BAIRD 167

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Recently the picture disappeared overnight and
the screen is blank, but the sound is quite all
right. I suspected the 1125 rectifier at first, since
there was no glow from the heater, but on replacing it with a spare, got the same result. I also
replaced the 20P1 and U801 without result. The
line timebase whistle is absent. Could you please
help me by indicating further steps I can take
to trace the fault and put it right. A further
point I would like help on is that prior to its
breakdown, a black band at the bottom of the
picture tended to creep up for about Zin. to
-Eric B. Hardy (Burton -on- Trent).
The 20P1 is self oscillating under conditions
of normal signal reception. If, however, the
vision signal is not present. the essential triggering
of the timebase will not take place. The 20P1 can be artificially stimulated by momentarily
shorting the screen of the 20P1 to chassis. If
this action commences timebase operation. check
the vision signal stages, valves and resistors, etc.,
to ascertain where the signal is lost.
If. however, the H.T. voltage is very low, check
the 100 µF reservoir capacitor and the surge
resistors of the U801.

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