XEGS color problems

[evietron]

I got a pristine like-new XEGS off eBay, and the color doesn't work. I tried all kinds of adapters and no color. Black & white works fine.

 

I replaced the trim pot, and still no go.

 

I went through a "Super Video" mod which includes fixes to luma, chroma, and a s-video mod, and while the picture quality is fine, there is still no color.

 

One thing I did notice, since adding the s-video mod, is that if I rotate the trim pot quickly, I'll get a flash of color on the screen occasionally, but it rarely stays for long.

 

If I swap the pins on the s-video adapter, putting luma into chroma and vice versa, I see the "color" screen in black and white but the vertical sync is terrible. Actually, when I remove the 75 ohm resistor for the s-video chrominance, it becomes more stable, but still has an offset.

 

SEE VIDEO OF SYMPTOMS HERE...

 

Any help would be appreciated. I really want to see color on this Atari!

[Sugarland]

Might it be a loose solder joint? A faulty chroma resistor? You could work around the issue by installing a UAV video upgrade.  The new Sophia video upgrade is due soon as well.

[StickJock]

I had a no color issue on an 800 that was caused by a bad 3086 transistor array (A104 on Atari 800 schematics).

I don't have the XEGS schematics, but the 130XE schematics don't show this part.  They do, however, show a transistor in the Chroma video path (Q2, 2N3904).  Maybe there is an analogous part on the XEGS that has gone bad?

 

[evietron]

Attached schematics. I replaced the Q5 transistor and now get slightly more color bleed but it still is not stable at all. I just get flashes of various color on the screen.

 

I'm picking up the color signal from the output of Q5 for the chroma output. I already replaced R55. So basically the only thing left is L5 and then the 5V power supply.

 

One other question .. the video upgrade mentioned a tantalum capacitor between R78 and R79. I am using an electrolytic instead. Could this cause problems? (Although that is on the luma circuit which seems fine)...

XEGS System Schematics.pdf

[mimo]

Try a CRT TV and see if you have the same issue. The Atari signal is way out of spec for LCD TV's and many display nothing or wont sync or drop colour etc etc 

[evietron]

Unfortunately I don't own a CRT TV. I use a converter box that goes from composite or S-Video to HDMI. Is there a good guide for studying a chroma signal through a scope?

[evietron]

I hooked up a scope and discovered the "color burst" signal. That helped me configure the right values for R55 and the color pot. The inductor seems to be filtering AC voltage properly.

 

So that pretty much leaves my converter box as the suspect. That, or the luma circuit is somehow out of phase with the chroma.

 

Has anyone had good luck with RetroTink2X with an XE machine?

[+DrVenkman]

5 minutes ago, evietron said:

Has anyone had good luck with RetroTink2X with an XE machine?

I use the 2X-Pro with my Atari 8-bits although I don't have an XEGS. But the BASIC Ready prompt or Missile Command if you don't have a keyboard attached should come through just fine. 

[gilsaluki]

4 hours ago, evietron said:

Unfortunately I don't own a CRT TV. I use a converter box that goes from composite or S-Video to HDMI. Is there a good guide for studying a chroma signal through a scope?

Thrift store shopping for a CRT.

[Mr Robot]

When I got my XL I only had black and white. Turned out to be a cheap converter from amazon was to blame. I got a better one and it solved the problem. Ambery was the brand I ended up with. 
 

 

[ChildOfCv]

3 hours ago, evietron said:

I hooked up a scope and discovered the "color burst" signal. That helped me configure the right values for R55 and the color pot. The inductor seems to be filtering AC voltage properly.

What frequency is the color burst output?

 

Straight out of the GTIA, it should be a square wave.  After R55/L5 it should look more sinusoidal.  At Q5 emitter, it should be the same signal, just shifted down by about half a volt.  It mixes into the luma signal on the other side of C44.

 

The color pot only changes the width of the color spectrum.  Unless it's so low that the color shift circuit never propagates, you should see some colors, even if they're not correct.

[evietron]

Thanks Mr Robot, I think I'll try the Ambery converter.

 

From the scope, the signal looks great. Up until now this converter box (Tendak) has worked on everything.

[ChildOfCv]

1 hour ago, evietron said:

Thanks Mr Robot, I think I'll try the Ambery converter.

 

From the scope, the signal looks great. Up until now this converter box (Tendak) has worked on everything.

Yes, but the question is, is it at the right frequency?  For NTSC that's 3.58MHz and for PAL that's 4.43MHz.

You should also check the composite signal to see what the color carrier's amplitude looks like in relation to the rest of the signal.  The color carrier during horizontal blanking should be about the same peak voltage as the sync tip.  So you should see a flat line on the sync, then back to blanking level, then a sine wave that reaches back down to the sync tip and then back up the same distance above the blanking level, at least 8 cycles.  Then a little more blanking level and then image data.

[+DrVenkman]

So a couple of caveats here with this example image. Number one, obviously, is it's a 7800 outputting the video from a 2600 test pattern ROM, so it's the TIA output, not the later GTIA used in your XEGS.  Number two, the signals shown here are the TIA LUM signals (basically the same as those produced by the GTIA) plus COL, as conditioned and turned into standard Chroma and Luma signals by a UAV board I had just put into the system. I thought I had similar images from one of my XL machines handy but I can't find them.

