Triggered Holdoff Stage; Bright Line Auto; External Horizontal Amplifier - Tektronix 7B70 Time Base Instruction Manual

Circuit Description-7B7O (SN B205000 and Up)
VR358, the sweep gate tunnel diode. VR358 switches to i t s
high-voltage state applying a positive signal to the base of
0360 (part of the bistable sweep gate multivibrator). The
collector voltage of 0360 decreases, producing the negative-
going step used to enable the Sweep Generator. When 0360
turns on, emitter-coupled 0368 is turned off to produce
the positive edge of the sweep gate output. The sweep gate
multivibrator remains in this stage, 0360 on and 0368 off,
until i t is reset by the sweep reset multivibrator (part of the
Sweep Generator) a t the end of the sweep.
At the end of the sweep, the negative-going lockout gate
from the sweep reset multivibrator i s applied to the base of
0326, turning i t off. Emitter-coupled 0328 is turned on
taking current from the sweep gate tunnel diode, VR358,
reverting
it
to i t s low-voltage state. This switches the sweep
gate rn~~ltivibrator back to i t s quiescent state (0360 off and
0368 on). The circuit remains in this condition (VR358 in
its low-voltage state) until the lockout gate returns to i t s
high-level at the end of holdoff time.
Triggered Holdoff Stage
The Triggered Holdoff Stage re-arms the sweep gate TD,
VR358, at the end of holdoff time. During holdoff time
VR322 is in i t s high-voltage state, biasing 0326 off. At the
end of holdoff time the lockout gate from the sweep reset
multivibrator returns to i t s high level. This switches VR322
t o i t s low-voltage state, turning 0326 on. 0328, which is
emitter-coupled to 0326, i s turned off to re-arm the sweep
gate TD, VR358.
to its high-voltage state, automatically retriggering the
sweep each time the lockout gate goes to i t s high level. The
result is that the sweep generator circuit is automatically
retriggered at the end of each holdoff period and a free-
running trace is produced. Since the trace free-runs at the
sweep rate of the sweep generator circuit (as selected by the
TIMEIDIV switch), a bright reference trace is produced at
all sweep rates. The sweep TRIGGERED lamp will not be
lit, indicating an untriggered condition.
The sweep TRIGGERED lamp is lit each time an ade-
quate trigger signal i s applied to the Pulse Generator. When
the trigger signal switches VR304 to i t s high-voltage state,
0309 is turned off. This turns on 0331, Q338 and 0339.
The top of C339 becomes less positive, turning off CR342.
Q346
i s
turned on and its collector goes positive, turning on
0348. 0348 collector current lights the sweep TRIG-
GE RED lamp. When 0346 turns on i t s emitter voltage de-
creases, and emitter-coupled 0350 is turned off to discon-
nect the bright line auto circuit from the sweep gate stage.
When the TRIGGERING MODE switch is in the NORM
or SINGLE SWEEP position, the anode of CR355 i s
clamped to ground. This reverse-biases CR355 and discon-
nects the bright line auto circuit from the sweep gate stage
but allows the TRIG'D light to function.
EXTERNAL HORIZONTAL AMPLIFIER
When the DISPLAY MODE switch is set to AMPLI-
FIER, the + I 5 volts isdisconnected from the base of 0370
turning off the sweep gate multivibrator current source.
A t high frequencies, double triggering may occur due t o
This disables the Sweep Generator circuit. The Trigger
the finite risetime of the lockout gate a s
it
returns to i t s
Amplifier is disabled by removing the +15 volts from the
high-level. To reduce the incidence of double triggering of
junction of
R148-R149.
indicator oscilloscope knob
the circuit, the trigger pulse from 0317 i s differentiated by
readout for
the 7B70 is also disabled by applying +15 volts
C324-R324 and
On
the lockout gate
at the
through R197 to the column current output, interface con-
cathode of VR322. Now the sweep is re-armed by one trig-
nector
A37.
ger pulse and triggered by the following trigger pulse, main-
. . -.
taining the proper time relationship between input signal
and the start of the sweep.
Bright Line Auto
The operation of the sweep gate stage in the P-P AUTO
and AUTO MODE i s the same as for NORM MODE just
described when an adequate trigger signal is present. How-
ever, when a trigger signal i s not present, a free-running
trace i s produced in the P-P AUTO and AUTO MODES.
With no trigger signal applied t o the circuit, Q331, 0338
and 0339 are off. This allows C339 to charge toward the
positive 15 volt supply. As the top of C339 becomes more
positive, CR342 is turned on making the base of 0346
move positive to cutoff. Emitter-coupled 0350 is turned on
and the additional current through VR358 switches the TD
The signal, external or internal as selected by the
SOURCE switch, i s applied push-pull to operational ampli-
fier U192. A DC level adjustment (R186). located in the
input to U192, provides a means of adjusting the amplifier
for minimum horizontal movement of the spot as the
VARIABLE control is rotated. U192 converts the push-pull
input to a single-ended o ~ ~ t p u t for use in the Horizontal
Preamp.
The output of the External Horizontal Amplifier is
passed through the VARIABLE control (R194). This con-
trol varies the output of the amplifier by a factor of
LIP
to
10. With the VARIABLE control fully clockwise the deflec-
tion factor is about 25 millivolts per division, and with the
VARIABLE control fully counterclockwise the deflection
factor i s about 250 millivolts per division.
Scans by ArtekMedia O 2006