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A streaky magenta cast is present on transitions from dark
to light areas, while on light to dark transitions there’s a
streaky green cast. The fault is not very consistent however,
sometimes changing every few seconds, remaining
sometimes for an hour or so, while on other occasions it
may occur only once or twice during an evening. The
transistors in the green channel have been replaced. Is there
anything else to check before trying a new tube?
Incidentally, the only other faults we’ve had over the past
six years have been a defective PCL805 field timebase valve
and output stage cathode decoupling electrolytic C427
(went short-circuit), R429 in the focus network going open –
circuit, and a dry joint in the i.f. preamplifier circuit (caused
a noisy signal).
The fault is often due to C143 (0.0015µF) which adjusts
the frequency response of the G output stage. Check this
capacitor, and its 27Q series resistor R176, by substitution.
If the fault persists, the tube could well be at fault.
Interchange the blue and green drives to the c.r.t., set up the
grey scale, and watch in monochrome: if the fault still
shows up in the same way, a new c.r.t. will be necessary.

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