Vigo Posted May 17, 2003 Share Posted May 17, 2003 Hello guys! I am currently going to add a composite video output for my PAL Atari 7800 from 1991. I succeeded in re-building the luma output, which gives me a real sharp noise-free black & white picture for both MARIA and TIA video output. However, in 7800 mode, the MARIA chroma output seems to suffer from crosstalk video noise, which doesnt look quite sophisticated. I connected it directly via a resistor to the luma output. The crosstalk becomes worse when i put my finger on the 4.43Mhz colour oscillator. Has anyone any idea how to fix this? Regards Vigo Link to comment Share on other sites More sharing options...
Miqorz Posted May 17, 2003 Share Posted May 17, 2003 PM a dude named Edit5 he knows all about PAL 7800 hardware. Link to comment Share on other sites More sharing options...
+Mitch Posted May 17, 2003 Share Posted May 17, 2003 Hmm, is this a combined 2600 and 7800 Chroma? Mitch Link to comment Share on other sites More sharing options...
Vigo Posted May 17, 2003 Author Share Posted May 17, 2003 My circuit currently uses the 7800 chroma signal only. I´m directly taking it from the Maria chroma pin. Here is a grab from this modification. Note the wavy pattern in the green and the purple colours... Link to comment Share on other sites More sharing options...
Vigo Posted May 17, 2003 Author Share Posted May 17, 2003 ...and here with disconnected chroma signal. Link to comment Share on other sites More sharing options...
+Mitch Posted May 17, 2003 Share Posted May 17, 2003 What size resistor are you using on the Chroma line? Are you using a capacitor on it at all? How does the picture look with a S Video connection? Mitch Link to comment Share on other sites More sharing options...
Vigo Posted May 17, 2003 Author Share Posted May 17, 2003 Well, i´m using a variable resistor on the chroma line (1k), but no matter how far i adjust it, the waves do not disappear (they get weaker, but also the colour saturation). Hmmm, i do not use any kind of capacitor.... The problem is that all video mods are NTSC only, and the layout of the PAL 7800 differs quite a lot (not mentioning the technical differences between PAL and NTSC). Even the schematics on AtariAge do not resemble my late 1991 model. When i´m finished with this, i´m going to write it down how the modification is done.... Link to comment Share on other sites More sharing options...
+Mitch Posted May 17, 2003 Share Posted May 17, 2003 Try dropping a 470pf ceramic capacitor on the Chroma line between the resistor and the luma connection. Also, do you have a S-Video TV you can try it on? This will help eliminate if it's crosstalk from the luma line or something in the Chroma signal. Oh, and you are pulling the 7800 Croma signal from the right side of the resistor(toward the center of the board), right? I've also found a newer PAL 7800 schematic but there are several errors on it. So, use with caution. Mitch Link to comment Share on other sites More sharing options...
Clock Posted May 18, 2003 Share Posted May 18, 2003 Having spent a couple of weekend trying to get my Pal 7800 to use a s-vid connection, I gave up, at least you get a picture, I just just got a constant fast rolling diagonal mess which you could just make out sprites moving. I found a German site which had great pictures of the Pal circuit board but I can't read a word of German. I used the S-vid circuit found from AAs FAQ and just hooked hat up to the same points found on the German site, and like I said it doesn't work. I would like to see your circuit diagram used so I can use it. Especially if Mitch's advise works. Link to comment Share on other sites More sharing options...
Vigo Posted May 18, 2003 Author Share Posted May 18, 2003 Try dropping a 470pf ceramic capacitor on the Chroma line between the resistor and the luma connection. Also, do you have a S-Video TV you can try it on? This will help eliminate if it's crosstalk from the luma line or something in the Chroma signal. Oh, and you are pulling the 7800 Croma signal from the right side of the resistor(toward the center of the board), right? I've also found a newer PAL 7800 schematic but there are several errors on it. So, use with caution. Mitch Thanks, Mitch for the valuable advice! The schematics are also the right ones! Right now, i dont have a 470pF capacitor right there, but tomorrow, i´ll give it a shot! Well, i´m taking the croma signal from the left side of the resistor, since on the right side, it´s much too weak to get recognized.´ But right now, i dont have a S-VIDEO cable handy to try this, but i can definatively say that the noise is in the croma signal, since the luma is 99% noise free. As i said, when i´m done with this modification, i´ll post the instructions how to do it here on AtariAge. Thank you very much Vigo Link to comment Share on other sites More sharing options...
hagbard Posted May 18, 2003 Share Posted May 18, 2003 I found this site in the web. It's text is in German. I haven't tried it yet, so I can't say that it'll work. http://134.155.59.2/~wi01196/colude/retrob...d/a2600mod.html Greets, hagbard Link to comment Share on other sites More sharing options...
