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There are two intermittent faults on this set. First, red
patches appear at the centre edges of the screen. They are
most noticeable in white areas of a monochrome picture,
though they can be seen on some colour pictures. They
don’t vary when the colour control is adjusted. The purity
has been adjusted, improving matters, but the fault is still
visible. The tube has also been degaussed. Secondly,
patches of green and black vertical lines appear even more
erratically. They are visible on black backgrounds only, and
only one patch at a time appears.
The patch problems are certainly due to impurity, and
are a function of the shadowmask temperature. This
accounts for their intermittency. The two remedies which
we find seldom fail are to readjust the purity when the set
has been running for some time, and to run the set at lower
brightness and contrast levels. This will keep the mask
cooler and also provide a much better picture.

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