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59
3.Service data
3.1
BLOCK DIAGRAM DESCRIPTION
3.1.1
Y Channel
The vertical channels A and B for the signals to be displayed are identical, each comprising an input coupling
switch, an input step attenuator, an impedance converter and a preamplifier with trigger pick~ff.
A channel multivibrator, controlled by the display mode pushbuttons, switches either channel A or channel B
to the final Y amplifier via the delay line. The channel multivibrator is operated by a pulse at the end of the
sweep, and offers an uninterrupted display of the A and B waveforms in the ALT mode. In the ADD position,
both switching amplifiers couple the signals through, thus adding channels A and B. By inverting the B channel
amplifier (PULL TO INVERT B) the A — B mode is obtained.
The AMPL/DIV switches provide x 1 or x 10 gain control of the preamplifier, which offers in conjunction
with the step attenuator a full range of deflection coefficients in a 1-2-5 sequence.
,..,
3.1.2
Main time base triggering
To initiate sweeps, trigger signals can be derived from the A and B vertical channel preamplifiers, from an
external source, or internally from the mains supply (LINE triggering) as selected by the trigger source switch.
,~
Composite triggering (A and B depressed) is derived from the delay -line driver stage. The polarity of the trigger
signal, negative or positive~oing, on which the display will start is determined by changing the output polarity
of the impedance converter.
With the AUTO switch depressed, the peak-to-peak level detector comes into operation. The peak-to-peak level
of the signal then determines the range of the LEVEL control.
With AC or DC depressed, the range of the LEVEL control is fixed.
In the TVL and TVF modes the LEVEL control is inoperative and the TV sync separator is switched into
circuit, thus initiating sweeps with line or frame puises depending on the setting of the TVL and TVF switches.
3.1.3
Main time base circuit
For normal internal time base operation the horizontal amplifier is fed by sweeps from the time base circuit.
With AUTO depressed, in the absence of trigger signals, the output of the sweep generator is fed back via the
hold-off circuit and gate to its input. This causes sweeps to free-run and a resultant trace is displayed on the
screen. As soon as the AUTO control circuit detects a trigger (i.e. a change in the output of the sweep-gating
logic) the sweep is fed back to the sweep-gating logic. This causes the circuit to revert to the normal triggering
mode in which sweeps are initiated only by trigger pulses at the input of the sweep-gating logic.
With AC or DC depressed, AUTO control is made inoperative. Sweeps are then only produced provided a
trigger signal is present and the LEVEL control appropriately set.
The display can be magnified in the horizontal direction by increasing the gain of the final amplifier by a
factor of x10 (also the X DEFL mode).
When the X DEFL pushbutton of the horizontal selection switch is depressed, the sweep generator output to
the final amplifier is inhibited and the impedance converter is connected directly to the final amplifier. In this
way, the signals normally selected for triggering, or an external source, can now be used for horizontal
deflection.
3.1.4
Hold-off circuit
The hold-off stage, as its name implies, 'holds-off' triggers from the input of the time base circuit until the
trace has completely returned and the time base circuits are completely reset.
3.1.5
Z Axis
The Z amplifier provides for the blanking of the trace during the fly-back and hold~ff time. In addition, it
blanks the sweep in the CHOP mode during the switching transients.
The I.f components of the blanking signal are modulated and demodulated before they are applied to the
Wehnelt cylinder together with the a.c. coupled h.f. component<
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