Composite output - no color

[coonsgm]

Greetings!

I'm working to try and assist someone troubleshooting getting composite output from at 1040STF (without modulator). He is using an Atari video output to composite video cable and gets a good signal from a 520STM he owns and uses as shown here.

520ST comp.jpg (222.76 KiB) Viewed 77 times

We added the sync signal circuitry to his 1040STF and no output on the same screen and cable

1040ST circuit.jpg (377.19 KiB) Viewed 75 times

We then added a modulator and now we get output but there is no color

1040STF.jpg (202.42 KiB) Viewed 75 times

I figured I'd test with another 520STM and a 1040STE (both with modulators) and both of these also showed with no color as well. I'm baffled by this one...

Any thoughts?

[exxos]

I think the confusion is that transistor isn't composite video, its composite sync. RGB scart cables tend to use composite sync to avoid the "jail bars" on using the actual H&V syncs.

Composite video is generated from the RF chip... So i'd say it's going to be almost impossible to get composite video unless you have the whole RF circuit... Some machines doesn't even have the RF circuit which you can add yourself.

In fact years ago, I had a "blank" RF modulator area. I fitted all the parts and it didn't work.. I gave up with it in the end. But this was like 30 odd years ago.. So based on that, id advise not even going down that path.

Basically you need a machine with RF output to get composite video output. Otherwise your only going to get composite sync output and some machines don't even have that I think.

[coonsgm]

Got it, that answers that first question very well.

We removed the sync circuitry and added an RF modulator which led to the composite output without any color....which was the same result I got testing two other ST's with modulators.

[exxos]

No colour with a normal stock ST with RF modulator seems Odd. The monitor may simply not like the ST video output.

I had similar issues a while back.. https://www.exxosforum.co.uk/atari/last ... /index.htm

But when you said you added the modulator, what exactly did you add? You would need the RF chip and all the RGB lines etc connected.

But if your seeing the same issues as with an ST with RF output, then something is maybe wrong with the cable or monitor.

Not sure if the NTSC PAL thing is a factor as I think the frequencys are a bit different... I half recall issues with NTSC on PAL TVs resulting in no colour. Theres a circuit for both in the original ST schematics.

[coonsgm]

I had removed a full modulator from an STM a while back and simply added that to his STF. It's a complete unit and the STF has all the same points to connect it to...