The Chief, Part 15 - More Improvements, part 2 

The monoscope and the vidicon tubes work on different principles. Technically, the target in the monoscope is at high voltage. Compared to the cathode which is sitting at minus one thousand volts. I did this so the target would be at ground potential and not burn out the front end transistor in the preamp. In a vidicon tube, the target is typically operated at a relatively low voltage. Only a couple of dozen volts above ground. There is a mesh right behind that target which slows the electron beam down and makes it arrive parallel with the face of the tube. The voltage is high because it takes a lot of potential to get the electrons moving. It is also necessary to for the electric fields to be strong enough to focus the beam to a sharp point. The monoscope plate is just bare metal and insulating ink. If the beam hits the plate, a strong voltage comes out and if it hits the printing, no current can flow, no signal is produced. The "collector" electrode, running at roughly +25V above the target voltage, sucks up the left over electrons from the ink. Otherwise these fall onto the target plate and create spurious signals and unwanted shading.

@@videolabguy thanks for explanations. It is really helped me

I put that text on the schematic as just a precaution. It means to not connect your FET input multimeter there. During flyback, there is quite a large voltage spike... possibly. The size of the pulse depends on how has the retrace occurs. I may take a look at that voltage with the 100:1 probe now that you got me curious. Thanks!

This is a civilized audience.

@@vaclavtrpisovsky Well on the previous episode someone said that.