ronin47 wrote: ↑Fri Mar 29, 2024 3:17 pm
OK, so I tried a slightly different circuit to you, but only because I had it ready made in a drawer!! It's just a composite mod I made for an Atari 2600 (I think) that I abandoned and bought a ready made one for that included a nice cable and audio built in too.
IMG20240329145534.jpg
IMG20240329150554.jpg
And there is a definite improvement with the LCD.
This is before:
IMG20240329145249.jpg
This is after:
IMG20240329145340.jpg
However this is the CRT with it:
IMG20240329145453.jpg
And this is the CRT without:
IMG20240329145649.jpg
Personally I prefer the CRT without.
I'd be interested in what you think.
Hi, yes I think the signal level is a bit too high with that particular mod circuit and the B&W CRT looks sharper without it.
The common collector (a.k.a. emitter follower) configuration has a unity Voltage gain, its main purpose being a buffer which requires little input current but can drive a greater output load. But the unloaded composite video from the original output of the Superboard II is over 2 volts p-p and too high for a standard 1V, 75 Ohm monitor composite video input.
In the circuit I have drawn the 75 Ohm output resistor is in series with the emitter output so, assuming a 75 Ohm termination across the monitor input that signal Voltage would be halved resulting in close to ideal 1V level. So it may be worth lifting that 75 Ohm resistor from ground on your circuit and trying it in series with the output rather than parallel.
Apart from that if the brightness / contrast levels can't be adjusted sufficiently on the controls of the TV itself or by backing off R58 on the OSI 600 PCB it may suit that particular TV better to use the unbuffered original output from the Superboard II.
One of the reasons I added the buffered Composite video output to a different pin of my J2 connector was so the original output could still be used, depending on the type of display connected. For an original style black and white TV or monitor the unbuffered output would probably be fine however my colour CRT TV and LCD monitor appear better with the buffered output.