Instructions for Adjusting Ion-Trap Magnets


The functioning of the ion trap necessitates the use of a small permanent magnet mounted on the neck of the tube, and this magnet must be correctly adjusted when the television receiver is installed, or when a new tube is fitted; otherwise the picture will be poor, and the tube may even be seriously damaged.

Correct adjustment of the magnet will be ensured by following the simple 'drill' described below.

  1. Switch of the supplies to the receiver.
  2. Remove the base socket from the picture tube.
  3. Slip the ion-trap magnet assembly over the tube base and on to the neck of the tube taking care that the arrow on the magnet points towards the screen.
  4. Rotate the magnet assembly until the arrow on the magnet is immediately over the line marked on the neck of the tube.
    NOTE-This line is normally approximately in line with the position corresponding to Pin No. 3 on the base.
  5. Move the magnet along the neck of the tube until it is only slightly in advance of the tube base.
    NOTE-In some receivers, for mechanical reasons, the ion-trap magnet may be fitted with the arrow diametrically opposite the line on the neck. In such cases the arrow must point away from the screen i.e. towards the base of the tube.
  6. Replace socket on tube
  7. Switch on the supplies to the receiver.
  8. Adjust brightness control and, if necessary, move magnet on the neck of the tube until the raster is only just visible.
    NOTE-This adjustment is best carried out with a stationary test pattern on the screen.
  9. Without altering the brightness control, and keeping the arrow over the line on the neck of the tube, move the magnet gradually towards the screen until the focused raster is at maximum brightness.
  10. Without altering the position of the magnet, adjust the brightness control to give normal brightness on the peak white parts of the image.
  11. If necessary, slightly re-adjust the position of the magnet to obtain maximum brilliance.
  12. Check the centering of the raster on the tube face and correct if necessary by adjusting the position of the focussing coil in the usual way.
    If it is found impossible to centre the picture correctly by means of this, rotate the ion-trap magnet slightly. This will probably assist in centering, but the position of the magnet must not be altered if it results in any decrease in brilliance.
  13. When these adjustments ave produced best results, clamp the magnet in position by means of the thumbscrew, taking care not to alter the position of the magnet.

Notes :-

  1. If a clear position for maximum brilliance cannot be found, the magnet is probably faulty, and a new one should be tried.
  2. Never re-adjust the magnet to remove a shadow if, by doing so, the brightness of the image is reduced. In such cases the shadow should be eliminated by adjusting the position of the focus coil and / or the deflection coils.
  3. Handle the magnet with care. If subjected to a very strong magnetic field or mechanical shock, its magnetism may be reduced or destroyed.

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Page copyright (c)
J.Evans 2001
Last updated
17th September 2001