PYE V4: Vision motorboating
At any position of the contrast control that was sufficient to provide a signal strong enough to hold a picture, if the brightness control was then adjusted to give a desired level of tube brightness, vision motor-boating would occur, the picture brightness varying quite rapidly up and down, giving an effect reminiscent of motor-boating in a sound receiver.
Decoupling in the vision receiver was found to be in order but if the A.P.C. circuit was put out of action by connecting the anode of the A.P.C. pulse rectifier to chassis (V13b in Trader 1120) thus short-circuiting the A.P.C. bias line, the fault cleared.
Component checking in the A.P.C. circuit revealed that the value of the resistor R75 which is connected between anode and cathode of the sampling diode V13a, had increased from 2.2MΩ to 5MΩ,and replacement of this defective component cure the trouble. Another difficult fault in this TV is an open-circuited contrast-control, which in my experience produced a different symptom each time it occurs.