Ion Trap Service Tips

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Many thanks to Frank Ferraro for posting this on Facebook last night.

From The Bag Of Tricks, Tips, and Past Experience.

WHAT ABOUT THE ION TRAP?

Service technicians took the ion trap magnet and the newer PM centering assemblies casually, as a rule. However, measurement of their field strength was an important factor that should not be overlooked when certain types of service trouble are encountered. Either of these units may be single or dual magnet assemblies. The better manufacturers of ion trap and centering magnets at the time used Alnico V. These magnets had a silvery color, while other materials didn't. This choice of material and the importance to the service technician is indicated because both ion trap and centering magnets operate pretty much under open circuit conditions and thus are subject to demagnetization due to heat, handling, or vibration. In addition, the centering magnet is close to the deflection yoke with its heavy demagnetizing effect, which may seriously affect its strength.

Appreciable reduction in the field strength of the centering magnet assembly may reduce its ability to center the beam or raster to the point where it has to be discarded. A simple check of its field strength would indicate whether the trouble is in this component or if a breakdown has occurred in the deflection yoke circuits.

Replacement by another unit, off the shelf, may not remedy the trouble If this new unit should be of a type employing magnetic components which also might have a weak field (from one of the causes previously indicated). The centering magnet produces a field which moves the electron beam before it reaches the yoke. The ion trap magnet presents a different problem. Normally where field strength is correct it is possible to bend the electron beam through the proper course until it again hits the gun structure.

Cases of ion trap magnets being overly strong are just about an impossibility, as the trap can be positioned. An inexpensive field strength meter can be used to check relative strength of magnets. Deflection is due to field of the near by ion trap far back if necessary. The usual trouble is a weak magnet, in which case there is lack of brilliance, difficult centering, neck shadow, and danger of the screen developing ion burns. For a CRT utilizing electrostatic focus, it is important that the field strength be close to the given value. A field strength appreciably higher than that specified distorts the focused spot, changes the focusing voltage of the tube, and also requires a shift in the position of the magnet on the neck of the tube.

In the electrostatically deflected cathode-ray tube the ions and electrons produced by the cathode of the tube are both deflected equally and the screen is thus bombarded evenly and not burnt. In the electromagnetically deflected tube, due to difference in mass, only the electrons are deflected appreciably, and the ions hitting the screen at one point in a steady stream would produce a burnt spot. This was remedied in later CRT's by directing the ions and electrons against some portion of the electron gun, as in the bent gun design.

The ion trap magnet straightens out the electron stream and allows it to go down the tube to the screen. This magnet does not affect the ions, which continue in a straight line into the tube structure where they do no harm. The magnet is adjusted until the electron beam is just straightened out. Improper adjustment or weak field strength will allow the beam to hit the gun structure (aperture), limiting screen brilliance and producing secondary ion burns due to disintegration of the structure.

The magnetic field strengths employed in these assemblies run between 5 and 40 gausses.

The types and forms of ion trap and centering magnet assemblies vary widely, as does the field strength of the magnets. There are ion trap units with single bar magnets (mounted either across or parallel to the tube neck), double ring types, bar and ring, and double bar, to mention some of the most common kinds of that time. The field strengths of these magnets vary widely among the different types. The Two-magnet assembly delivers a strong and a weak magnetic field, whether of bar or ring construction. The apparent value of the field strength will also vary with the point at which it is measured. This means that regardless of the type of field strength indicator used, some measurement configuration must be used for each type of assembly if comparative strengths of assemblies are to be measured.

Therefore some position and distance between the magnet assembly and indicator must be selected and noted on a chart along with the value of the flux meter reading for each type of assembly.