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Color on composite doesnt work unless long cable used

falter

falter

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Weird one.. my Mindset VPS - I can't remember for sure but I'm pretty sure the composite put out color before on a short high quality RCA cable I had. After using it for video titling, it now only shows mono. However, when I was troubleshooting I grabbed a different, much longer (like 15 feet) cable.. and suddenly I had color again. Does that make any sense? Switching back to short cable.. we are back to greyscale.
 
No it doesn't, unless the short cable is somehow going faulty.

By putting a 75 Ohm resistor across the 'far end' of the cable (or making a suitable socket with the resistor fitted so you can just plug the cable in) you can then use an oscilloscope to measure the video signal across the resistor.

Make sure the oscilloscope GND lead is connected to the end of the resistor that is connected to the cable shield/screen. The resistor simulates the impedance of the monitor.

You can then 'see' the video signal and compare it between the two cables. Look (in particular) at the colour burst signal (just after the line synchronising pulse).

Dave
 
Perhaps there's something going on related to the larger (distributed) capacitance of the long cable. I wonder if a small cap of a few puffs at the far end might make the difference. I'd probably start with a gimmick.
 
I had something in mind as well this morning: length = delay. But it used to work before and if there is no damage to the short cable, something else is to blame IMHO.
 
It could be that the color burst signal is marginal in amplitude, if it is the VDU will see the signal as monochrome and deploy its color killer. Somehow the longer cable is tuning the level up. It sounds suspicious that the cable might not be properly terminated at the VDU end into 75 Ohms. I would suggest doing what Daver2 said. That will also help determine if there is any problem with the buffer stage driving the cable, or the relative burst (& chroma) level are correct. If the output stage driving the cable is ok you should see about 2v pp un-termianted and 1Vpp terminated into 75 Ohms and the color burst level in that case should be about 0.3V pp, about the same pp amplitude as the H sync pulse.
 
Thanks guys. I'll check it out. I'm trying to think if I ever saw color out of this particular machine but i'm not sure now. I'm pretty sure when I was doing testing I had it on a regular 3 foot composite cable. I then had it hooked up to the video production module and actually did some video titling with it, put it away.. and then it was after when I fired it up I saw it was only coming up black and white on my composite monitor. I recall when I was using the titling system the video was rather unstable and unhappy - so when I only got black and white recently I thought maybe it'd damaged something. RGB is still fine. A couple of days ago I wondered aloud if it was the monitor acting up, so I grabbed a nearby cable and plugged into my 1701... and it was color. But that was a long cable again. I'm going to see if the long cable trick works on the IIe monitor I was originally using.

I recall reading somewhere that Mindset has a key bit or something that, when doing video production, can turn color on and off. But I'd assume it would default back to the original state as soon as power was off.
 
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