Theory of Operation—2213 Service
Step Balance potentiometer R138 (at pin 1 of R139) is
adjusted to compensate for input offsets reaching U120
pins 2 and 3 when switching between VOLTS/DIV switch
positions.
In the fast path, the input signal is ac-coupled to input
FET Q122 through C121. The input FET is arranged in a
source-follower configuration used to drive complementary
emitter followers Q133 and Q134. The combination of
Q125, R126, R131, R132, VR130, and R130 sets a
constant current through R125 in the source lead of Q122.
The voltage drop across R125 biases Q133 and Q134 for
about a 10-mA idle current.
A bootstrap circuit composed of Q139, VR122, and
R122 connects the Q122 drain to the Q122 source. This
circuit forces the bias voltage across Q122 to remain con
stant, which in conjunction with the constant bias current
supplied by R125, keeps Q122 operating at a constant
power level to prevent distortion due to changing signal
currents.
Complementary emitter followers Q133 and Q134
supply drive current to the -M, -f2.5, and -f5 voltage dividers
and provide impedance matching between input FET Q122
and the divider network. The bias levels of Q133 and Q134
are stabilized by emitter resistors R139A and R139E
respectively. Average voltage changes occurring in the out
put of Q133 and Q134 are sensed through R139B and
R139D which are connected to the point of lowest
impedance (the emitters of Q133 and Q134). Resistor
R139C provides a path that completes the feedback loop
to the slow-path portion of the Buffer Amplifier.
Volts/Div Var Circuit and X1/X10 Amplifier
The appropriate voltage divider signal output (-5-1, -^2.5,
or f-5) is selected by VOLTS/DIV switch S105B and routed
to the Volts/Div Var circuit composed of C141, R141,
and R143. Changes that occur in the Buffer Amplifier
output impedance due to setting R141 or switching the
divider output are sensed via R139M. These changes modify
the slow-path feedback signal to cause U120 to again match
the gain of both paths.
From the Volts/Div Var circuit, the signal is applied to
the input of the X1/X10 Switchable-gain Amplifier U145.
Amplifier U145 produces a differential output signal from
the single-ended input signal. The gain of the amplifier is
controlled by the setting of VOLTS/DIV switch S105.
Amplifier gain is changed by switching between two
pairs of transistor amplifiers contained in U145. Gain of the
X10 amplifier pair is adjusted by R145 to obtain the
correct deflection factor for the 2m, 5m, and 10m VOLTS/
DIV switch positions. Resistors R146, R147, and R148
act to balance any dc offsets between the XI and X10
amplifiers. Trace shift occurring when the VOLTS/DIV
Variable control is rotated is minimized by resistor R142
which stabilizes the input bias current to U145.
VERTICAL PREAMPS
The Channel 1 and Channel 2 Preamp circuitry, shown
in Diagram 2, includes the vertical preamplifiers, the
internal trigger pickoff amplifiers, and a common-base
output stage for each channel. Vertical positioning of the
channel display is incorporated in the common-base
amplifier stage.
Channel 1 Vertical Preamplifier
The Channel 1 Vertical Preamplifier produces differ
ential output signals to drive the Vertical Output Amplifier
and internal trigger signals to drive the Trigger circuitry.
Differential signal current from the Attenuator circuitry
is applied to common-base transistors Q157 and Q167
through cable-terminating resistors R151 and R161
respectively. The collector currents of Q157 and Q167 will
flow through R158 and R168 to produce level-shifted
signals which drive U170D and U170E. Balance potenti
ometer R154 is adjusted to balance the dc level of the
Channel 1 output with the Channel 2 output by setting the
bias levels of Q157 and Q167. Channel 1 frequency
response is matched to Channel 2 response by adjusting
Cl 67.
Transistors U170D and U170E form a common-emitter
amplifier. The gain of U170D and U170E is set by R180
(connected between the emitters), and the high-frequency
response is compensated by C l80. The emitters are also
connected to the bases of U170C and U170B respectively
to provide an internal trigger signal pickoff point. Vertical
signal output current flows from the collectors of U170D
and U170E to the emitters of common-base amplifiers
Q177 and Q187. A shunt resistor gain network (R176 and
R186) sets the gain of the common-base stage. Channel 1
POSITION control R190 supplies a variable offset current
to the emitters of Q177 and Q187 which allows the trace
to be vertically positioned on the crt. The common-base
amplifier stage converts the differential signal input current
to a differential signal output voltage that is applied to the
Diode Gate circuitry (Diagram 3).
Channel 2 Vertical Preamplifier
The Channel 2 Vertical Preamplifier functions the same
as the Channel 1 Vertical Preamplifier previously described.
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