CATHODE RAY TUBE AGEING AND TESTING (iii) - FINAL TEST

A number of tests were undertaken on the finished CRT under actual operating conditions, below, we shall discuss each of them in turn: -

Cathode emission -  insulation resistance between all electrodes was checked, anode current, Ia was checked as was "cut off" which is the negative grid potential required to nullify emission

Vacuum integrity - the degree of vacuum was determined using an ionisation gauge.

Final visual check - the focus was checked, the raster was checked, the faceplate was checked for any blemishes, unevenness of colour or stray emission, the whole tube structure was checked for mechanical defects.  Only then was the tube given a final polish clean, marked with a type number and then packed in it's sale carton for storage in the stockroom.

Above, we have described the Quality Control (QC) tests that were carried out on each and every CRT that Mullard manufactured but in addition, Mullard were early exponents of Quality Assurance (QA) and from each production batch of CRT a sample was taken (√n+1 sampling plan) then independently checked for the same range of tests as the on sale devices. Each test was logged using Shewart charts such that trends in performance could be seen and corrective action taken to ensure product consistency.  Pretty impressive and all implemented long before our American colleagues reinvented this form of quality management and renamed it Six Sigma!

Of the QA samples taken, a portion were stored as batch retained samples whilst the remainder were subject to Accelerated Life Testing where operating conditons were chosen such that a 1000 hour continuous run would mimic the typical domestic useage experienced in approximately 6 years of operation.   This data was used again to assess product consistency but also to give valuable data on expected product Life Cycle and End of Life product performance.