130xe - No video via Composite, SVideo is fine

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I recently purchased an Atari 130XE, in "perfect" condition. 

 

The computer boots, but composite video has no output (screen is completely black). 

The SVideo output provides nice and crisp video. 

The composite cable works on my 800xl, so the cable is fine.

Before I open it up, any ideas what might be the culprit?

[+Nezgar]

That's not a failure symptom I've seen before. First thing I'd do is go ahead and open it up to see if any mods have been done by a previous owner, because for instance if a stereo pokey upgrade was installed, the composite video pin may have been repurposed for the right audio channel if the previous owner didnt want to make any new holes in the case...

[Gavin1968]

I have to agree with Nezgar, it probably has been modified and that connection either disconnected or repurposed.  Need to open it an explore.  

[+hueyjones70]

I have an 800XL that has no composite video. No mods. Which POKEY pin is the composite video.

[ApolloBoy]

16 hours ago, hueyjones70 said:

I have an 800XL that has no composite video. No mods. Which POKEY pin is the composite video.

The POKEY isn't responsible for generating video, that would be the GTIA. Even then there isn't a pin for composite video on the GTIA, it's generated through additional circuitry as the GTIA outputs a chroma signal and four digital luma signals. Looking at the schematic, you may want to try replacing U20 (a 4050 hex inverter) as that's the only IC involved in generating the 800XL's video signals outside of the GTIA. Failing that, you may want to take a look at transistors Q2 through Q5 as those are also part of the video circuitry.

[+hueyjones70]

Thanks for the info. I will look at the transistors, I have already tried replacing the 4050 (U20).  I realize now that the previous reference to the video pin was for the composite pin for the monitor cable.

[ApolloBoy]

By any chance is the RF output working? If so, the problem might be elsewhere as the composite video signal is also piped into the RF modulator.

[ChildOfCv]

From what I can see, an unmodified 800XL should already have composite output.  The schematics show that it's connected.  J2 (the DIN plug), pin 4 IIRC.

[+hueyjones70]

2 hours ago, ChildOfCv said:

From what I can see, an unmodified 800XL should already have composite output.  The schematics show that it's connected.  J2 (the DIN plug), pin 4 IIRC.

I know that an 800XL has composite video, the issue is that the composite video is not working on the 800XL that I am currently repairing.

[ChildOfCv]

Oh, well your initial question made it sound like you were looking to mod it.

 

Okay.  Do you have RF output?  Most of the outputs are interconnected through passive parts (resistors, capacitors, and chokes), so it's not likely that you'd lose only one output.  It's possible that the DIN plug has a bad connection though.

[+hueyjones70]

54 minutes ago, ChildOfCv said:

Oh, well your initial question made it sound like you were looking to mod it.

 

Okay.  Do you have RF output?  Most of the outputs are interconnected through passive parts (resistors, capacitors, and chokes), so it's not likely that you'd lose only one output.  It's possible that the DIN plug has a bad connection though.

I chose this topic because it sounded similar to my situation. I don't know if I have RF or not I don't have a TV that I could connect to. I use a monitor. I will check the monitor DIN.

[+hueyjones70]

The problem was with the monitor DIN.