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The colour content of the picture intermittently breaks up
into wide horizontal bars of desaturated magenta and cyan.
The bars remain locked and coincide with the red and blue
castellations of the test card. The picture can be restored to
normal by depressing the tuning button or by detuning and
retuning. The fault is not temperature related since it
sometimes appears when the set is first switched on.
The trouble is almost certainly due to a late or mis-
shapen burst gating pulse. Check the components around
the pulse polarity splitter transistor VT308, especially the
two clipper diodes W315/W323 (type 0A91) in its base
circuit, also the input pulse coupling components R351
(220kS2) and C334 (82pF). Finally adjust the pulse width
control R354 as necessary. This should normally be fully
anticlockwise but if there is loss of colour on the left-hand
side of the screen it should be turned slightly clockwise for
full colour scan. (Thorn 3500 chassis.)

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