BUSH TV128U

The set works normally on 405 lines but when switched to
625 lines the picture and sound fade out after some minutes,
with a hissing noise as though off signal. On repressing the
u.h.f. tuner button both picture and sound reappear for half
an hour or more. Both valves in the u.h.f. tuner have been
replaced.
There is almost certainly a dry joint in the tuner.
Resolder all suspects or disturb each with a plastic tool to
see which is causing the effect.