THORN 1500 CHASSIS

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There’s a good monochrome picture when the set is first
switched on, but after a few minutes something clicks and the
picture goes foggy, with a tendency to field slip and line
pulling. Sometimes the set wil click back to normal, but at
other times the picture remains foggy. I’ve replaced the
coupling capacitor to the video output transistor, and its
emitter decoupling capacitor, the tube’s first anode decoupler
and also the smoothing capacitors in the a.g.c. line, and the
electrolytics which smooth the various supply lines. When the
picture is foggy the brightness and contrast controls don’t
seem to alter it much: at other times they work normally.
We suggest you first check that the video output
transistor’s two collector load resistors R40 and R41 are in
good condition and well connected, then suspect the video
output transistor itself (VT9). If this fails to cure the problem
check for dry-joints around the vision detector and its
associated coils (L9/L10), also the condition of C32 which
decouples the video driver transistor’s base bias. Faults in the
i.f. strip transistor emitter decoupling disc ceramic capacitors
C8/C 13/C25 sometimes occur to give strange efects on the
picture.

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