Ferguson 406T series: Sound muted

Sound and vision were very good in this set until the signal was interrupted ( in the customers house the aerial lead was intermittent), when the effect of pulses of signal was to cut the sound right off for a period of several seconds. The sound would then come back gradually, just as if it were being faded in.

A series of rapid pulses would mute the sound for about ten seconds, but a single pulse * as when changing channels) caused only momentary fading. By elimination this fault was traced to the sound i.f. amplifying valve ( V11 Trader sheet 1276), where a pulse seemed to cut off anode current.

The i.f. stage seemed unstable both on or off fault as the presence of a test prdo near the valve produced howls from the speaker. Decoupling on screen, anode and cathode seemed in order, and attention was turned to the sound A.G.C. circuit where normal voltage was found until checked at the control grid of V11. It was then found that a 1MΩ A.G.C. decoupling resistor R75 was open-circuit. Replacement cured the fault