THORN 1690 CHASSIS
The symptoms are no raster and no sound except for a hissing sound from the loudspeaker. As there was no line whistle, we checked for e.h.t. and as expected found this absent.
Both fuses are intact. A new line output transformer/e.h.t.
rectifier combination was tried, but this made no difference.
The 11.3V rail is correct, but the 100V rail is not there
because of the lack of line output. The line driver transistor’s collector feed resistor R90 (12Q, 0.2W) was found
to be very high in value, starving the transistor of collector
voltage. As a direct replacement wasn’t available, a 1052 fw
resistor was tried but got hot within seconds, as did the
driver transformer T3. Continuity tests on the driver transistor have been carried out, but it seems to be satisfactory.
The line driver transistor VT16 is obviously hard on.
This could be due to leakage in VT16 itself, or in the
preceding oscillator transistor VT15. It’s best to check these
by substitution. First however ensure that the tuning
capacitor C84 (0.0068µF) is not leaky, as this would upset
the bias conditions.