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13
SERVICE NOTES
Time base B
This plug-in unit comprises a complete time-base
generator, a trigger unit and adelay-pick-off circuit.
The delay-pick-off circuit delivers a pulse, after an
adjustable delay, to front-panel socket BU4 and to
time-base A for delayed triggering (see Fig. 1).
Time-base B, like time-base A, is based on a Miller
run-up circuit with the following modifications:
Delay pick-o f f circuit
Valves B13, B14 and B16p constitute along-tailed
pair circuit (Fig. 9) which acts as a voltage com-
parator.
In the quiescent state of the voltage comparator,
B13 is cut off and the cathode voltage of B14 is
approximately equal to the voltage applied to the
grid of B 14 from helical potentiometer (R4) "Delay
time-multiplier 1-10".
The range of R4 is preset by means of potentiometers
R129 and R132.
The ramp voltage from the output of time-base B is
applied to the grid of B13, which will start to conduct
when its cut-off voltage is reached.
However as B 16p is aconstant-current source, sup-
1~
814a
+330`J
Schmitt -trigger
+130v
— the "Sweep-length" control is no longer a preset
adjustment, but afront-panel control "Length".
Continuous sweep speed control is not possible.
— the lock-out multivibrator (single-shot facility) is
not duplicated.
The working of the time-base generator is not de-
scribed due to its similarity with that of time-base A.
plying the cathodes of both B13 and B14, a rise in
current through one (B13) will cause an equal drop
in current through the other (B14). Furthermore, the
use of a constant current source in their cathodes
makes the current in B14 independent of the setting
of R4 (the "Delay-time Multiplier 1-10"), which is
used to determine the point at which the take-over
of current by B 13 from B 14 occurs. In this way the
output from B14 to the Schmitt trigger starts from a
constant level and provides a constant amplitude step.
The voltage rise of the anode of B14 is applied to the
Schmitt trigger circuit, (Delayed trigger multivibrator)
consisting of valves B17' and B17". (Fig. 10).
In the quiescent state B 17' is cut-off and B 17" is
+130V
Bl6t
Del d trigg
BU4
20 BUX
Del ~ t ri gg
time -base A
Differentiator
Cathode follower
Fig. 10. Schmitt
-Trigger,
Dif ferentiator and Cathode follower
FIG 3
PEM 2872
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