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F.E.T. Miller circuit, then runs up linearly charging up the hold
off capacitor via R110 and D106 and resetting the bistable via
R112.
When TR102
switches off, TR101 conducts and
discharges the timing capacitor until D103 conducts and reduces
the current in TR101 to the value required by the timing
resistor. At this point the flyback stops. During the flyback the
hold-off capacitor discharges through R112 until 0108 conducts,
the circuit then "clamps in 3 quiescent state and remains so
until the arrival of the next triggering pulse.
If the Trig level control is in the Auto position and no trigger
pulses are
present,
TR105
and
D109
conduct
and
reduce
the
potential
at
the
anode
of
D108.
This
allows
the
hold-off
capacitor to discharge further and re-trigger the bistable. The
timebase then free-runs. If trigger pulses are applied at the anode
of D113 to TR106 base, TR106 then conducts during the period
of the pulse, switching
off TR105,
via C106. TR105
collector
potential then rises, back biasing D109 thus inhibiting the free
run timebase sweep which is then dependant upon the trigger
pulses.
The collector current of TR104 which is cut off during the
sweep, is fed to the input of TR107. The collector of TR107
goes negative at the beginning of the sweep driving TR108 on,
bringing the unblanking plate positive, and unblanking the trace.
If the vertical channels are in the chopped mode the chop
transitions are blanked via TR109 which pulls the unblanking
plate negative in response to the chop blanking pulses fed to its.
base via C111. At the end of sweep, TR104 turns on, turning
off TR107, TR108, and turning on TR109, returning the trace
to the blanked condition.
Negative pulses from TR104 collector are fed to CHOP/ALT
bistable to switch channels in the ALT mode at the end of each
trace.
3.6
HORIZONTAL AMPLIFIER
figure 5
TR111,
TR112,
TR113
and
TR114
form
a balanced
symmetrical amplifier system to drive the CRT X plates.
Shunt
feedback
amplifier TR111
accepts sweep generator
currents via R138 and a current from the horizontal position
potentiometer R137 via R139. An output voltage proportional
to the input currents is then applied to TR112 base. TR113 is
longtailed with TR112 to form the output stage with X1 and
X5 gain adjustment located between the emitters. Balance for
TR113 is obtained from TR114, a similar stage to TR111, but
with an effectively grounded input.
3.7
CATHODE RAY TUBE
figure 6
301 is a flat face, mesh cathode ray tube. The astigmatism
electrode is fed from a low impedance supply, TR302, avoiding
the necessity
for re-adjustment
of the astigmatism with change
of brilliance.
Cathode-grid potential is stabilised by zener diode D303 and
pre-set intensity control R309 takes up spread in tube character-
istics. The geometry, mesh and focus electrodes are fed from a
resistive divider which, with R313, provide bias currents for
D303. A coil L301 is provided to take up variations in trace
alignment with the graticule. It is fitted to the CRT neck, and
supplied with a stabilised current, from TR301. The rear panel
rotation control is in the base circuit of TR301. C303 and C304
provide inter plate decoupling to avoid unwanted Z modulation
effects.
3.8
CALIBRATOR
figure 6
Transistors. TR315 and TR316 form a bistable switch,
compensated against ambient temperature variations by D316
and
D317,
and
supply
stabilised by
zener diode D315.
Switch
3/2
over is controlled by the sweep voltage applied via R315, to
TR315. In the absence of sweep voltage, TR315 conducts and
TR316 is switched off and DC voltage at SK315 is therefore zero.
An increasing voltage applied to TR315 base reaches a level equal
to TR316 base voltage, switch over then occurs, TR315 switches
off and TR316 switches on. Collector current of TR316
determined by R316 and R318 then flows through precision
resistor R323, providing
300 mV DC output level to SK315.
Switch over of TR315 and TR316 is arranged to occur at
approximately half sweep, so that the CAL signal when applied
to the Y amplifier, provides a zero volt datum level and a
calibrated +300 mV level during the period of the sweep. R316
enables accurate setting of CAL voltage.
3.9
BATTERY CHARGER
figure 7
Mains transformer T401 supplies current via fullwave rectifier
D401
— D404, to TR401
and TR402
which provide a constant
current charge to the batteries with the instrument switched off,
or a trickle charge with the instrument on. Line voltage switching
100-125
V AC
or 200-250
V AC
is set by S401
located under
the instrument
via a small access hole, and connected
to T401
primary. Range switching selecting the lower or upper 10% of
each line voltage is set by S402 located at the rear of the
instrument and is connected to T401
secondary. C402 smooths
the rectified output. 0405 and D406 stabilise TR401 base
emitter voltage, bias current being provided by R401. TR401
with
R402
in
its emitter
provides
a constant
current
to
the
negative rail via D407 the "ON"
panel
LED.
and part of R404.
Forward
bias
voltage
developed
between
R404
wiper and
the
negative rail causes TR402
to conduct and draw current from the
positive rail via D408 and hence form the charge current for the
battery pack. R404 therefore controls the charge current rate.
$403 and S404 form the 'ON' switch and are ganged together.
Current demand in the 'ON' condition is over twice that in the
'OFF' condition, increased current is obtained from TR402 by
switching in an additional parallel resistor R403, TR401 current
is therefore increased, thereby increasing TR402 current. Battery
charge rate in the 'OFF' condition is 400 mA and varies between
zero and 100 mA in the 'ON' condition dependent upon actual
line input voltage. D408 prevents battery discharge when
instrument is stored.
3.10 CONVERTER
POWER
SUPPLY
UNIT
figure 7
All supply rails, with the exception of the battery charger,
are derived from a push-pull, C.R. timed, DC to DC converter.
TR411 and TR412 with protection diodes D411 and D412 drive
ferrite cored transformer T411 at approximately 14 kHz, feed-
back being obtained from a single winding in series with C412
the timing capacitance. R411 and R412 are forward bias resistors
decoupled by L411 and C411. C413 decouples supply lines from
voltage spikes generated by switching action. +13 and —13 volt
supplies are obtained from the two 14.5 volt windings on 7411,
rectified by D413, 0414, D415 and D416 and smoothing is
provided by R418, C418, L412 and C423 for +13 V and C421,
L413 and C424 for -13 V.
An additional 23 volts winding added serially to a 14.5 V
winding, rectified by D421 and smoothed by C422, L414 and
C425 supplies +36 V. Additionally +80 V is obtained from the
same windings by a voltage doubler C417, D418, D419 and
C419.
CRT negative supplies for the gun are half wave rectified, from
523 volt winding,
by
D417
and
smoothed
by C415 and C416
in series.
PDA
potential of 2.4 kV is obtained from the gun supply via a
conventional quadrupler, comprising C431, C432, C433, C434,
D431, D432, D433 and D434,
3/77
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