THORN 1500 CHASSIS
After about an hour the picture begins to break up horizontally, with bright flashes across the screen. After this the screen goes faint with only traces of a picture. The picture
returns to normal when the set is switched off for about half
an hour and then switched on again. A fault that’s been present for some time is that the picture suddenly looses contrast, light pressure on the contrast control bringing it back.
This doesn’t cure the first fault however. The contrast control and its connections seem to be in order.
It would be advisable to replace the contrast control to
start with, then check the electrolytic capacitor (C37) which
is in series with it and feeds the base of video output transistor. Another electrolytic to check is C38 (12µF) which
smooths the HT2 supply to the video output transistor’s
collector circuit. If necessary check the video output transistor VT9 by substitution, and ensure that its two series –
connected load resistors R40/51 are of the correct value.