Vigo Posted May 21, 2003 Author Share Posted May 21, 2003 Hi again. So today, i connected a 470pF capacitor between the chroma resistor and the luma output, but sadly there is no improvement. I have the feeling that the noise is due to poor engineering from Atari... Am i the only one with a PAL 7800, which managed to get some sort of coloured picture at least? What if i would replace the 4,43Mhz oscilating curcuit with a 4,43Mhz oscillator (the square, blocky ones, which dont need any extra components to ocillate)? I´m really trying to be the first PAL 7800 user to get it right...... Regards Vigo Link to comment Share on other sites More sharing options...
Spirantho Posted June 2, 2003 Share Posted June 2, 2003 Hi everybody / Vigo, Just wondering how you're getting on with your PAL 7800 mod.... Yesterday I completed modding my spare PAL 7800 and on my Sony Trintron TV I can get a lovely 7800 picture (although as most of my 7800 games are at work I haven`t checked it thoroughly). However, on my AverMedia JoyTV tuner I get (chroma?) stripes, particularly on certain colours like green. I`m going to hook up the 7800 to the box via S-Video rather than composite and play with the pot to see if I can get rid of some of the noise (though there was hardly any on the Sony). I`d be interested in knowing how you did your mod, though.... I used the second one in the FAQ, but I had to use different resisters as the PAL layout is different. Maybe between us we can find an even better solution and document it. Let us know how you get on... Link to comment Share on other sites More sharing options...
Vigo Posted June 2, 2003 Author Share Posted June 2, 2003 Hi! Well, i soldered a second 74LS32 on top of the 74LS32 which is near Maria. I connected the luminance outputs from the LS32 to a resistor ladder (1k, 0.47k, 0.22k). The fourth lumance bit was directly taken from MAria pin 42, and wired to this ladder using a 0.1k resistor. The output of this ladder was directewd through a 1K variable resistor. The sync output from the LS32 is directly wired with a 0.47 resistor to the output of the variable resistor. This forms the luminance part, which is crystal sharp, and noise free in both 7800 and 2600 mode. Now i´m taking the Maria chroma signal directly from Maria pin 43, send it through a 10K variable resistor, and then connect it via a 470pF capacitor to the luminance output. The resulting composite video signal suffers from tze same diagonal distortion lines you describe, no matter how i adjust the colour resistor. I tried it also via SVIDEO, and the result is ofcourse better, but the noise is still visible. The Atari 2600 chroma signal does not suffer from this kind of distotions, in fact, the 2600 display is crystal clear. Strange... Btw, i´m testing this circuit on a C= 1084S Monitor. Link to comment Share on other sites More sharing options...
Spirantho Posted June 2, 2003 Share Posted June 2, 2003 Hi, Sounds like what you've done is basically what I did - but you used different values to me (I used 820R, 1K6, 3K3 and 6K8) and I didn`t use another chip - why did you need to put more OR gates in the circuit? And why put it on top of the other one - why not just take the outputs directly (which is essentially what I did, except I soldered the joints onto the resisters connected to the pins). I used a 10K preset, BTW. My luminance is also pretty fine. The 470pF capacitor you mention didn`t have any effect did it? I`m not using a variable resistor myself - just a 200Ohm one, and a 1K2 on the 2600 chroma output, joined together and put through a 1uF capacitor. Just so we know we're talking the same problem - the worst game I have for this effect (that I've found so far) is Commando - the green display ripples quite visibly. The best I think is Choplifter (so far). The 2600 games are fine (albeit dark on this telly - need to tune that preset!) Do you get a nasty flickering on Tower Toppler too? IIRC I get that on the TV out too - I think that's a 7800 "feature". Link to comment Share on other sites More sharing options...
