C ircuit Description—434
Circuit is changed to decrease the deflection factor (see
Preamp C ircuit discussion).
For the CH 1 VO LTS /D IV switch positions above 10
mV, the attenuators are switched into the circuit singly or
in pairs to produce the vertical deflection factor indicated
by th e V O L T S /D IV switch. These attenuators are
frequency-compensated voltage dividers. In addition to
providing constant attenuation at all frequencies w ithin the
bandwidth of the instrument, the Input Attenuators are
designed to maintain the same input RC characteristics (one
megohm X 24 picofarads) fo r each setting of the CH 1
VO LTS/D IV switch. Each attenuator contains an adjustable
series capacitor to provide correct attenuation at high-
frequencies and an adjustable shunt capacitor to provide
correct input capacitance.
NOTE
Each attenuator is a hybrid encapsulated assembly;
therefore, replacement o f individual components is
no t possible. Should defects occur, the attenuator
must be replaced as a unit.
Scale-Factor Switching Circuit
The vertical deflection factor of Channel 1 is indicated
by back-lighting the appropriate figures imprinted on the
flange of the CH 1 VO LTS/D IV knob. When a X I probe is
connected to the CH 1 input connector, the base of Q200 is
connected to +15 volts through R200. Q200 is biased into
saturation and conducts current through indicator DS20.
DS20 indicates the correct deflection factors fo r X I probes.
When Q200 conducts, the voltage level at its collector is
very close to zero volts; therefore, there is insufficient bias
at the base of Q202 to cause it to conduct, and X I 0 indi
cator DS22 remains dark.
When a X I0 probe w ith a scale factor switching
connector is attached to the CH 1 input connector, the base
of Q200 is grounded. Q200 does not conduct, and its
collector rises to approximately +15 volts. X I indicator
DS20 is dark and Q202 is forward biased into conduction.
Q202 conducts current through X10 indicator DS22, which
now indicates the correct vertical deflection factor, includ
ing the X I0 attenuation ratio of the attached probe.
Input Stage
The Channel 1 signal from the Input Attenuator is
connected to the Input Stage through C121, R121, and
R122. R120 provides the input resistance for this stage.
R121 limits the current drive to the gate of Q124A. Dual
diode CR122 protects the circuit by clamping the gate of
Q124A at about —15.5 or +15.5 volts if a high-amplitude
signal is applied to the CH 1 connector. FET Q124B is a
relatively constant current source fo r Q124A. and also
provides temperature compensation of Q124A. R125
isolates the input of the Preamp Stage from the source of
Q124A.
Preamp Stage
The Preamp Stage U210 is a multiple-stage integrated
circuit am plifier. Adjusting the gain of this stage sets the
overall gain fo r Channel 1. Basic gain of the amplifier is set
by adjusting R244. VAR control R245 permits continu
ously variable uncalibrated deflection factors between the
calibrated settings of the V O LTS /D IV switch. R212 adjusts
for no base line shift of the CRT display when rotating the
VAR control.
In the 1, 2, 5, and 10 mV positions of the V O LTS/D IV
switch no attenuation is used in the Input A ttenuator stage.
The correct vertical deflection factors are obtained by
changing the gain of the Channel 1 Preamp Stage. This is
accomplished by changing the value of the gain-setting resis
tance connected between pins 7 and 8 of U210. The STEP
ATTEN BAL screwdriver adjustment R22 located on the
instrument fro n t panel adjusts fo r no baseline shift of a
CRT display when switching between the 1, 2, 5, and 10
mV positions of the CH 1 V O LTS/D IV switch.
Variable capacitors C217, C220, C223, C225, and C226
are compensation adjustments to provide optimum high-
frequency response through the channel amplifier. A
sample of the signal being amplified in Channel 1 is taken
from pin 19 of U210 and connected to the Trigger Pickoff
and Generator circuit to allow Channel 1 only triggering
operation. Q255 and Q265 in the O utput Am plifier stage
are connected as common-base amplifiers to provide low
input impedance load for the Preamp Stage. They also
provide isolation between the Preamp Stage and the
Channel Switch circuitry.
CHANNEL 2 INPUT AMP AND PREAMP
General
The Channel 2 Input Amp and Preamp circuits are
basically the same as the Channel 1 Input Am p and Preamp
circuits. Only the differences between the tw o circuits are
described here. Portions of this circuit not described in the
following description operate in the same manner as for the
Channel 1 Input Am p and Preamp. Fig. 3-3 shows a
detailed block diagram of the Channel 2 Input Amp and
Preamp. A schematic of these circuits is shown on diagram
2 at the rear of this manual.
Preamp Stage
Basically the Channel 2 Preamp Stage operates as
described fo r Channel 1. However, the Channel 2 Preamp
3-5
To Next Page
Loading...
Need help?
General
