Hi Helder, first off, thanks for going the DIY route instead of buying a board which was the intent of this thread, to not have to buy some JROK or similar board and learn to build it yourself. Very few participants have tried to do so and everyone else just resorted to buying turbokon's boards. I found that disappointing, but I understand, not enough has been done to reduce the first impression complexity of the circuit. The OP is very unfinished and more work on better presentation and guiding is needed which I hope will happen over time.
Anyway, I have a quick experiment for you to try. I dunno if it'll help, I wanted to be set up with my scope before getting there, but I am behind on it.
So, for relevant background, the formula for producing a proper Luma/Y signal for YPbPr when the resolution is 240p/480i/480p is something like this: Y = .299 (Red) + .587 (Green) + .114 (Blue). It's different if the resolution is 720p and up and the TV's circuitry would expect a different mix as a result.
Given that formula for Y, those first 3 resistors (R1-R3) for the RGB lines are what condition our Luma/Y mix to get to that standard which most TVs will expect. Now to translate it to precise resistance values is the trick/thing!! The way I've seen it done was based on the first feedback resistor to the first amplifier component which inverts the signal (thus, gives you a 180 degree out of phase signal, hence "-Y" and why the P [for Phase shifted] notation is used for P_blue and P_red). The properly selected feedback resistor is 324 Ohms in that case, so for R = 324/.299 = 1070 Ohms, G = 324/.587 = 549, and B = 324/.114 = 2940 Ohms.
Well, Steve's chosen feedback resistor is 500 Ohms and so if blindly using that same formula, then:
1) Red = 500/.299 = 1672 Ohms
2) Green = 500/.587 = 852 Ohms
3) Blue = 500/.114 = 4386 Ohms
So to get to the point, for R1 on Steve's schematic (page 1 of thread), I want you to try 1672 Ohms, R2, 852 Ohms, and R3, 4386 Ohms. R2 going to 852 Ohms will weaken the green by a bit from the 500 that it's at now, but since this calls for weakening the others, still couldn't guess if that would bring the 3 signal levels to the proper balance that the spec calls for. You see, I don't have the special insight when it comes to the S8050 transistor and how that relates to the chosen feedback resistor of 500. Steve does, but he didn't closely follow the spec and went by his eyes in this area I believe. The circuit that I learned this math from was using an op-amp and the feedback of 324 Ohms was like a special sweet spot for optimal operation of it and what not... I am blindly applying it here with transistors so no guarantees obviously.
For thread reference/research, here is said circuit/schematic, but note that it's not complete as they do not inject H/V Sync (unlike Steve's circuit) to produce a working Luma/Y (left up to the user):
Well, if trying my idea, you'll obviously need like 2 solid resistors in series or 10k pots to get those non-standard values (like 4386 Ohms) for all 3 signals, but I wanted to offer those values up for a test since I haven't done it myself. The short answer is though you can simply increase the resistance value of R2 which is for green obviously. This assumes you did the whole circuit correctly as it is tricky and one soldering mistake will seriously throw things off!!
