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The trouble with this set is loss of colour. I managed to
restore the colour by replacing the delay line driver transistor T29, using a BC107, but the colour went again after
a couple of days. Using the correct BC171B transistor
produced no results. The voltages seem to be correct, except
for those at the output pins 1, 2 and 4 of the MC1327 i.c.
These are 7.5-8V instead of 6V. I’ve changed the varicap
diodes in the saturation control circuit, also C161 which
couples the signal to the base of T29.
First check that the bistable transistors are working – the
collector voltages of the two transistors (T36/37) should be
9-10V with a colour signal. Then check C165 which
decouples the emitter of T29 and C162 which smooths its
base bias. These are both 4.7µR tantalum electrolytics.
There are three other capacitors of this type in the chroma
amplifier circuit, C155, C153 and C151. All may need to be
checked – observe correct polarity if any need to be replaced. The chroma amplifier transistors T27 and T28 are also
suspect. Loss of colour is commonly due to faults in the
burst/reference oscillator section, such as oscillator drift,
but this would be betrayed by a rainbow pattern as the fault
comes and goes. If necessary, check that the output from
the ident stage is adequate – there should be 11V across
C 218 with a colour signal.

Previous ITT CVC8
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