Master system screenshots using this circuit in my genesis:
A looooong time ago I followed this internet guide on how to add s-video from the cxa1145 encoder built into most model 1 genesis consoles:
Here's my original worklog, I don't even remember when I posted this originally the worklog was copied and pasted to a new forum when bacteria changed servers:
This was one of my first mods ever and I've done so much and learned a lot since then. I had long ago abandoned working with the cxa1145 encoder and had been focusing my efforts on the much newer cxa2075 chip.
Either by fate or randomness I was forced to take a second look at the cxa1145 s-video circuit that I built such a long time ago, here's the reason why. I picked up a mega everdrive which is currently the best flash cart money can buy for a sega genesis. One cool feature about this flash cart is that it can play roms for the 32x and even sega master system roms by the flash cart setting your sega genesis into master system mode. Recently I discovered that you can s-video mod the 32x and get perfect s-video image quality for both genesis and 32x games:https://16bitgamer.canadian-forum.com/t129-the-sega-32x-s-video-modded
Since the 32x 100% cleans up the video distortion on my tv I wanted to use the 32x all the time so I had my genesis running with just the built in cxa1145 using the rgb output into the 32x. Unfortunately after picking up the mega everdrive I learned that you can't play master system games through a 32x. I talked to tiido and he said you'd have to rewire the whole cartridge slot. Normally pulling out the 32x when I feel like playing master system games wouldn't be a problem, except the 32x is still the only piece of hardware I have that 100% gets rid of the sega genesis video jailbars.
My genesis still has a header wired for me to plug in a cxa2075 circuit. For some terrible reason when my genesis has the cxa2075 circuit plugged in it causes the s-video image from the 32x to flicker, I have no idea why this happens. Even when I'm feeding rgbs from the cxa1145 into the 32x having the cxa2075 circuit plugged in causes the image to flicker. Unplugging the cxa2075 circuit fixes the image flicker....oookay then. Another problem I came across working inside the model 1 genesis is audio buzz caused by too much power usage. For example, if I wire in two cxa2075s into the system I get the audio buzzing, but if I wire in just one cxa2075 circuit there's no audio buzzing (don't ask me why I wired in two cxa2075 circuits it was an experiment). Since the mega everdrive uses a nice beefy fpga it uses a good amount of power too. The mega everdrive doesn't use enough power to create audio buzzing on its own, neither does the cxa2075 circuit, but combine a mega everdrive with a cxa2075 circuit and my genesis audio goes into buzz mode. Therefore wiring up my genesis with the cxa2075 circuit while using the mega everdrive wouldn't work since I would have constant buzzing going in with the audio. This combination of hardware flaws literally forced me to revisit the long forgotten cxa1145 s-video circuit. The cxa1145 s-video circuit is the only thing I could use in combination with the mega everdrive now to cause audio buzzing from the console but still giving me the ability to play sega master system games in beautiful s-video.
The last time I took a look at the cxa1145 s-video circuit I knew pretty much nothing about video circuits so I just built the circuit the way the website told me to and assumed it couldn't be improved upon. This was the first time I'd taken a look at this cxa1145 s-video circuit since building it way back when but I've learned so much since I did that very early mod. I wired up a luma amp / buffer using a npn transistor I had sitting around, in this case I used a 2sc1815 transistor. To test that the luma was working I plugged it in and got a picture. Right away I noticed that having only luma wired up gave me a black and white picture with absolutely no jailbars. I found this interesting since the cxa2075 circuit has loads of jailbars in the luma line but the chroma line actually makes the jailbars weaker. I next wired up the chroma from the cxa1145, I installed the capacitor in the opposite direction of the guide because the author tells you to wire it the opposite way that the datasheet tells you. The datasheet tells you to wire up a cap with positive facing the cxa1145 encoder so that's what I did. After wiring up chroma the jailbars were nice and super strong just how I remembered the cxa1145 circuit looking.
Now's the part where I did some experimentation and used what I've learned since I last tinkered with this mod....
I tried reversing the direction that the capacitor faces, no effect. I tried adding a transistor amp to the chroma line, the jailbars became worse. I tried adding a very strong resistor to the chroma line (470 ohms), the jailbars disappeared
. 470 ohms was a bit too strong it made the colours all faded and washed out, but not as washed out as I would expect from that strength of a resistor. I tried adding a 1 transistor amp after the strong resistor to the chroma line, the jailbars came back. The logical step was to decrease the resistor strength until the colours no longer looked washed out but you still had the jailbars reduced to the point where they were no longer annoying. Final resistor strength, 220 ohms. Now you might be asking yourself "so what's the difference?". Here's a before and after screenshot, first screenshot has no resistor on the chroma line, second has a 220 ohm resistor on the chroma line:
The image quality isn't as perfect looking as what I get from the 32x but it's certainly good enough for sega master system games, or anyone who's too broke to buy a 32x.
Here's my revised circuit: