How Can We Help?
The set has given faultless service for some four years, apart
from one unsoldered joint. From new however low signal
level inputs have produced snow, while even with a local
signal and a good aerial faint noisiness can be seen when the
screen is closely inspected. The noise is not random
however. By selective tuning with a low signal level it
becomes a pattern of diagonal black -and -white lines
superimposed on the picture at roughly 60 cycles per line. I
suspect that this is sound -on -vision, but adjusting the 6MHz
sound trap L44 makes no apparent difference.
Tune in the test card and examine the display. If the dot
pattern you describe is present only on coloured areas (the
the centre circle) it is a 4.43MHz colour subcarrier dot
pattern. If the pattern is present over the entire screen
however, roughly diagonal, moves in sympathy with the
sound and reverts to straight lines during silent periods, it is
6MHz intercarrier sound patterning. In the latter case adjusting L44 should minimize the patterning: if it has no
effect, its 390pF parallel tuning capacitor C66 is suspect.