Video Polarity and Direct Injection

[kf4rca]

That means that the circuit is designed for a 75 ohm input 1 volt video source (standard).

[vts1134]

Originally Posted by kf4rca

That means that the circuit is designed for a 75 ohm input 1 volt video source (standard).

Ok.... so it's not an actual part, just a way of noting (in dummy terms) "video is supposed to go there"

[N2IXK]

You probably want to actually install the 75 ohm resistor, so that your DVD player or other source is seeing the proper load impedance.

The exception would be if multiple inputs are "daisy chained" from a single video source. Then you install the 75 ohm terminator across the last input in the chain.

[Electronic M]

Originally Posted by vts1134

Ok.... so it's not an actual part, just a way of noting (in dummy terms) "video is supposed to go there"

Transmission line theory 101 terminate your line with it's characteristic impedance to prevent reflections, ringing, reduced signal at terminals etc...Might be a good idea to brush up on (or look up the basics of) transmission line theory. If you are ever running a wired signal more than a few feet (especially if it is more than a tiny fraction of wavelength), or transmitting, T-line theory is crucial.

[kf4rca]

What is the polarity at the CRT? Are you looking at the schematic or a scope?
The CRT should be positive. Which means the driver tube input should be negative.
The output of the detector is usually negative, because it is transmitted that way. (Sync is peak power.)
Some of those detector circuits produce 5V of video.

[Electronic M]

Originally Posted by kf4rca

What is the polarity at the CRT? Are you looking at the schematic or a scope?
The CRT should be positive. Which means the driver tube input should be negative.
The output of the detector is usually negative, because it is transmitted that way. (Sync is peak power.)
Some of those detector circuits produce 5V of video.

There is no fixed polarity that MUST be sent to a CRT. The polarity of video you feed a CRT depends on whether it is cathode injected video or grid injected video....Both injection schemes were used back in the day*, and those 2 schemes want the opposite polarity of each other.

*In fact color sets used both at once to effectively mix the color difference and monochrome signals into RGB in the electron gun to save a few signal tubes in the design.

[vts1134]

Originally Posted by kf4rca

What is the polarity at the CRT? Are you looking at the schematic or a scope?
The CRT should be positive. Which means the driver tube input should be negative.
The output of the detector is usually negative, because it is transmitted that way. (Sync is peak power.)
Some of those detector circuits produce 5V of video.

The output of the detector is positive on this set. The video is amplified once and then fed to the grid of the kinescope.