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The initial problem was intermittent loss of sections of the
picture, and sometimes also brightness and colour content
variations. Subsequently a more regular loss of the field
scan occurred, a short interruption of the supply restoring
this. The first problem was traced to the luminance output
emitter -follower transistor T151 having an intermittent
short-circuit. But it’s difficult to trace the field fault because
the load imposed by the test meter restores the scan.
We have known this fault to be caused by intermittent
failure of the field blocking oscillator timing capacitor
C312 (0.01µF) and by 8322 (330kS2) which is in series
with the slider of the height control. We assume that you
have replaced the PCL805 field timebase valve, and probed
around the field timebase for loose connections and dry –
joints. Failure of the 265V h.t. line (U5) which powers the
field timebase can also occur, due to intermittent
connections in the power unit.

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