CRT TV S-Video mod, I added Chroma, how should the C be filtered or grounded? - Page 1
Forum section: electronics / projects

author: kyle
author_rank: Contributor
timestamp: September 26, 2018, 04:57:37 am
content: Hello,I have a Philips 25PS40 CRT TV from around year 2001, it only has 1 AV Composite input. Few days ago I took it apart (my first ever CRT TV disassembly) and I found that it uses a TDA9587H IC chip, not much inofrmation on internet about it, but on the IC I found that it has spare RGB lines and (C)hroma line!I soldered RGB cables to pins 51,52,53 and S-video cable to pins 44 and 45.The TV's circuit board is highly integrated, besides 2 ICs there's almost nothing there, not the easiest TV to work. Stuff like OSD and teletext features are right inside of the TDA9587H IC itself. I have spent last few days reading about other people's mods, from people hijacking OSD RGB signals to people injecting signal just before the CRT gun on the neck board, at this point I'm not keen on getting RGB input working since I don't have any device that can output RGB (but I'm planning to mod my Sega Genesis Model 2 soon to RGB), but I have been very interested in getting S-Video to work. So far I'm stumped as tohow signalshould begroundedorfiltered. I see recurring pattern of incoming signal gettingpulled to ground with 75 ohmresistor and then goingthrough 0.1uF ceramic capbefore it reaches the Jungle IC. Like this:Well I tried 80 ohm resistor (I don't have 75) and 104 ceramic cap, and this is what I get on my XBOX via S-Video:When I disconnected the Chroma line image stops jiggling but of course goes to black white. When Chroma cable is connected the colors are very dim and seem to bleed at some points. When I run Chrome straight from my XBOX to the TV without any other components the TV partially shuts of (the Tube goes off but 5V electronics in TV seem to be still on, I have to power cycle the TV to get it back on).So I'm asking eevblog folks for help, what is the science behind Chroma signal? How should the signal be handled before it reaches TV's video chip? How can I troubleshoot this issue with oscilloscope, what to look for? As a hobbyist I have never worked with NTSC signal so I don't know what to look for, but I have looked at Composite signal in the past and I was blown away, there's so much happening, I only have Tektronix 2225 scope and I'm not even sure how to set it up (the coupling, trigger, etc) for NTSC troubleshooting.Thanks.

author: bwXg
author_rank: Newbie
timestamp: June 12, 2019, 03:02:36 am
content: Hi,I know it is a long shot that this will get a response, but this is one of the few places I've seen any reference to the TDA9587H chip for this kind of project. I'm hoping to perform a similar mod myself. Did you ever have any success going either the RGB or S-Video route? I was able to dig up a "Tentative Spec Sheet" searching the web. Essentially I found a lot of the same information you did a second time around. I then found this post and followed along closely. I presume that is where you found the pinout, but if not, I can send a link, or share it.In your photo what are the two wires running amongst your RGBs set? I presume one is ground, but I'm not clear on what the other is for.I tried connecting my Sega Nomad (Genesis 2 essentially) to the RGBs pins. I used the schematic you outlined, and used the Nomad's sync pin. All I was able to get out of it was a scrolling set of diagonal black and purple bars. I had the TV set to the AV Front input when I got this. Is there some sort of trigger pin, or other input that this somehow needs to be set to?I haven't delved too deep, and unfortunately I haven't an Oscilloscope, nor the proper know how to use it. I'm thinking of measuring voltage of the various inputs to confirm they're within reasonable limits of the spec sheet. As a recommendation, I figure that may help with your S-Video issue as well.This unified IC seems like a blessing and a curse at the same time. It's nice that it appears to have RGBs, and S-Video inputs available, but the lack of project fore bearers makes it challenging. If things have developed any further with your project, I'd love to hear about it.Thank you.