THORN 1600 CHASSIS

About half an hour after the set has been on there suddenly
appear increased background mush and crackles on the
sound – quite loud in fact. The fault is not affected by
operating the volume control, though the tone control
reduces the h.f. noise in the same way as it attenuates the
h.f. signal.
Assuming that there is little or no effect on the picture,
the audio i.c. (IC3) is probably responsible for the trouble.
The preceding intercarrier sound i.c. (IC2), the supply line
smoothing electrolytic C82 (470µF) associated with IC3, or
the decoupler C78 (0.001µF), could be at fault. These are
less likely possibilities however.