PC-engine RGB mod - dark image

Hello! I realize this might be difficult to answer, but asking won't hurt right? I just built this RGB amplifier for my PC-engine, and I'm using it with a Sega Mega Drive 2 RGB scart. I've not included the capacitors and resistors on the R, G, B outputs of the amp since they're supposed to be included in the cable.

I get a good picture output, but it's quite dark. Is there an obvious answer to what might be causing this, and what I can try to do to improve it? I should mention that I used a 330 Ohm resistor on the collector of the Sync transistor, and not a 300 Ohm one.

 

What does your wiring look like?

 

I've wired just as the mini-DIN9 on the attached picture. I guess I'm not sure how the cable is wired, but it works perfectly with my Mega Drive 2.

Edit: Oh yeah, I also have another Mega Drive 2 RGB scart cable that gives me sound but a black image.

 

Assuming your MD2 cable is exactly what's in the diagram, the problem is that the amplifier circuit above doesn't actually amplify the signal's amplitude and the signal delivered to the TV has half the amplitude it should.

The only way for this configuration to work is to remove the series 75 ohm resistor in the cable. A better solution is to use a real video amplifier such as the THS7314 which has actual signal gain in order to deliver the correct amplitude to the TV.

Also I don't understand that MD2 cable--the resistors and capacitors must be placed at the SCART connector end because they surely wouldn't fit anywhere at the DIN end. This is problematic because the whole point of the 75 ohm resistor is to "back-terminate" the line, and for this to work it should be placed very close to the voltage source inside the console, or at the very least at the console end of the cable. It's very surprising to me that Sega wouldn't include at least the resistors inside the console if the console is meant to output TV-ready RGB.

 

Oh my, I just noticed what's wrong. The resistor going from +5V to the collector of the transistor should be 10 Ohms (for R, G, B) - I used 10k Ohms. I can't believe I missed that, stupid.

Eh, I'll just try correcting that mistake... ;-)

 

Now it seems to be working! However, the image is slightly shifted (black bar to the right). Sync issue? Is this a known problem?