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2-18
2.3.3.
Main Time-base (see Fig. 8.9)
For a fuller understanding of the functioning of the main time-base, important voltage waveforms in t h e MTB
control logic are given in Fig. 2.5.
a) Auto mode without triggering (free-running time-base)
Consider the situation a t the moment the main time-base starts.
With AUTO selected (S3A closed), NOR-gate output D209-13 i s L and the switching transistors V233 and V234
are turned off. The MTB i s therefor? able ro run and produces
by the Darlington pair V257, V258 and applied t o the X deflection selector via emitter-follower V224 and
switching diode V217.
The MTB is based upon the principle that
of generating
time-linear sawtooth voltage that is ideal for c.r.t. forward trace sweep and flyback.
a
Transistor V236 provides the current source, the base of which i s connected to a fixed voltage in the CAL
position of R10.This voltage i s only varied when the continuous control R10 i s moved from the GAL position.
The emitter resistance of V236 may have ten different values (R109
switch S15, which has 22 positions. Three timing capacitors are also selectable. Capacitor C226 is permanently
in circuit, capacitor C234 is selectable via switching transistor V238 and C241, C242 andC243 paralleled
capacitance via transistor V237. These transistors function in 'reversed' mode (collector-emitter reversed)
during charging of the timing capacitors and in the 'normal' way during the discharge period.
The following table indicates the capacitors and adjustment potentiometers that are brought into circuit in the
various positions of S15.
...
5
...
10 ps
2
...
5 ms
0,5
.
The L level from the inverting output pin 6 of flip-flop D212 i s applied to the base of switching transistor
off. The hold-off time now
it
The hold-off time-base circuit operates according to the same principle
capacitors are charged by
current can be adjusted in steps via the MTB TIME/DIV switch, which influences the voltage applied t o the
non-inverting input (pin 3) of operational amplifier D216. The HOLD-OFF potentiometer R11 permits
continuous adjustment of the current.
The voltage on the hold-off timing capacitors is applied to the inpu't of Schmitt trigger D214 (12,13,11) via
Darlington pair V286, V272 and the voltage divider R339, R308.
If this voltage has reached a level of +1,9 V, an H level is detected. In this event, output D214-11 becomes L.
This L level is applied t o the reset input D212-1, which gives an H on inverting output pin 6. Switching
transistor V271 conducts again and the hold-off time-base i s switched off: the timing capacitance i s discharge.
timing capacitance charged by
a
Timing capacitors
in circuit
C226
starts.
constant-current source to generate
a
a
time-linear sawtooth voltage. This is picked-off
constant-current source is capable
a
...
Adjustment point
R348
R347
as
for the MTB and DTB. Timing
time-linear sawtooth voltage. The charging
a
I
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