Advice on TG-16 RGB mod

[ApolloBoy]

So now that I've got my TG-16 working again with the 9-pin mini-DIN I installed, I'm planning on finishing it up soon and adding RGB in addition to composite. I've been looking up info on it and I'm leaning towards this mod as it's simple and I have the transistors needed for it: http://www.mmmonkey.co.uk/pc-engine-rgb-amp/

However, I'll be using it with my Genesis 2 RGB SCART cable which has 75 ohm resistors and 220 uF caps on the RGB lines. Would that cause any problems or do I still need to build a full-blown RGB amp as opposed to the three transistors?

[NightWolve]

> http://www.mmmonkey.co.uk/pc-engine-rgb-amp/

I think that's terrible, TBH. The voltage on the RGB pins is very high when it comes to the Hu6260 chip, like 4.6 VPP or something if I recall correctly from steve. That ignorant design doesn't take any of that into account along with what would happen if a transistor shorted!

> http://www.gamesx.com/grafx/pce_rgb.jpg


> http://www.console5.com/wiki/Hu6260
> http://www.console5.com/wiki/File:Hu6260_RGB_Amplifier_Schematic.png (Same circuit, just bigger, clearer by console5)

This one does it right! It kills off the voltage and you got some protection to the chip, I'd say, by that 10 uF capacitor that connects first to the RGB pins before anything else.

With your cable providing some of that design, the 75 Ohm resistors to ground and the 220 uF caps to help kill off voltage, it's a step in the right direction, but not enough/wise by itself... You'd be putting stress both on the 75 Ohm impedance resistors with a 4.66 VPP RGB input, so they're gonna get hot and on the chip too (current flow). I'd add the three 10 KOhm resistors and 10 uF caps to the equation following that Japanese amp if it was me. Just try to complete the rest of the circuit internally given this cable of yours.

Notes: I came to understand not too recently what it means that a TV/monitor's video inputs are of 75 Ohm impedance. It's as simple as this: whatever input connector there is, be it a RCA jack or whatever, there is a 75 Ohm resistor connected to its ground. Picture a female RCA jack, and on the other side you immediately find that the ground (-) is connected to the inner connector (+) with a 75 Ohm resistor. The two are always connected with a 75 Ohm resistor. So, if you don't kill off the voltage, and it's high like ~3-4.66 VPP, that resistor is gonna heat up and you're gonna get current flow from the chip outputting the signal, etc. Take say the (+) red wire and the (-) black wire of a 5 V power supply and connect both to the leads of a 75 Ohm resistor, what happens ?? It's gonna get hot if not burn out... That's exactly what this guy is doing with his ignorant mod to the 75 Ohm resistors inside the monitor since as far as I can tell (that's what they're for, protection from this!), he wires it all directly to the transistors!

Your cable will protect the monitor here somewhat, at least, but yeah, to immediately ground 4.66 VPP with 75 Ohms is gonna equal a hot resistor (in the cable in this instance) I should think. That's why the Japanese amp does a few tricks before that point by starting it off with that 10 uF cap... FYI, I'm no expert, I just started learning this stuff from steve last year as you know, so don't take my opinion as authority here, etc. But yeah, I do NOT like the short cuts that are being attempted with that simple transistor-only mod there... It strikes me as a guy that thinks he's clever/slick because he's saving on parts, using the least amount possible, and it suffers from the "Well, durr, it seems to work, so I guess it's good enough!" syndrome (just cause you get a working picture doesn't mean you did it safely/properly for the long-run!). Bottom-line, whoever designed the Japanese amp actually knew what the hell he was doing and took more things into account, so that's the one to trust!!!