[NJARC] Dim eye tubes - How to make them brighter...

[Thomas Lee]

Thought this might be of interest...  Taken from a link now long 
forgotten...

One method that can brighten a dim electron-ray tube is to increase the 
voltage applied to the fluorescent target. Small changes in supply voltage 
produce disproportionately larger luminance changes. As a last-ditch effort 
in some sets, especially where the very expensive 6T5 is involved, the 
target could be rewired directly to the rectifier cathode to boost B+ by as 
much as 100 volts. This method is influenced by a number of factors in 
receiver design and operation however and may not work in every set.

One published method described using a technique similar to that applied to 
rejuvenate cathode-ray tubes. The problem however is not emission, which 
this method addresses, but target contamination. CRT rejuvenation techniques 
have no influence on the condition of phosphors.

Electron-ray tubes remain bright for about a thousand hours of operation. 
The heaters in electron-ray tubes can fail or weaken before the targets 
become dim, but this is not the norm. Target dimness is believed to be due 
to physical or chemical contamination of the willemite boundary layer by 
Barium and other ions that are boiled off through normal cathode emission. 
This contamination is visible as streaks of darker gray discoloration to the 
otherwise light gray coat of a new electron-ray tube target. If you’re 
rummaging through a box full of tubes at a flea market and spot a tuning eye 
tube, look for this discoloration before you buy. Be certain to examine the 
target surface in a bright light. The target of a good - or reasonably good 
- tube will have a consistent light gray tone. Look for two thin lighter 
gray streaks that are in the same line but opposite one another. These light 
streaks are produced by the shadow cast by light shield’s supporting pins 
and are evidence of a well-used tube.