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CRT Astigmatism Adjustment

Guy considers this as "one of the most difficult but potentially resolution increasing adjustments of a CRT projector. Most people never even try to get astigmation exactly dialed in.

Here are some tips:

  1. Optical focus MUST be very well done first to make it easier to see the beam shape.
  2. Turn up contrast considerably to make the halo light up enough to be easily observed.
  3. Recenter any electronic astigmation controls before adjusting the CPC magnets.
  4. This is one of the few cases where the built-in patterns are best to use, at least for the moment. =)
  5. Adjustment of the 2 pole and 4 pole magnets is a bit interdependent. You need to get the circularity corrected with the 4 pole to make observing the effect of the 2 pole easier. Conversely, it's easier to get the 4 pole set correctly with the 2 poles correctly done. Expect to go back and forth between them to get it right.
  6. The angle of the two rings changes the intensity of the effect. So yes, there is a position which nulls out the effect. Just set the rings to get the effect you need. Don't worry about the 180 degrees opposing direction being equivalent. They are so it doesn't matter which position you end up at physically just as long the beam gets centered in the EM lens and shaped circularly.
  7. Set the 2 pole controls using the Center Cross pattern rather than dots pattern. You'll see a bright central line with a halo on each side when the EM focus is overfocused. Adjust the 2 pole magnets to get both the vertical and horizontal bright line centered in their haloes. This is easier to see than the centering of the bright point in a Dot pattern.

    Once this is done go back to the Dot pattern and run the EM focus up and down. The dot should change in focus but no in position. Do fine tweaks of the 2 poles until the dot stays fixed in position as EM focus is changed.
  8. Slightly EM underfocus a Dot pattern and use the 4 pole magnets to make the CENTER dot exactly circular. Change EM focus to slightly overfocused and again retweak to keep the haloing circular. Don't go too far in either direction with EM focus or else the final 4 pole setting will differ tremendously from under and over focused EM positions. Just defocus enough to clearly get the shape visible.
  9. Recheck 4 pole and 2 pole settings and iterate as needed.
  10. If you happen to readjust the centering magnets on a CRT, you'll also need to redo the 4 pole magnets as the two interact.
  11. If corner focus is still a problem, EM focus coil position on the yoke might be incorrect. That is set by centering all EM focus controls for center, corner and edges and then physically moving the EM focus coil. That's the smaller coil just in front of the CPC magnets. DANGER DANGER!!! You can snap off the tube neck if you are not careful. Too much torque while loosening or tightening the mounting bands is very bad news.

Basically, the coil needs to slid back and forth until you get best overall focus. You'll find that forward tends to get screen center dead on while further back gets the corners more focused. I tend set it between those extremes with a slight bias towards the corner. That gives more uniform focus, especially since the electronic controls have greater effect on the center focus than corner focus.

Of course, you'll have to redo 4 and 2 pole magnet adjustments if you move the EM coil."

Additional Information

"I think the internal pattern is more suitable because the dot pattern has a smaller dot than available from external sources (save a HTPC or progressive scan test disc). The small scan rate differences between the source and not quite genlocked pattern should not be enough to affect astigmation. However, a finer pattern does make it easier to see beam shape.

There is a caveat with internal dot patterns. They aren't always a dot and may actually be a short dash. This can sometimes be confusing when setting circularity with the 4 pole magnets. The greater horizontal length of a dash means the electron beam "blob" should be slightly longer horizontally rather than perfectly circular if the built-in pattern has dash shaped dots."

Addressing adjustments on a Barco 808

"Some tips on setting beam and optical focus very well.

Display a dots pattern. Set dynamic astigmation controls to center position. This allows you to set the magnets to do most of the work and just fine tune later with the dynamic controls. Binoculars are very handy but must be of large light gathering power.

You'll need to significantly increase contrast setting to above normal to see things clearly during astigmation adjustment. Also work with one gun at a time. Cut-off the other two. Once astigmation is complete, don't forget to return contrast to normal before final beam focusing.

Turn e-beam focus to max one direction then the other. In one direction (underfocused)the dots will become large circular or oval blobs with uniform brightness within the blob. In the overfocused direction, the spots will have a central luminous portion and a dimmer halo. These both need to be examined when doing astigmation.

The 2 pole magnets are the rearmost of the shaper magnets. These are used to center the electron beam in the middle of lens. Place the system into overfocus to show the central luminous point and surround halo. The 2 pole magnets deflect the luminous point. Twist the knob to alter the amount of deflection. Rotate the knob about the tube neck to alter the angle of deflection. 

If you use a center cross pattern instead of a dot pattern when centering the e-beam it's easier to see when the beam is centered in the lens. A center cross shows up as fat lines with a central luminous line. Just adjust the 2 pole magnets to make the luminous portion of the vertical and horizontal lines centered within the fat linear halos. It's easier to see than staring at a point with a halo.

The 4 pole magnets are frontmost of shapers. It is used to alter beam shape to a circular shape. One sets the tube to underfocus and displays a dot patter. This turns the dots into a larger, uniform blob whose shape is adjusted using the 4 pole magnets. Twisting the knob adjusts degree of oblongation. Rotating the knob about the CRT neck alters the axis of the effect. Make things as dead round as possible for
the center dot. 

Turn focus up and down and make sure things track correctly. You may need to make some minor tweaks to the magnets to get things just right.

Focus the dots as well as possible. Then go on to the next gun. When all three guns are completed, you now have "static" astigmation optimized and this means the dynamic astigmation system is under less stress. Perform dynamic astigmation for each zone as well as beam focus. 

Now that beam focus is optimized, it's time to recheck optical focus for center, corner, and Schempflug (horizontal and vertical flapping).  You can check lens flapping by using the center lens focus (rear wingnut). Display an inverse + pattern and adjust center focus to make the center of the top screen edge perfectly focused. Note the lens control position. Next adjust the lens to make the center of the bottom screen edge perfectly focused. Is it the same position. If yes, vertical lens flapping is dead on. If the two positions are not identical, you need to readjust vertical Scheimpflug (flapping). Similarly compare the perfect focus positions for left vs right edge. Finish by adjusting center focus to perfection and touching up the corners using corner focus (front wingnut).

Now both e-beam and optical focus are perfected and you can proceed with convergence and redoing grayscale.

Addressing adjustments on an NEC

Vertical and horizontal lens flapping (Scheimpflug) I think is done on the NEC's using the focus rings between lens and CRT. If I recall correctly, the A knobs (rear) adjust horizontal flapping and the B knobs (front) adjust vertical flapping. Don't confuse these with the lens focus windnuts. The focus rings are adjusted by loosening the four lens mounting screws, moving the knobs and then retightening the mounting screws.

Does the older 6 PG not have focus rings and only have fixed lens spacers? If so, you might be forced to try washers on the lens mounting hardware to flap the lenses and simulate formal Scheimpflug adjustments. I guess one could also create a wedged lens spacer to set the flap angle."

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