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This applies only to B&W sets with focus coils:
Once you have repaired a few sets of this era, the below will be done automatically, without even thinking. For a first-timer with a knowledge of electronics from a different era, or from a different medium, it is very easy to mess up in these adjustments.
General instructions for eliminating neck shadows (round shadows on edge of the image) for post WWII through 1950's Black and White TV's with a chassis mounted CRT that have a focus coil. This also is useful for cabinet mounted CRTs with focus coils where the yoke must be removed for any reason. (If any of this contradicts the manufacturer's instructions, ignore that part.)
Since the CRT is usually secured by padded clamps on the edge of the faceplate and this padding may well have shrunken over the years, when you turn the chassis on its side to work on it, there is the possibility for the CRT to shift slightly. This can shift the yoke and focus coil alignment.
When you turn the set back right side up, before doing any centering adjustments, try to wiggling the neck of the CRT to see if there is any play. This also helps to reseat everything in back in its normal operating position.
The yoke generally should lightly touch the CRT bell -- I like to have them just loose enough to slide a scrap of waxed paper between the yoke and bell. This lessens the chance of the yoke sticking. Most focus coil sets have some slotted screw hole adjustment on the mounting bracket or support to position the yoke assembly.
As a rule of the thumb, the further away from the screen the yoke is on the CRT neck, the greater the chance of neck shadows.
First, adjust the ion trap for maximum brightness.
On sets where the centering is done with the focus coil, focus, but do not attempt centering until the horizontal (and to a lesser extent the vertical) linearity correctly adjusted as these interact. It is better to have the width and height slightly undersized if you cannot determine the correct width and height.
Use the centering adjustments (usually on the focus coil's mounting bracket) to center the image. Then gradually increase width while readjusting linearity to maintain a good image, again doing the horizontal before the vertical, until the image fills the screen without curved shadows that start in the corners.
The first time you work on a B&W TV with permanent magnet centering, you will be amazed how much that simplified this adjustment!
A few got-chas on ion traps:
Dual magnet: The black or darker magnet goes nearest to the tube socket. If there is a colored dot, usually blue, the dot goes away from the tube socket.
(Someone please double-check me on the above. It has been almost 50 years since I serviced a dual magnet set.)
If a single magnet, if maximum brightness either is completely off centered or usable brightness cannot be achieved, then the ion trap may be on flipped.
And the worst got-cha of all is on the very few CRT's that did not use an ion trap, where some well meaning person has installed one. If you don't have original service info, always look up the CRT in question!
Also during the B&W focus coil era, permanent magnets were not as permanent as we have today and sometimes the ion traps had to be replaced when they were only a few years old.
If while servicing, the set must be run for more than a few minutes with the ion trap incorrect, keep the brightness very low to avoid brownish burns on the CRT.
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