copied from my forum:
Lately I've discovered that the cxa2075 in the model 2 genesis isn't generating an image that's as perfect as I thought. So I decided to revisit messing around with that video circuit in the model 1 genesis. My modding / hardware knowledge has come a long way since I last worked inside the mdoel 1 so as to be expected the results have been pretty great so far.
I first tried working with my va5 model 1. I removed all the previous wiring which was a huge mess mostly because it's one of the first systems I ever mucked around with. After cleaning the internals up I remembered that the model 1 is terrible when it comes to rainbow banding. I've had a lot of people swear up and down that replacing a capacitor fixes the rainbow issue but nobody seemed to know which capacitor it was. Earlier I had been mucking around with the model 2 genesis. I noticed that when feeding the colour subcarrier line from the system into the video encoder that the image stability was much improved but the rainbow banding appeared. When running the video encoder off of a separate crystal oscillator I didn't really notice any rainbow banding but there was other stability issues with the image. So this lead me to believe that the rainbow banding is mostly caused by the colour subcarrier line that comes straight out of the vdp chip.
I figured if there was a capacitor to replace that would fix the rainbow issue the capacitor would probably be somewhere near the vdp chip. Ironically the capacitor that was closest to the vdp chip where the colour subcarrier came out from had been crushed. I have no clue what crushed that capacitor....but maybe it was gods way of pointing me to the right part to replace. The cap was a standard 10 uf cap so I replaced it, wired up everything nice and neat, turned the system on, surprisingly there was no smoke coming from it. Next I attached the cxa2075 circuit into the header I wired up, turned it on, everything worked fine! First thing I noticed....the vertical bars were pretty weak...pretty much as weak as my model 2. Second thing I noticed, there was barely any rainbow banding....I don't know if this's because I replaced this one capacitor or something else.
The VA5 vdp outputs rgb that's pretty good but the va6 and later vdps output noticably better video quality. So today (when my client stops hounding me) I'm going to do the same thing to my va6 and see if I get the same results. Also with the va6 I'm hoping to get rainbow banding before replacing that capacitor (hopefully it's not crushed) so I can do a before and after comparison. I'm glad that I got such good results as I love the look of my model 1 megadrive.
As promised I did some farting around today and again had some great results. I cleaned up my va6, fixed a few things to restore it back to factory condition, and went to work on it. Installed the cxa2075 circuit and indeed the s-video picture looked a little better than my va5. The rainbow banding on the va6 was actually just as good as the va5 but no caps had been replaced so I guess that had to do with the cxa2075 circuit.
With the cxa2075 I got a bit of jailbar effect going so I decided to see if I could fix that. I tried adding a low pass filter to the luma line in the s-video output but that made the bars worse (strange). I then tried adding a low pass filter in the chroma line and that made the bars weaker. The filter isn't strong enough to completely get rid of them but it was strong enough to make them not annoying or even noticable unless I'm infront of the tv. I would have tried a stronger filter but I used my strongest capacitor so I'd need to buy stronger ones to do that.
Anyway here's a screenshot comparison between the s-video from the built in cxa1145 vs the cxa2075 when I bypass the built in encoder. Keep in mind both of these screenshots are taken from the same system just running different video encoders with different circuits for each.
Strangely adding the low pass filter to the chroma line made the rainbow banding a little worse but since it got rid of the jailbars it's a good trade. Also the rainbow banding isn't strong enough to be annoying when playing games so no big deal. I absolutely love the colour encoding of the cxa2075 it has an even bigger pallette range than the cxa1645 which is amazing.
Another thing to take note of is the model 1s which came with a cxa1145 work a lot better when you bypass the built in encoder to a cxa2075 circuit. My model 2s which came with the garbage samsung encoder are possible to bypass to a cxa2075 but when I plug the s-video output in my capture device from the model 2 with the cxa2075 I had all sorts of strange interference going on. This's probably because the signals were designed to be fed into a samsung chip instead of a sony chip like the model 1s that came with sony chips. The bizarre thing is that the model 2 with the bypassed encoder looked fine on my tv, but on my capture device it was insanity. The cxa1145 based model 1 with the same mod looks just as good on my tv and capture device.
I consider this job done.