Coleco Composite Video = blurry picture

[maddox]

hi there,

i'm new here

I just finished the Composite Mod to my PAL ColecoVision but the picture on my TV is very blurry.

Ok step by step

First, there is no R156 (1k) on my Coleco Board (different revision ?)
Ok I tryed it without R156 (connected to R154 (1k) wich leads to the empty points of R156) = blurry Picture
I added a 1k resistor = blurry Picture

R140 is changed to 3k3 ..

everything seems to be soldered right, but the picture is damn blurry (shaddows to the right)

I hope anyone here could help me with the problem.

[viletim]

What does your circuit construction look like?

You should try to minimize the distance between Q24, the 3k3 resistor to +5v, and the BC548 transistor as much as possible. Disconnecting Q24's collector from the PCB helps too. Try shorting across R140 with a wire link to see if that helps.

And you're right, there is no R156. It should be R154.

[maddox]

I think the Picture says everything

Maybee you could paint in some different point for +5V.


EDIT :

shorting R140 = black/white picture, no colours.

[viletim]

Maybee you could paint in some different point for +5V.

It doesn't really matter that the +5v supply is located some distance away but you could probably find it on one of the nearby logic ICs (usualy pin 14/16 (the one on the end) is connected to +5v). Point to point wiring is the best construction method here.

EDIT :

shorting R140 = black/white picture, no colours.

It doesn't surprise me that you loose colour altogethor when shorting out this resistor... But does it fix the blurryness? If it does then poor construction (wires everywhere) is the likely cause of your problem.

[maddox]

shorting doesn't "kill" the blurrynes ...
ok, i check my construction again and remove the wires.

[maddox]

here we go ...

removed all the wires except +5V and the wires to the Video-Jack.
But still blurry ... not better ...

[maddox]

*push*

still blurry, nothing better.

Is there maybee a difference between the BC548 transistor cause this device is available in an A, B or C version.

[l_oliveira]

It's just a suggestion. Colecos use the Texas TMS9928 video processor, right ?
How about taking it out of the board and replacing with a TMS9918 which has ready to use composite video out ?
The TMS9928 is B-Y R-Y Y format which requires another chip to make the conversion....

[Bostich]

This looks a lot like the problem I had with my RGB modded NES at first.  Turns out I had the transistor hooked up wrong.

[maddox]

I think everything is hokked up right, but still the same ol blurry problem ...

Any further sugestions ? or alternative video modifications ?

[viletim]

maddox, I'll try to have an alternate (op-amp based) method ready in the not too distant future.