Tektronix 310A, Instruction Manual, Page: 31 / 132

Circuit Description-Type 31OA
INPUT
SW 305
VOLTS/DIV
/ \
Vertical signal to V 401 ,
Vertical Amplifier
(0.1 and above)
Vertical signal to V 408 ,
Vertical Amplifier
(0 .0 1 , 0 .0 2 , 0 .0 5 )
Fig. 3 -2 . V ertica l Pream plifier detailed block diagram.
V E R T IC A L PREAM PLIFIER
General
Input signals fo r vertical deflection on the CRT can be
connected to the vertical INPUT connector. This circuit
provides control o f input coupling, vertical deflection
factor and DC balance. Fig. 3-2 shows a detailed block
diagram o f the Vertical Preamplifier circuit.
Input Coupling
Input signals connected to the vertical INPUT connector
can be AC coupled or DC coupled to the Input Attenuator.
When Input Coupling Switch SW300 is in the DC position,
the input signal is coupled directly to the Input Attenuator
stage (except when the VO LTS/D IV switch is in the .01,
.02 and .05 positions). In the AC position or when the
VO LTS/DIV switch is set to the .01, .02 or .05 positions,
the input signal is connected through capacitors C315 and
C316. These capacitors prevent the DC component of the
input signal from passing to the Input Attenuator.
Input Attenuator
The effective overall vertical deflection factor o f the
Type 310A is determined by the VO LTS /D IV switch,
SW305. In the V O LT S /D IV switch positions above .05, the
basic deflection factor is 0.1 volt per division of CRT
deflection, and in the .01, .02 and .05 positions the basic
deflection factor is 0.01 volt per division. To increase the
basic deflection factor to the values indicated on the fron t
panel, precision attenuators are switched into the circuit.
These attenuators are switched into the circuit singly or in
pairs to produce the vertical deflection factor indicated on
the fron t panel. These attenuators are frequency compen­
sated voltage dividers. For DC and low-frequency signals,
they are prim arily resistance dividers and the voltage
attenuation is determined by the resistance ratio in the
attenuator. The reactance of the capacitors in the circuit is
so high at low frequencies that their effect is negligible.
However, at higher frequencies, the reactance o f the capaci­
tors decreases and the attenuator becomes prim arily a
capacitance voltage divider. Each attenuator contains an
adjustable series capacitor to provide optim um response for
the high-frequency components o f the signal and an adjust­
able shunt capacitor fo r optim um response fo r the lower-
frequency components. In addition to providing constant
attenuation at all frequencies w ithin the bandwidth o f the
instrument, the Input Attenuator is designed to maintain
the same input RC characteristics (one megohm X 40 pico­
farads) fo r each position o f the V O LT S /D IV switch. The
signal from the Input Attenuator is connected directly to
the Vertical A m p lifie r circuit in the positions o f .1 and
above. In the .01, .02 and .05 positions o f the VO LTS/D IV
switch, the output signal is connected to the Preamplifier
stage.
Preamplifier
In the .01, .02 and .05 positions o f the VO LTS/D IV
switch, the signal from the Input Attenuator stage is con­
nected to the control grid o f V320. The gain o f this stage is
determined by Preamp Gain adjustment R322, which sets
the cathode degeneration o f V320. The signal at the plate
of V320 is connected to the control grid o f V329. V329 is
connected as a cathode follow er stage and the output at its
cathode is connected to the Vertical A m plifier circuit
through C340, L341 and R341. C320, L325, L341 and
R345 are adjustable to provide the correct signal response.
The preamplifier is by-passed at VO LTS/D IV positions o f
.1 and above.
DC Bal adjustment R344 sets the grid level o f V408 (on
Vertical A m plifier schematic). This adjustment is set to
establish the same DC voltage level at the cathode o f V408
as at the cathode o f V401. Then, w ith the cathodes o f
V401 and V408 at the same DC voltage level, there is no
current flo w through the Variable V O LTS /D IV control and
the position o f the trace w ill not shift as this control is
rotated.
®
3-3