So I seem to be a bit obsessed with trying to get nice component video out of my recreated Funvision. I thought I might be happy just getting black and white working, but component video output is an interesting puzzle. Poking around the net you can find an op-amp based colecovision component mod and an MSX RGB mod. I also found a great technical discussion about 9929 component video on the NZ Vintage computer forums . And the TI 9928/29 Interface to Color Monitors doc is really quite interesting too, even though it shows a 9929 to RGB interface.
So I didn’t have any op-amps spare, but I did have an LM1881 sync separator and 4066 analog switch and heaps of resistors and BC547 NPN transistors so I thought I might attempt some of the detail described in this post but with transistors instead of the op-amp. I am not an expert when it comes to analog electronics, so this is mainly me having a play to see how it goes.
So originally with my Funvision recreation, I made a very simple transistor buffered interface to the component inputs on my Gonbes 8200 (I never tried this on a TV).
But based roughly on that NZ Vintage forum post, I’ve hacked together the circuit below. The sync separator’s back porch (pin 5) is inverted (using one of the spare 4066 switches) that then controls all the other analog switches to diode key clamp them. In theory I should have a trimpot on all the diode anodes, but I only had one spare 200 ohm trimpot so it goes on the Y clamp so that I can adjust the sync tip (NB: I originally tried a 10K trimpot … but it didnt work as well). The 2.2K/6.8K pairing on the B-Y and R-Y is purely through trying out different resistor pairs repeatedly until I I found a divider that ‘looked ok’
So this circuit works mostly OK (However I do keep playing with the Y sync clamp stuff. The 3.3uF cap is after trying lots of different caps, and I am thinking one of those multiturn trimpots would allow a finer adjustment). The picture is still blue-ish (but nowhere near as bad as before) and yellows still appear a bit washed out. I did most of my testing on the Gonbes, but discovered later (when I tried it on a 2012 Sony TV) that the Gonbes must be very forgiving. Often the Gonbes would give me a picture regardless of what the trimpot was set to, but the TV required me to carefully adjust it to get proper sync … and even then I still get the occasional picture ‘bump’ as though the sync is still not quite right. The pics below are all of my Sony LCD TV.
So the output is not too bad, but I’m still not entirely satisfied with it. Black is definitely not black at the moment. Maybe I need to invest in some op-amps. I also find I still have to adjust the trimpot too much, and often between games (almost like the set of colours onscreen have some leaning as to where the trimpot bias point is set). Makes me think the sync clamping is not working properly.
Another thing I’ll note is about the vertical banding. With my very first attempts at colour I was getting really bad vertical line interference like the picture on the left below. A suggestion from my brother was to make sure the ground leads for all the RCA connections for the component outputs are connected as close as possible to the GND lead of the 9929 on pin 12 (I found later on that getting the GND of the diode clamp voltage dividers close to the 9929 GND also helps). If you can’t put them physically close, try running additional wires directly between the 9929 GND and these other GND points ( a sort of fake ground plane). This all makes an enormous difference (pic on right). I was quite surprised. I honestly thought I would end up with far more screen interference having put the Funvision together on a breadboard … but it’s actually pretty good.