This is a PAL coder that can be used with CoBra, to display the color video out on a PAL TV screen. I used this version with the first CoBra mainboard (grey) I built in the '90s. "Successfully built", means that even if I built it once back then, I built it once again now, on a board designed from scratch and fabricated, with recently manufactured ICs. Below is a picture of the coder the way I built it and installed it on the grey mainboard back in the day.
This version uses 4 TTL ICs. I got the schematic from others in the '90s, so I don't assume any credit or responsibility for it. If anyone knows the author, I would very much like to put his name here. The only weak point of this schematic are the somehow unnatural colors. To be more precise, I'm showing below a snapshot of the CoBra logo generated through this PAL coder and displayed by a Hauppage TV tuner for PC with a BTTV chip.
Schematic - v.0
The original schematic I had back then (see the paper scanned on the "Documents" page) used resistors with different values. Now when I built it again, I wanted to use LS TTL circuits, but with a 74LS00 the quartz oscillator would not generate a clearly cut waveform between "1" and "0", but rather between "in air" and "0". I had to change U4 with a 74AS00 (Schottky) and bring the value of resistor R3 to 750 ohm in order to get a correct output signal. I also changed the values of the other resistors, basically bringing them all to the same value: 750 ohm.
YO3GHM used a little modification, replacing R4 and R6 in the schematic with 1N4148 switching diodes (see schematic on the left) in order to add the two color components ("U" and "V") with equal levels. I'm quoting below his own explanation, the way he wrote it to me by Skype:
"By eliminating the two summing resistors (R4 and R6) I did not have to adjust colors independently anymore... I would just do a general color adjustment and that would be that. Because... no matter how I would struggle to do them correctly... with other TTLs... I always needed different values. Because I would not make coders with only one kind of 7400... I would use whatever I would find, Russians, Romanians, normal, LS, ALS. The best results I had with the ones in the "S" series, that is K531LA3 (74S00). You know we would pick up whatever we could scavenge... back in the day. And back then I got rid of a problem... didn't matter how."
I tested this modification personally but I did not notice any change, but then again I use a TV tuner for PC as display for CoBra, which seems to be a little more tolerant to errors. Maybe if I used a TV set as display there would be some visible difference.