Ferranti T1002: Lack of Width
Despite the substitution of a new line output transformer, boost diode, line oscillator and rectifying valves, it was not possible to get sufficient width. After a period of twenty minutes or so the picture shrank still further, and distortion crept into the sound reproduction. This drew attention to the sound channel and it was noticed that the 30P12 sound output valve ( V15 in Trader sheet 1326) was looking hard-worked. This valve was replaced and the width spread out immediately, and with it the sound distortion disappeared.
Apparently the fault in V15 was loading the H.T. circuit heavily, but the H.T. voltage was not low enough to be suspected in the first place.