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differentiated
by C15 and
R38. The resulting
bidirectional
pulses
are applied
to the series clipper
01,
which
provides
the
collector
of TR68
in the
sweep
circuit
with
negative-
going trigger pulses.
In the TV positions
of Sla
and b, R25 is disconnected
from
the emitter
of TR4; TR4 converts
into a sync separator
with
C12 being switched
across R31. TR5 changes into an inverter
with
decoupling
capacitor
C16
being
switched
across
the
emitter
resistors
R36, R25 and R34. In the TV F position
of
Sl a, the
differentiating
time-constant
of C15
and
R38
is
increased
by the addition
of R39.
3.4.4
With Slc
set to
HF,
R32
is added
in series with
R34
across
C14; this converts
TR4 and TR5 into a free-running
oscillator
whose
frequency
is adjusted,
by R15 the LEVEL
control,
to synchronise
with the H.F. trigger input.
The sweep genArator, reference
Figure 6, consists
of a Miller
integrator
TR71
and
emitter
follower
TR72;
and
also
bistables,
a gating
bistable
TR66,
TR68
and
hold-off
bistable
TR73,
TR74,
connected
between
the Miller output
and input.
3.5.1
Initially,
for an incoming
trigger
pulse to fire the sweep
the following
conditions
apply:-
Diodes
067,
068
and TR69
are conducting
and clamp the
drain
of TR71
at
+
2.5 V approx.
The hold-off
bistable is
held with
TR73
off, TR74
on and the gating bistable
with
TR66
on, TR68 off.
3.5.2
A negative-going
trigger pulse causes TR66
to switch off,
TR68 on, and 066 to conduct.
Hence current flowing through
R84 diverts from 067,
068
to 066. This open circuits
067,
068
and releases the gate of TR71. TR71 drain starts to rise,
due to Miller action,
taking
TR72
base and emitter
with it
and
cutting
off
TR69.
This
rising sawtooth
voltage
passes
through
071 until eventually
TR73 base becomes sufficiently
positive to switch the bistable over. Hence TR74 switches off,
TR73
on and
the
negative
voltage
step at TR73
collector
causes
TR68
to switch
off
and TR66
on. TR68
collector
goes positive,
switching
066
off,
067 and 068
on; starting
the flyback.
3.5.3
Current
flows through
R84, 067 and 068 into the timing
capacitor
Ct to commence
flyback.
When TR72
emitter
has
fallen
sufficiently
taking
TR69
emitter
with
it then
TR69
conducts
and
clamps
Ct at the initial start
potential.
This
potential
is determined
by the resistor
ratios R85, R86, R94
and R95.
3.5.4
During the flyback
period,
071
is off due to the charge
on the hold off capacitor
Ch' This charge leaks away through
R104,
R105,
R106,
R107 and Rl12
until eventually
TR73
switches
off,
TR74
on;
the initial conditions
(3.5.1 above)
are restored.
3.5.5
When the sweep
is switched
to single-shot
mode,
TR73
base is prevented
from
switching
at the end of the flyback
and clamped
by diode
072.
The bistable
is switched
over by
pressing
RESET,
this applies a negative-going
pulse to TR73
base and causes the collector
current
to switch off and TR74
to conduct.
The circuit
is then ready for the next incoming
trigger pulse to fire the sweep.
3.6
HORIZONTAL
AMPLIFIER
(X-AMPLIFIER)
The
horizontal
amplifier
reference
Figure
6, consists
of a
pre-amplifier
TR76,
followed
by a cascode
connected
long·
tailed pair output
stage, TRn,
TR78, TR79 and TR81.
3.6.1
The
pre'amplifier
TR76
is a shunt
feedback
stage
in
which
the
sweep
and
shift
voltages
are
mixed
via Rl03,
R121 on its base. In the EXT X position;TRl
is connected
in
place of a sweep signal, which converts
the high impedance
external
input
into a low impedance
suitable for mixing with
the shift voltage at TR76
base. The TR76 collector
output
is
fed to the base of TR78 via 076.
TR78 and TR79 form the
bottom
half of a cascode
amplifier,
their
collectors
driving
the emitters
of TRn
and TR81;
tail current
being supplied
via TR82.
3.6.2
Gain control
is provided
in the Xl condition
by R132,
in the
X5 position
by R131.
The output
from
TRn
and
TR81
collectors
driving
the CRT
"X"
plates.
3.6.3
In the
sweep
and
EXT X positions
076
and
081
are
conducting;
077
and
082
are not
conducting,
the
signal
being
fed
to
the
base
of
TR78
with
075
and
079
not
conducting.
074
and
078
are
conducting
shorting
the
collectors
of the X-V pre'amplifier
TR751 and TR752.
3.6.4
In the X-V mode
074 and 078 are not conducting,
075
and 079
are conducting
allowing the push pull output
from
CH2 to be fed to the bases of TR78 and TR79. Also 076 and
081
are
not
conducting;
077
and
082
are
conducting
shorting
out the signal on TR76 collector.
The amplifiers
for unblanking
comprise
TR65, TR67 and for
chopped
blanking
TR62 and TR64, reference
Figure 6.
3.7.1
In the absence of a sweep TR66 conducts,
causing current
to flow through
TR65 making the TR65 collector,
the TR67
emitter
and
the
CRT
Mod
Plate
electrode
negative
with
respect to the CRT A 1 electrode
so blanking
the trace.
3.7.2
When the sweep starts TR66 switches
off; TR65
current
ceases;
hence its collector
goes to h.t. causing TR67 emitter
and CRT Mod Plate to follow. The potentials
of A1 and Mod
Plate electrodes
are equalized
so unblanking
the trace.
3.7.3
Chopped
blanking
pulses
are
fed
from
the
vertical
amplifier
via C642 to the cascode
circuit
TR62
and TR64,
which
amplifies
the
pulse.
The
collector
of
TR64
falls;
allowing
064
to conduct
and pass blanking
pulses, via TR67
to the CRT Mod Plate electrode
to blank the trace.
The
CRT
is of the direct
viewing bistable
storage type and
contains
special
storage
electrodes
additional
to
a
conventional
CRT
To make
the circuitry
comprehensible
two figures are included
in this manual.
Figure 8 shows the
conventional
mode circuit.
Figure 9 the storage.
Summary of Contents for DM64
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