The following artical is largely as it appeared in the book "TV Data Publications" in 1966. The voltages involved are potentially lethal and hence the procedures used should only be attempted by people with suitable experience of such voltages.
During the life of a cathode ray tube, the cathode emission will fall gradually. Eventually this slow deterioration will reach a point where it begins to have an adverse effect upon the brightness of the picture. Providing no other defect has developed within the tube, it can probably be rejuvenated when it reaches this stage by one of two methods.
The first is perhaps the simplest method and is the one most likely to give the best results. It is to increase the heater voltage by about 15%. This may well extend the life of the tube by several months, whereupon a further small increase to, say 25% may be tried. Generally, these increases in heater voltage will provide a very worthwhile extension in the tube life, and only in a few isolated cases will the heater wire fuse due to the overload. C.R.T. "booster" heater transformers are commercially available, their function being to increase heater voltage in this way. They may be temporarily plugged in between the receiver circuits and the tube base.
The other method of rejuvenating tubes is the procedure which is usually termed "reactivation". This involves temporarily overrunning the heater whilst, at the same time, a positive potential is applied to the tube electrodes. The result of this procedure is that a new supply of emitting oxides are formed on the surface of the cathode, and the tube should then be good for a further period of use under its normal working conditions.
A simple reactivator could consist of a heater transformer which is capable of providing a voltage which is about 30% in excess of the working heater voltage of the tube. The positive potential for the electrodes is obtained from a separate power supply, or directly from the a.c. mains via a suitable rectifier and potentiometer. This voltage is fed to the tube electrodes via a current limiting resistor and milliammeter, the complete circuit being shown below:-
These supplies are fed to the tube which is undergoing treatment, and if the procedure is going to be successful the emission will be initially very low, but will rise gradually until it reaches a peak as shown on the meter. The time required for this part of the operation may be anywhere between a few minutes and an hour or so, but once the peak has been reached, the reactivator should be disconnected and the tube heater run at its normal voltage for at least half-an-hour with no voltage on the electrodes. After this, the tube may be returned to its normal operating conditions and, if the exercise has been successful, a good picture will be obtained.
The procedure outlined is not that dissimilar to that used during the production of cathode/CRT at the factory. If you want all the heavy details, then refer to the artical "The Materials and Shapes of Vacuum Tube Heaters" elsewhere on this site.
It will be appreciated that success cannot always be expected in repairing tubes which have poor emission, but as the tube is useless when it reaches this stage, there is everything to be gained in making an attempt at prolonging its life.
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17th September 2001