1084sd-1 slight wobble on very top of picture

[bigsanta]

This is another bloody 1084sd-1 problem,which i assume to be caused by it getting a ntsc-j rgbs signal from my newly acquired ,official jap mega drive.

I am feeding the monitor r g b cvid gnd/Vss ,from my scart to db/e9 male plug .The scart plug is internally wired as so, r g b with a 75 ohm resistor in series on each line, cvid ,5Vcc going through a 100 ohm series resistor ,Vss and mono audio connected to both L & R scart pins .

Now when i feed the 1084 a pal I rgb signal from a uk master system,uk ps1 and 2 ,then i get a rock steady image ,but using the ntsc-j mega drive 1 ,gives me a tiny bit of wobble on the very top of the image,it bends to the right .I can fix this by using the horizontal control, to shift the image further to the right or left,but doing this causes dirty great big black borders down one of the sides of the image.So if i want a full screen image ,i can't ,only with a PAL I console connected will i get full screen perfection .

I do remember seeing something on this site regarding the lm1881 and not connecting something to ground with a 1084 ,as you would get a wobble ,but i'm not using a sync stripper ,so ?  

My rgb scart cable is around 3 metres long ,just incase this could be effecting the video stability.

[bigsanta]

One more thing,if i plug in the audio lead,the wobbling gets a bit worse and the image goes a bit brighter.

[ninn]

Hi bigsanta. 

You know, yeah, I did the same mod like you, 1084, sync-hack and PAL Megadrive, and yeah, .... Does it look like that?



(Pic1: This is Sonic in 60hz on the 1084. Notice the checkerboard on the loop, and the bent palm tree.)

Now ... compare the upper image to this picture:


(Pic 2: This is Sonic in 50hz on the same monitor. Notice how the checkerboard and the palm look like they should.)

... thou, the slight bending occours now at the middle of the screen, look:



(Pic 3: bending in the middle of the screen, at 50hz sonic)

... whereas at the 60hz, the bending in the middle is not existant, but at the very top - see Pic 1.





Pic 4: Sonic is confused.


... i checked this behaviour on several (PAL) consoles, - cause I thought that I destoyed something inside the console first  -  ... but it was noticeable on all of them. Same characteristics, same bending.

Did this help you?

ninn

p.s.: My RGB-Cable does not have 75ohm resistors / 220uF caps added, like a scart-lead would have needed. Would this matter?

[bigsanta]

Yep,mine shifts to the right like that,but not as much.The caps,well i've never needed them before,and my pc engine also has a rock steady image ,and i've always used 75 ohm resistors in my md/sms rgb scart leads ,so ?

I think it might have something to do with the jap signal .As with everything else PAL i that i've used ,the whole image is solid ,top, middle and bottom .And this 1084 i have ,it has a few other faults , it gives me small electrical shocks sometimes,when touching things like the headphone jack,the screws on the back and the other input sockets(when it's unplugged) and the sound has a background fizz ,aswell as sound still coming through even when the audio is tuned completely off .

It's still hanging on after all these years though.

The only time i used to get any centre problems ,was when i had more than 1 console plugged into my multi scart box.That was down to the grounds,so i just kept one thing plugged in to fix it.

[ninn]

... if you find a fix, or need something tested, just let me know.

My horizontal shift is very small too: It is about 6mm ... the images were just in macro-mode. 

I think it has nothing to do with the JAP signal, cause my Megadrive is a PAL one with a 60hz switch.

... meanwhile, I will hunt for a LM1881.

ninn

[krullo]

(first post!)
Hi there.

I don't know if it's a good idea or not, but I fixed this problem by connecting pin 7 of the LM1881 (odd/even output) directly to ground. I have no technical explanation for this, as I figured it out by experimenting. I also added a 100nF decoupling cap between V_CC and GND on the chip, but it was the pin 7 fix that made the real difference. Try at your own risk! Hope this helps.

[ninn]

There is no LM1881 in this solution. 

[bigsanta]

It does wobble even with an lm1881,when i tried one out about a week ago .So i made a new one up yesterday ,along with a smoothing cap .So far it's rock steady.Now if i remove the series resistors on the R G B lines ,then i get some problems,mainly the horizontal centre line of the screen.Strider shows this off best when you slash with his sword,the right edge of the screen moves out.This is down to the contrast/colour  levels being too high without the 75 ohm resistors.Using the monitor's contrast control's ,the problem goes away ,but the image is practically black .Add the resistors back and that should fix the problem .

[bigsanta]

Added back the resistors on the R G B lines as standard and the image is PERFECT !Seems the smoothing cap worked and i placed it right on top of the IC.Then i whacked it inside a switching multi scart box,so i've no need to mod the 1080 again if this holds up .

[ninn]

whats a smoothing-cap? 

[bigsanta]

Smoothing/decoupling capacitor ;Removing noise ,in essence i believe .
http://en.wikipedia.org/wiki/Decoupling_capacitor

[ninn]

I just modded an NTSC Dreamcast to RGB, and plugged it to the monitor ...

... slight wobble on very top of the picture. 

[ninn]

I guess I found the solution.
read http://nfggames.com/forum2/index.php?topic=4438.msg29856#msg29856
.. looks like it is a linearity-problem of the monitor.

Maybe you can confirm this by looking for a sweetspot too, bigsanta.

[bigsanta]

Calibration tests for those specific consoles is a huge help ,so many people don't even know about calibrating the monitors/tvs.Some just keep to the factory settings  ,but i'm not going to bother using those tests,instead, i'm sticking with the sync IC and the perfect synced picture it's giving me on everything and i have already calibrated my colour/blacks and the white levels .

Good gear though!

[bigsanta]

One more thing (problem)i encountered.Right,all my consoles are now dot on with the lm1881.But  when i finished my cmvs ,i found it had sync problems.But i fixed it fairly easily.I opened my rgb scart lead,popped the sync pin out ,hooked up a 10K pot on my breadboard .Then ,using the cmvs's cross hatch option,i adjusted the pot to about 2k7 ohms to get a rock steady image and then just swapped that for a 2k7 ohm resistor and that's finally the end of all my sync problems once again 

Ninn ,you should try that with your internal mod ,as when you're adjusting the sync pot ,this has an action of moving parts  of the screen horizontally(like horizontal sections until it syncs better,or goes completely out of sync).