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For some time now periodic adjustment of the vertical hold control has been necessary to stop
the picture rolling. Recently, however, another fault has developed. Approximately 15 minutes after being switched on from cold, the picture  suddenly disappears completely, leaving a good
raster and sound.
If I switch the set off and on again after about half an hour, a perfect picture is received (apart from the occasional roll) for a further 15 minutes. Most of the valves (including the tuner valves) have been changed without clearing the fault.- D. Jennings (London, E.3).
We advise you to check the PCL84 video amplifier (if this valve is not among those already
changed) and the coils to the left of this valve; open wound chokes. Then check the 0A70 crystal diode inside the coil cap near the coils and rear edge, which is
smaller than the other three.

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