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This set was purchased secondhand and has a green picture
– even with the colour control at minimum. The green
output transistor, also the feedback resistor in the green
clamp circuit, have been checked and I now suspect the
SL901B demodulator/matrixing i.c.
The simplest step to take is to interchange the red and
green leads from the top of the decoder panel to the c.r.t. If
the fault now comes up in red, this proves that the fault is in
the green video channel. Since you’ve checked the output
transistor, the suspects are the clamp diode 3D6, the driver
transistor 3VT5 and the clamp reservoir capacitor 3C48.

The SL901B is also a “possible”. If the screen remains
green, either the green first anode supply circuit
(7RV12/7R9) is faulty or the c.r.t. has internal leakage in
the green gun. This assumes that the c.r.t. base connector
panel is in good condition.

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