 

But at any rate, this image is useful because this vertical-bar test pattern creates an entire screen of the same scanlines all the way down the screen - it therefore doesn't matter which particular line of the ~240 or so per frame that you're catching on the scope. The scope traces will be the same.  Anyway, as you can see, the Luma signals for the progressively darker bars drop across the length of the scanline, and since this is a B&W image, the Chroma signal is essentially dead-flat until the very end of

the trace, after the low Sync pulse on the Luma line during the horizontal blanking period. That squiggle in the Chroma that repeats every 15 Khz  should be more or less what you're looking for. 

[ChildOfCv]

Huh.  That's a fairly weak chroma signal in comparison to the luma.  I wonder if that's why the converter is rejecting it?  It should be 40IRE, just like the sync tip.  It looks like maybe 30, from my admittedly bad estimation capabilities.

 

Edit:  Oh wait... looking closer, they're on far different scales, so the direct comparison is not valid.  Why is the luma only at 50mV/div?  Is your yellow probe actually switched to 10x?

Edited July 15, 2020 by ChildOfCv

[+DrVenkman]

12 minutes ago, ChildOfCv said:

Huh.  That's a fairly weak chroma signal in comparison to the luma.  I wonder if that's why the converter is rejecting it?  It should be 40IRE, just like the sync tip.  It looks like maybe 30, from my admittedly bad estimation capabilities.

 

Edit:  Oh wait... looking closer, they're on far different scales, so the direct comparison is not valid.  Why is the luma only at 50mV/div?  Is your yellow probe actually switched to 10x?

Could be, I don’t recall. 

 

That’s not really the point though - the picture was to show Evietron what to look for with regard to an un-modulated colorburst during the horizontal blank after the Luma SYNC pulse at about 15khz for an NTSC signal.

[evietron]

I hooked up the Ambery converter and it works flawlessly. No more cheap no-name boxes for me! Thanks Mr Robot, that was an awesome suggestion.

 

I'm going to mount an S-video connector where the RF modulator is (I've already destroyed the thing), and then re-hook up the composite so I can use that as an option.

 

Up until now I've been using a rewired composite RCA as luma and RF RCA as chroma.

 

DrVenkman, I will try some test patterns once I get an SD solution on my machine. For now I am just using Missilie Command and the built in test.

[kyokorn]

Hello , i have an Atari XE, with the same problem, on my case i replaced the transistor Q5 and the potenciómeter (500k) with that i fix the issue 

[M M]

Hey there kyokorn; my family has begun having the same trouble with our Atari XE, namely that it has gone from outputting color to just black and white video. Is there any chance you'd be willing to post some more details about the transistor you had to replace? Like maybe, what type it was? I'm pretty sure if I open up the unit I can find where "Q5" is written, and I can solder and desolder components... would just want to make sure I put the correct transistor (PNP? NpN?) back in place...

 

(And one last question... what is the range of resistances used on the potentiometer you replaced, for exactly? Color saturation level?)

[kyokorn]

7 hours ago, M M said:

Hey there kyokorn; my family has begun having the same trouble with our Atari XE, namely that it has gone from outputting color to just black and white video. Is there any chance you'd be willing to post some more details about the transistor you had to replace? Like maybe, what type it was? I'm pretty sure if I open up the unit I can find where "Q5" is written, and I can solder and desolder components... would just want to make sure I put the correct transistor (PNP? NpN?) back in place...

 

(And one last question... what is the range of resistances used on the potentiometer you replaced, for exactly? Color saturation level?)

Hello friend, 

 

I used a NPN Transistor 2N3904-

 

For the potenciometer I'm not sure what was the value my friend, but you can try just replacing the potenciometer (500 Kohms) and move it until you have on the screen a good blue and green (running the memmory test).

 

I really hope you can fix your Atari, let me know if i can help with some else 😃

 

P.S: Here a picture of my little Atari collection, Good Luck!

 

 

Edited August 2, 2023 by kyokorn

[Beeblebrox]

Yup, the NPN Transistor 2N3904 is a common one and usually is one of the values in bulk bought muti packs of transistors. 

 

For the Potentiometer here ya go. This one is here in the UK on ebay but I am sure you can find an equivalent where you are:

Get the 500k one

 

I just installed one in a 130XE with a damaged pot. 

 

10 Pack Trimmer Preset Potentiometer Carbon Variable Skeleton Pot 100R to 1M

https://www.ebay.co.uk/itm/113667124066

 

[M M]

Thanks kyokorn and beeblebrox, I'm going to open it up today and will report back after a visual inspection / trying every bit of easy tinkering / that doesn't require a purchase. And, beautiful computing device collection there, kk!

 

BTW, both of your usernames go really well together (imo)... maybe you two could start a YT channel, 'the Kyokorn and Beeblebrox show' ? 😃 Where you help total strangers identify and replace faulty components, on their beloved old gaming devices?