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Model 5065A
Circuit Diagrams, Theory, and Maintenance
AC AMPLIFIER A7 THEORY
The Ac Amplifier is a low noise, high gain amplifier.
This amplifier receives a low-level signal
from A12
RVFR Assembly and provides an ac output proportional
to amplitude and phase of the fundamental frequency
of the input signal. At resonance, the input contains
a large amount of 274 Hz second harmonic and a small
amount of 137 Hz fundamental. The amplified 137 Hz
fundamental is separated from the second harmonic,
amplified, and applied to A8 Phase Detector Assembly.
The 137 Hz output is also routed to A14 Logic Assembly
as one logic input.
The 274 Hz second harmonic is
amplified, rectified, and routed to A14 Assembly as an
other logic input.
These logic inputs are two of the
signals which control the CONTINUOUS OPERATION
lamp on the front panel. The Ac Amplifier also furnishes
PHOTO I and 2ND HARMONIC inputs to the CIRCUIT
CHECK meter.
The input signal, some 137 Hz but mostly 274 Hz at
resonance, couples through J1. Input stage Q1 and Q2
form a low noise differential amplifier which feeds the
push-pull inputs of IC1 that provides high gain.
Note
that the input at IC1(2) is ac-shunted to ground by C4.
Thus the ac input to IC1 is single ended while the dc
input is a balanced differential input.
In addition, the
feedback from IC1 (6) to Q1 base treats ac and dc
separately.
This technique results in a preamplifier
circuit that constitutes a transfer impedance; i.e., input
is current and output is voltage.
This impedance
amounts to 2 x 10
7
ohms for ac and 10
5
ohms for dc.
Thus for a 1ju a input signal, the output voltages is 10~
6
x
2 x 10
-7
= 20 volts. R3 adjusts the input to zero volts dc.
Zeners CR1 and CR2 provide +14.7 volts for IC1. Feed
back to Q1 base restricts frequency response to approxi
mately 6 kHz.
There are three IC1 outputs: (1) the twin-T filter con
necting to Q5 and Q6, (2) the 2nd harmonic adjustment
R19 and, (3) TP2 and the connection through A17(6) to
the PHOTO I position of the CIRCUIT CHECK meter.
The twin-T filter circuit works with Q5 and Q6 to give
sharp rejection to the 274 Hz component of the input
signal. The filtered 137 Hz component is amplified in
IC2 and routed to A8 Phase Detector Assembly.
Q8
serves to impedance match the notch filter circuit to
the loop gain control R32. With R32 properly set, the ac
amplifier (Q8, R32, and IC2) gain is about 40 dB.
Zeners Q6 and Q7 provide ±9.1 volts for IC2. Dc feed
back from IC2(6) to IC2(2) sets IC2 gain.
The signal at 2nd harmonic adjustment R19 (which is
mostly 274 Hz at resonance) is amplified, rectified, and
then dc amplified in the 2nd harmonic detector circuit
for a logic output to A14 Logic Assembly. Q3 and Q7 act
as forward amplifiers, with Q4 functioning as a feed
back amplifier.
The RC components in Q7 collector
circuit provide frequency compensation.
Diodes CR4
v _ ^
and CR5 provide a rectified dc proportional to the input.
Emitter follower Q9 feeds the 2nd harmonic output to
_
the A14 Logic Assembly.
A7 MAINTENANCE
NORMAL OPERATION
a.
Preamplifier A7Q1, Q2, and IC1 act as a trans-
—
fer impedance, i.e, this circuit is designed to work from
a current source and deliver a voltage output pro
portional to the current input.
The equivalent transfer
impedance of this preamplifier circuit is 2 x 10
7
ohms for
ac signals and 10
5
for dc signals.
b. The notch filter circuit is set to notch out the
—
second harmonic content of the modulation frequency
(274 Hz).
c. The ac amplifier circuit of Q8 and IC2 has a
"gain of 40 dB adjustable by R32.
d. The second harmonic detector circuit amplifies
—
and detects the preamplifier output, which is mostly
274 Hz at resonance.
The dc output of this circuit
routes to A14 Logic Assembly and to the 2ND HAR-
_ —
MONIC position of the CIRCUIT CHECK switch.
OPERATIONAL CHECK
NOTE
This check need only be performed if trouble
_
is suspected in the A7 Assembly.
a. A quick check of the A7 Assembly can be made
by monitoring the output (yellow lead) with an oscillo-
scope, removing the input cable from A7J1 and, using a
small metal tool, touching the center conductor of A7J1.
The hum signal thus induced will cause a saturated or —
clipped signal to appear on the oscilloscope. This maxi
mum signal output will peg the CIRCUIT CHECK meter
when switched to 2ND HARMONIC.
__
b. A more precise test can be made by use of the
following procedure.
1)
Set up equipment as shown in A7 Test Setup.
Use Micon-to-BNC test cable that is supplied,
for the connection to A7J1.
2)
Set oscillator frequency to 137 Hz and output
^ ~
level to .5 V peak-to-peak.
7-30
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