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I recently came into possession of a vector monitor, as shown below (Vector Monitor, Model: IO-1128, from Heathkit). Its got a red, blue and green "gun" connections. However, I am not sure about its third green connection.

Heathkit Vector Monitor, Model: IO-1128

From the schematic ((https://archive.org/details/HeathkitIO1128VectorMonitor), the red and blue connections are related with vertical and horizontal display positions respectively. What would be the purpose of the "green gun" connection?

enter image description here

plu
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  • are they not for connecting to the three electron beam guns in the crt, one for each color? – jsotola Jun 30 '20 at 20:12
  • @jsotola nah, judging from the picture, it looks like it's for checking the chrominance of analog video. (R-Y, B-Y) would be the signals for red and blue difference. Green is probably not green but actually luminosity? Like maybe on a component YPbPr scheme. – nabulator Jun 30 '20 at 20:15
  • @nabulator, I'm going by the switches on the front of the scope – jsotola Jun 30 '20 at 20:23

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I think what you're looking at is a tool for calibrating chrominance (color burst) signals in old analog TV signals. That would explain the labels of on the axis. The vertical axis measures the (R-Y) the red difference with Y, luminosity and the horizontal axis measures the (B-Y) the blue difference. Thus, the red and gun switches seem to ground entirely one of these axisis so you view just a single component of the your 2D color space.

I am guessing that the input is RGB and the output is the color space, thus jsotola is correct in that the "green gun" is in fact just the input for the green input signal. I'm guessing that the green signal ouput on the vector display is actually just the magnitude of the signal.

nabulator
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  • No need to guess, you are correct. Back in the 1970's and early 1980's I used these and pattern generators to calibrate tube and transistor color TV's, still all analog and always drifting out of spec. –  Jun 30 '20 at 21:25
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    Looking at the schematic, I don't think the green gun connector is actually an input, but instead merely provides a means of disabling the green gun. If one is attempting to display a pure green signal on a TV set, looking at the TV set and saying "yeah that looks green" won't indicate much about the calibration, but if one displays bars that should render as a yellowish green, green, and bluish green, while disabling the green gun, one would be able to ensure that one sees a dim red bar, a black bar, and a dim blue bar. – supercat Jun 30 '20 at 21:27
  • @supercat Yes, I think you are more clear. The connector here is really just a on-off switch. Although not shown in the OP's cropped image, there is a green signal going into the CRT, seemingly powered directly by the landline AC, which is why I believe there still is an input. – nabulator Jun 30 '20 at 21:41
  • @nabulator: The "Grn" label on the plug refers to the safety-grounding wire which, by the convention of the time, was colored green (European standards nowadays require safety-grounding wires to be green with a yellow stripe, and I think many other standards also allow that even if solid green would also be acceptable). – supercat Jun 30 '20 at 22:10
  • @supercat gotcha. I was confused for a second because green can also be shortened to grn and I see gnd for ground more. – nabulator Jul 01 '20 at 00:39
  • @nabulator: I think the label on the safety conductor means "green", and refers to the color of the wire in the cable. – supercat Jul 01 '20 at 17:49