Circuit Description Section
4
Figures 2, 3 and
4
show the circuit details of the Y ampli-
fier, timebase and power supplies respectively. Fig.
5
shows their inter-connection but for clarity some details
of this inter-connection are included in the detailed
diagrams also. For convenience, the circuit reference of
any component can be used to determine its location in
the instrument.
1
-
99 Frame or interconnection
100
-
199 CHI Pre-amplifiers,trigger amplifier
beam switch, beam switch bistable.
200
-
299 CH2 Pre-amplifiers, trigger amplifier.
300
-
399 Delay line drive, equaliser, Y O/P
amplifier, bistable drive and X/Y
amplifier.
Trigger Timebase, delay and X O/P
600
-
699
700
-
799) Power Supply
800
-
899 Front Panel Control Board
900
-
999 E.H.T. Supply.
e.g. R309 is part of the Delay Line Drive.
The circuit diagrams are generally arranged for each
printed board assembly or group of boards. These
diagrams include some of the switches, potentiometers
etc., closely associated with the circuit although these
components may be mounted on the frame.
Consequently not all such components appear on the
interconnection diagram Fig.
5
and where it makes the
circuits easier to follow, some components appear on
more than one circuit.
4.1
THE BLOCK DIAGRAM
The block diagram for the complete instrument is shown
in Fig.
1.
It is not intended to be a full working diagram
but gives details of the functional points of the circuit
and their inter-relation. The circuit can be readily
divided into three main sections which are:
Y deflection circuits, X deflection circuits (including
timebase delay and trigger circuits) and Power Supply
which includes Bright-up functions.
Y
CHANNELS
The switched attenuator,pre-amplifier and trigger
amplifier are identical for the two Y channels except
that CH1 provides pick-off and amplification for the X/Y
signal and CH2 provides invert facilities. The selection
of the decade steps of the attenuator and 1.2.
5.
sequence of pre-amplifier gain switching are determined
by the sensitivity selected.
The use of the fine gain facility on either channel allows
a 2:1 increase or decrease in sensitivity.
The channel switch is a fast electronic switch with the
equivalent of a change-over action as shown. It selects
either the CH1 or the CH2 signal to be passed to the sub-
sequent stages and is controlled from a J-K flip-flop. The
state of the flip-flop and hence the channel selection is
determined by d.c.voltages applied to its preset and clear
inputs. In the chop mode, the J-K flip-flop is driven via
its clock input and a signal gate from a free running
monostable, switching the beam between CHI and CH2
signals as the X sweep progresses. In the alternate mode,
end of sweep pulses are fed from the timebase to the J-K
via the signal gate and reverse the beam switch at the end
of each timebase sweep, giving alternate CH1 and CH2
sweeps. In the ADD mode, both the CHI and CH2
switches are closed and the two signals are summed
algebraically. On CH1 or CH2,the appropriate switch is
closed, allowing only that signal to pass.
The signal from the selected channel is passed via a delay
line and amplifiers to the Y deflection plates of the c.r.t.
The delay allows examination of that point in the wave-
form which triggered the sweep since the deflecing signal
reaches the Y plates after the timebase sweep and bright-
up have been initiated.
THE TIMEBASE
The purpose of the timebase system is to generate a linear
ramp to deflect the spot in the X direction. The trigger
system initiates each sweep from the incoming or other
signals, normally to obtain a stationary display of a
repeated waveform.
The internal or external trigger signal as selected is
amplified by the trigger amplifier which is biased by the
required trigger level, and the resultant is amplified and
passed to drive the trigger circuit. If the timebase is
ready to commence a sweep, a transition of the trigger
circuit will set the timebase bistable which in turn initiates
the ramp. This signal is passed via the X amplifier to the
X
deflection plates of the c.r.t.
At the end of the sweep, the bistable is reset, returning
the ramp to the original level. During the period of sweep,
the trigger gate is prevented from passing trigger pulses to
the bistable and this inhibition is maintained by the hold-
off circuit until the ramp generator is fully recovered,
ready for the next sweep to commence on the next trigger
pulse, when the cycle is repeated.
In the delay mode the timebase bistable is triggered in the
same manner but the bistable output to the ramp
generator is inhibited for the period of the delay. To
allow time for the delay monostable to fully reset there
is an additional hold-off control from the delay circuit
into the trigger gate. In Single Sweep, the relevant
bistable normally inhibits the trigger pulses from reaching
the timebase bistable by biasing the hold-off circuit, and
hence closing the trigger gate. When the single sweep
bistable is armed the trigger gate is opened and the next
trigger pulse initiates a sweep. At the end of the sweep
the single sweep bistable is reset, preventing the timebase
from sweeping again until the manual ARM button is
pressed once more.
The trigger signal for the timebase, selected from either
internal, external, or line frequency sources, is passed
through the required processing filter selected by the
Scans
by
Arte