Vigo Posted June 2, 2003 Author Share Posted June 2, 2003 Hi, Sounds like what you've done is basically what I did - but you used different values to me (I used 820R, 1K6, 3K3 and 6K8) and I didn`t use another chip - why did you need to put more OR gates in the circuit? Hi, I did it because i wanted to avoid crosstalk in the luma output. mI just wanted to be pn the safe side.... The 470pF capacitor you mention didn`t have any effect did it? I`m not using a variable resistor myself - just a 200Ohm one, and a 1K2 on the 2600 chroma output, joined together and put through a 1uF capacitor. No, it didnt had any visible effect... Just so we know we're talking the same problem - the worst game I have for this effect (that I've found so far) is Commando - the green display ripples quite visibly. The best I think is Choplifter (so far). The 2600 games are fine (albeit dark on this telly - need to tune that preset!) Yeah, the green display in Xevious and the brown Lucasfilm screen in Ballblazer are really bad.... Do you get a nasty flickering on Tower Toppler too? IIRC I get that on the TV out too - I think that's a 7800 "feature". I dont own a tower toppler, but i know what you mean. Other than the TIA, the MARIA fully supports the PAL phase-shift feature, which is why the edges of colours are "rolling" (like a real TV transmittion). When you put your finger on the resistors on the left side of the 4.43 Mhz crystal, the crosstalk gets worse. So i really guess the colour signal catches interferences from something else on the board. Strange, since the TIA uses the same signal source. Or perhaps Atari really fucked up the PAL circuit of MARIA (since it doesnt take the colour frequency from the main clock, like the NTSC 7800). Perhaps i should try disconnecting MAria´s colour clock from the mainboard, and solder a 4.43 Mhz crystal-oscillator directly to it.... Link to comment Share on other sites More sharing options...
Vigo Posted June 2, 2003 Author Share Posted June 2, 2003 Hmmm, it seems that MAria needs the TIA to make the colour clock oscillating. One hint that the TIA might be the problem..... Link to comment Share on other sites More sharing options...
Vigo Posted June 2, 2003 Author Share Posted June 2, 2003 I´v done it!!!!! I disconnected MAria from the TIA by removing R74, ran the clock signal through both inputs of an AND gate (to buffer it), and the output via a 1k resistor to the MARIA. The crosstalk is gone!!! Link to comment Share on other sites More sharing options...
Vigo Posted June 2, 2003 Author Share Posted June 2, 2003 Here is the trick: Remove R74. The output should be black and white now. Solder a 74LS08 on top of a 14 pin chip. connect only pin 14 and 7 to the chip beyond. Now connect a cable from the right side of R44 (under Q9) to pin 1 and 2 of the 74LS08. Now connect pin 3 via a 1k resistor to MAria pin 3. You have got an excellent quality now! Link to comment Share on other sites More sharing options...
Spirantho Posted June 2, 2003 Share Posted June 2, 2003 Wohoo! Of course that leaves us with a bit of a tricky moral dilemma - I don`t want to modify the original circuit in any way, as I want the change to be totally reversible - so are the improvement worth the sacrifice of the "purity" of the machine..? I wonder what your 7800 looks like at the moment.. probably like the inside of mine. :-) (wires all over the place!) How are you mounting your veroboard (I presume you're using veroboard)? Mine's just hovering inside with insulating tape on it at the moment... Link to comment Share on other sites More sharing options...
Vigo Posted June 2, 2003 Author Share Posted June 2, 2003 Wohoo! Of course that leaves us with a bit of a tricky moral dilemma - I don`t want to modify the original circuit in any way, as I want the change to be totally reversible - so are the improvement worth the sacrifice of the "purity" of the machine..? From the moment you open the machine, and remove the metal shield, it´s not pure anymore. The video is now crystal clear, without any inteference, i think its worth it! I wonder what your 7800 looks like at the moment.. probably like the inside of mine. :-) (wires all over the place!) How are you mounting your veroboard (I presume you're using veroboard)? Mine's just hovering inside with insulating tape on it at the moment... When i get the hands on my difital camera back, i´ll post some photos here. This weekend, i´´m going to write the article, so that PAL useres can at last now have the superiour video quality NTSC users have been enjoying for quite some time.... Special thanks to Mitch, whose 7800 schematic find was an extremely valuable contribution to this mod! Now i can put the board back into the case again! Link to comment Share on other sites More sharing options...
Vigo Posted June 3, 2003 Author Share Posted June 3, 2003 UPDATE: The 1k resistor between the LS08 and Maria pin 3 is not needed. In fact, the 1k resistor weakens the clock signal. Link to comment Share on other sites More sharing options...
Lumpbucket Posted June 4, 2003 Share Posted June 4, 2003 Great! I can't wait for the full write-up of this mod. Are both the TIA and Maria outputs crystal clear? From the RF output I have at the moment, both are bad, but the TIA is significantly worse than Maria (darker, noisier, just horrible), so I'm looking forward to a nice, clear output Link to comment Share on other sites More sharing options...
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