What is the theory behind low voltage focus CRTs?

[LukeSimon]

I applied the previously mentioned steps, which are based on the theory of using G1 minus cathode voltage as an iris, by maxing cathode DC bias, and minimizing G1 DC bias. Focus voltage is zero, and G2 DC bias is maxed out. I had to slightly increase the DC bias of G1 to get black levels calibrated and to keep the moire effect from being overly noticeable.

With a low voltage focus CRT, the lower the focus anode voltage, the more the cathode ray is squeezed into a thinner beam of electrons, as the negatively charged anode repels the negatively charged electrons. A high voltage focus CRT is a bit different in that it accelarates the electrons towards the center of the beam and forward towards the phosphor screen.

As you can see in the first picture, which is a test pattern of 240 alternating white and black horizontal lines. Before the tuning, the adjacent horizontal lines blurred together, making it impossible to distinguish between alternating lines. Now there is enough separation to be able to distinguish every line. The white lines are still too thick compared to the black lines, but if the cathode ray is made thinner, the moire effect becomes terrible.


This color bars pattern shows a subtle moire effect caused by the cathode ray’s spot size becoming too small relative to the phosphor mask dot pitch. If you zoom in to the horizontal white and black lines test pattern above, the white lines are one line of video, so the vertical thickness of the white line is the cathode ray spot size’s diameter. You can see that the spot size is just about 1.5x the dot pitch. This is why moire effect is becoming noticeable.