While reading through some old copies of 'Television' I came across an interesting reply regarding moisture ingress of capacitors.
The set in question dates from late 1954, the article is from 'Your problems solved' December 1972.
Quote:
MASTERADIO TE7T
The height decreased until the picture occupied a
strip about 2in. high across the centre of the screen.
This was some three years ago and the set has not
been in use since. Although it is old it gave a good picture until the fault developed and being a well built chassis I am thinking of trying to put the fault
right. As the set has not been in use I do not expect any deterioration to have taken place.-T. Ryle
(Worthing). We do not agree that the set would not have
deteriorated while out of use for three years. Normally disuse for this length of time would result
in paper capacitors absorbing sufficient dampness to make leakage develop. This usually shows up in
the field timebase, giving the symptoms of lack of height, bottom compression and so on. Which leads us to your fault. The likely cause of this is that
the supply to the field output transformer is much
below the correct 275V. The set is unusual in employing
a 12BH7 double triode as the field oscillator and output valve, with the supply to the output triode taken from the boost line via a 4.71w 1W
resistor decoupled by a 16uF 450V electrolytic. Check both these components and the 50,uF electrolytic
decoupling the cathode (pin 8). Then if neces- sary turn attention to the blocking oscillator section
of the 12BH7 where the 471w resistor in series with
the height control and the 8,uF (200V) field charging
electrolytic should be checked.
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I was quite surprised on the comment that the capacitors in question were only out of use for three years, it just goes to show that the problems were well known 46 years ago!
Mark