting of with a lower value e.g. 220ohm vs 500ohm?

[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:


Attachment 7991

Lets hope it will be worth it.

Recieved this yesterday and have been testing all evening. So far the results are excellent - no inteference, no colour shifts etc. Picture brightness is simply ideal also.
The cable fired up straight away on my machine, it takes it sync from csync, which I have wired internally to csync out and not csync in on the encoder, but my feeling is that this wouldn't make much difference (can test this since I have another unmodded machine if anybody is interested).
Bearing in mind the official cable has a whole box of stuff attached to it, my feeling is that an OEM spec cable is more than simply a few resistors / capacitors soldered onto the lines.
The only downer is that the cable is mono, but this is no big deal since one should be able to attach two wires to the L and R pins in the Scart plug and just run a audio cable with a 3.5mm male jack from there to connect to the Mega Drive. Haven't opened the scart plug though - at this moment I just want to enjoy the picutre / games and not mess about with soldering irons etc.
I will keep you guys updated, but at this moment it seems that for anybody with RGB problems picking up the OEM cable is the way to go.

[MtothaJ]

Just to check with regard to the pull down resistor values - what is the relationship between value of resistor and transmitted current - is it that the higher the pull down resistor the lower the current transmitted to ground hence the higher the current going to the monitor? WHat are the consequences of starting of with a lower value e.g. 220ohm vs 500ohm?

Just to say that before I got the OEM Sega cable I gave the above a try - didn't work for me I used 500ohm pots on the pull downs and all it really did was act like a resistor wired in series, so I could control the screen brightness. Maybe I wired it wrong (however don't know how since I did this exactly as discussed).

[retrospiel]

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.

seems this is related: http://www.sega-16.com/forum/showthr...l=1#post623291

[omp]

My scart lead has no caps in it for rgb (resistors are there though), it seems fine through my rgb to scart converter. Reckon it would be worthwhile adding the caps? Perhaps the converter acts as a filter of sorts?

[MtothaJ]

My scart lead has no caps in it for rgb (resistors are there though), it seems fine through my rgb to scart converter. Reckon it would be worthwhile adding the caps? Perhaps the converter acts as a filter of sorts?

Lack of resistors - too bright picture
Lack of capacitors - possible interference on certain displays when there is more power going through the lead into the display (e.g. white / bright background) - the caps seem to smooth things out / eliminate surges

I say that if your lead works fine then there is probably not much point adding anything to it. The lead without caps seems to work fine on all manner of TV's both CRT and LCD.
I don't think console manufactures nor manufactures of broadcast / medical monitors assumed the two would be hooked up together hence the specific over sensitive problems I was experiencing.
I have actually modified my initial lead - removed all caps / resistors from the scart plug (since the caps were getting too big) and basically added a board near the console end plug onto which I have put 1,500uf caps and 500ohm pots for RGB and 470uf cap / 75ohm resistofor for sync. THis is actually my best lead - absolutelly no interference, no colour shift due to the mannly caps used and also the possibility to individually control the brightness for the RGB lines (although I have just set this at the standard 75ohm resistance).
I will be getting my Sony 21" CRT TV with Scart RGB out of storage this weekend, so I will try both this lead and the official Sega one and see whether the capacitors casue any problems. It will also be interesting to compare the picture quality between the BVM and a consumer CRT TV.

[fluxcore]

@MtothaJ

Do you have any idea what comprises your 'official' cable? I'm currently trying to run my PAL MD1 VA4 through RGB into a GBS-8200 RGB->VGA converter, and having problems.

My cable simply has:

MD R -> 75ohm res -> GBS 'R'
MD G -> 75ohm res -> GBS 'G'
MD B -> 75ohm res -> GBS 'B'
MD Csync -> 220uF cap -> 75ohm res -> GBS 'S' (with -ve leg away from the megadrive end)
gnd -> GBS gnd

I should mention there is also a 'VS' wire on the GBS harness - I've assumed the 'S' one is the correct one to use, but I suppose I could be wrong there (the manual certainly doesn't explain the header in english!)

I've tried the completely unmodded system, which seemed fairly stable, but had some slight artifacts in the display. I thought 'maybe these are due to the MD1's csync being from pin 10 rather than 11' and so put a straight wire from the encoder pin 11 to the csync pin on the DIN connector (I figure I don't need the 200uF cap/75ohm resistor here, if it's in the cable). Now the image is very jumpy, and blanks out constantly. On both setups, colours have looked pretty much fine.

From what I understood, the GBS-8200 is a pretty common device, and has been successfully hooked up to MD1s before (mmmonkey certainly seems to have).

A little bit later I might revert my pin 11 -> csync mod and then take a video, if that helps, too.

Any help greatly appreciated.