THORN 980 CHASSIS

The trouble with this set is very critical line hold, which has
to be constantly adjusted – every three to four minutes. A
new line oscillator valve has been tried but has not improved
matters.
The fault could be in the flywheel line sync circuit, where
the discriminator diodes W3/4 are the first suspects. A very
likely possibility however is a fault in the video output stage.
You will find a bias stabilising resistor (R26, 39kS2, 1W)
connected between the screen grid and the cathode of V5.
This commonly falls in value, upsetting the bias conditions
in the stage and therefore severly clipping the sync pulses.
The sync separator’s screen grid and anode load resistors
would also be worth checking.