AD725 rgb converter too bright

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Sprites Lover

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hi neo lovers,ive finally built my own rgb to ntsc converter with the ad725 chip.I only use composite as my tvs don't have S-video input.However the pictures on the games appeared too bright,the colors appear rainbow- like.i had to lower my tv brightness,contrast to even below 10 to make it look decent but overall still looks bad.I followed everything as shown in the ad725 datasheet.I read old treads some
suggested to add a pot?but which value?some said its due to bad grounding?Can u guys help me out?
 
ttooddddyy

ttooddddyy

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If you were to use pots, 470 ohm would do.
But you should not have to imho. me thinks
 
Xian Xi

Xian Xi

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make sure your capacitors are facing the right way.
 
Xian Xi

Xian Xi

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I usually use 124-150ohm resistors on the RGB input side.
 
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Sprites Lover

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oh so u use 150ohm resistors instead of 75 on each of the red,green,blue output?how about the resistor at the output near the composite?according to the datasheet its 75ohm too.
 
Xian Xi

Xian Xi

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Sprites Lover said:
oh so u use 150ohm resistors instead of 75 on each of the red,green,blue output?how about the resistor at the output near the composite?according to the datasheet its 75ohm too.
Click to expand...

The datasheet is for the circuit itself, which requires the 75ohm resistors for the picture. The resistors I am referring to are to adjust the RGB levels not alter the build of the circuit.
 
Murray

Murray

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Sprites Lover said:
hmmm so i just leave the 75ohm as it is and add the 150ohm on the 3 rgb lines?
Click to expand...
Correct. Leave the composite and s-video output resistors alone and just add some to the RGB inputs until it looks right. I think I uses 110-125 or so (in addition to the existing 75s) on the inputs on my AD724 encoder. It keeps the picture nice and bright but not so much that it's washed out.
 
Xian Xi

Xian Xi

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So did you add them on? Hows the picture now?
 
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Sprites Lover

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Murray said:
Correct. Leave the composite and s-video output resistors alone and just add some to the RGB inputs until it looks right. I think I uses 110-125 or so (in addition to the existing 75s) on the inputs on my AD724 encoder. It keeps the picture nice and bright but not so much that it's washed out.
Click to expand...

ok i tried out.The picture improves dramatically! WOW!no more 'overexposure' in the colors!thank you so much.
 
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Sprites Lover

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Xian Xi said:
So did you add them on? Hows the picture now?
Click to expand...

ok i add the 150s on the rgb inputs in series with the 75ohms..and at the composite output,instead of 75ohm as stated in the data sheet,i removed it and placed two 150ohm in series.The picture looks great!
 
segasonicfan

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Yeah, it was me who suggested adding pots way back when I first started working on the circuit a couple years ago (I even made a little schem on my site). It's a decent chip but the RGB lines aren't standard 75ohm termination (even though the datasheet says they are) which is just stupid. just use pots and its gravy :)
 
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Sprites Lover

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segasonicfan said:
Yeah, it was me who suggested adding pots way back when I first started working on the circuit a couple years ago (I even made a little schem on my site). It's a decent chip but the RGB lines aren't standard 75ohm termination (even though the datasheet says they are) which is just stupid. just use pots and its gravy :)
Click to expand...

wats ur webby address?i would like to look at ur own diagram.so which pot do u recommend?470ohm?
 
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Sprites Lover

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ive taken a look at ur diagram from the link.Now i understand..replace the resistors with the pots on the rgb.btw ur supergun is HARDCORE! Wow!
 
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