Kjmann's S-Video upgrade revisited - Atari 400

[biobern]

Getting out the hardware docs....

 

on the 400, these are the video 'ladder' resistors:

LUM0 - 62K R180 - 2K

LUM1 - 30K R178 - 1K

LUM2 - 12K R177 - 470 or 510

LUM3 - 120K R179 - 3.9K

SYNC - 15K R176 - 510

(I don't know their relative positions)

 

This gives a combined resistance of 172. So, if we ignore the 4050's internal resistance, we get a voltage swing of 2.3V which will translate to 1.15V as seen by the monitor. In reality, it will be little lower since the 4050 won't go rail to rail. If it looks a little dark, the 75-ohm output resistor can be reduced.

 

Your idea kinda remembers me to this mod which works very well for my Atari 2600 PAL:

http://www.console-corner.de/videomod.html

I don't know how much these both machines are similar.

[Kjmann]

Since S-Video is no longer available on most new TV's I experimented with some of the suggestions in this thread and came up with this.....

 

 

 

Simple, But seems to work well for me. seems to eliminate the shadowing quite a bit.

Edited June 2, 2013 by Kjmann

[biobern]

Since S-Video is no longer available on most new TV's I experimented with some of the suggestions in this thread and came up with this.....

 

 

 

Simple, But seems to work well for me. seems to eliminate the shadowing quite a bit.

 

Thanks! So, you just replace the capacitor, right? All the resistors remain unchanged?

I will try it!

[biobern]

Thanks! So, you just replace the capacitor, right? All the resistors remain unchanged?

I will try it!

 

I did try to replace the 22uF electrolitic capacitor by a 470pF disc capacitor. No good result. Same ghosting as before.

[SoundGammon]

Try a 150 ohm (2 x 75 ohm in series) from the video output to ground.

Edited June 9, 2013 by SoundGammon

[biobern]

Try a 150 ohm (2 x 75 ohm in series) from the video output to ground.

 

I tried some resistors between 50 and 500 ohms. Didn't help. Vsync got weaker, that was all.

 

I the last weeks, I tried around five NTSC-compatible CRT-TVs sold in germany around the year 2000. Most of them didn't even accept this as NTSC most of the time (the results are quite randomish) and thus show no color.

 

Yesterday, a friend of mine analyzed the signal with an oscilloscope and in fact, it is far away from NTSC standards. He tried some stuff with buffer-ICs instead of the capacitor but could not find an acceptable solution.

 

So, until now, my Sony Monitor from the early 80s remains the best solution despite the heavy ghosting.

Bern

[Magic Knight]

Are you still attempting Composite? or are you trying S-Video as part of your tests?

 

If you are trying out several monitors, then i would assume composite is no longer restricting your choice of output. and the trouble involved so far getting composite, the time may be better spent on getting the best from your machine via the better picture type.

Edited June 28, 2013 by Magic Knight

[biobern]

Are you still attempting Composite? or are you trying S-Video as part of your tests?

 

If you are trying out several monitors, then i would assume composite is no longer restricting your choice of output. and the trouble involved so far getting composite, the time may be better spent on getting the best from your machine via the better picture type.

 

True. I mostly tried composite, but also tried S-Video on my newer Sony Monitors from around the Year 2000. They didn't accept it as NTSC either and showed no color. :-(

[Magic Knight]

If you refer to the start of this thread and apply all the resistors and wiring AND your getting lack of color or fuzz then there is likely to be a defect on your machine somewhere.

 

Even the roughest application towards this shows up reasonable results so I don't think you are doing anything wrong. The only time I had similar was when I accidentally has chroma and Luna swopped around, the picture was still visible but not that good. Perhaps you have something similar?

[ScreamingAtTheRadio]

In case someone needs to find the long-gone archive containing the original instructions, Wayback Machine to the rescue...

 

http://web.archive.org/web/20130213183644/http://www.karaokeportland.com:80/svid.html

[Beeblebrox]

6 minutes ago, ScreamingAtTheRadio said:

In case someone needs to find the long-gone archive containing the original instructions, Wayback Machine to the rescue...

 

http://web.archive.org/web/20130213183644/http://www.karaokeportland.com:80/svid.html

Thanks for this. It has, among others, the Chroma/luma hookup points for the XEGS S-video upgrade, which is useful. (My U1MB enhanced XEGS only has it's stock av composite at present). 

 

Now to figure out whether, like the XE line, if you remove the XEGS' rf modulator to fit the Din5, it disables the video circuit altogether. (On the XL A8's if you remove the rf modulator you still have av composite and S-video, whereas on the XE line the rf modulator is a core part of the video circuit generation Iirc.). The only way I'll add the Din5 is if I can remove the rf mod and keep the case modding to an absolute minimum, using the existing rf hole. 

 

Anyways, not wishing to hijack this thread. 

Edited October 24, 2023 by Beeblebrox

[ScreamingAtTheRadio]

I just finished applying this mod to my 400.

 

I was a little confused about the bit in the document about the capacitor being between GTIA pin21 and ground to get rid of prison bars that everybody applying the mod seems to ignore (including @flashjazzcat in his video as far as I could tell). Just to test everything, I built S-Video and audio cables that I can connect to a breadboard to experiment (and also easily determine if the resistor value needs to be tweaked):

 

That enabled me to check that my taps were working. On the motherboard, I put in the same common values of resistors as others. To make the whole thing easier to disassemble, I put the signals on a triple Dupont cable that I spliced in two, using the middle to make connections on the motherboard and on the new video connector.

 

I mounted the DIN5 where the switch was, at the back of the power board, and I secured it in place with a ziptie:

 

Then I soldered the chroma cap, the 75Ohm luma resistor, the cable, and I made a connection between the middle DIN pin and the nearest ground.

Then I dremelled my way around the former switch window in the case:

The video signal I'm getting is pretty solid (ignore the 16:9 aspect ratio, that's the fault of the HDMI converter that sits behind it). This is one of the cleanest video outputs I've had so far on an Atari computer, (probably until I try that new lumacode board I bought).

[POCOAUST]

G'day Atari 400 peoples.....

 

I did the mod with the resistors and the Capacitor.  Did it all under the board, then cut the tracks as described, but I joined the luma and croma outputs back under the board with the Cap and Resistor,  and then ran a single wire that I then connected to the center pin of the RCA connector on the power board after cutting the existing track.  Retained the factory cable and speaker.  

 

Works great....

Paul