HP 435B Service Manual

SE RV IC E SHEE T 2 (Cont'd)
Second Amplifier
A 4U2 and U3
and associated components are operational ampli-
fiers
with voltage gains of a bout 25 each . Gain for A4U2 i s d eter-
mined by
M
R22 and R23 ; f or A4U3 by A4R27 andR28 . Bias
current
is
provided for A4U3 by M R25 .
The tuned a mplifiers upper bandpass l imit (240 Hz) i s set by t he
p
arallel
RC
n etworks of Α 4C11 and R 22, A4C14 and R 27 and
parallel RC
n etwork in the First Amplifier .
Th ird
Amplifier
A 4U4A a nd its
associated components f orm an operational
a
mplifier' stage with variable v oltage gain from 1 .3 to 3 .4 . The
front p anel C AL ADJ gain control is set to compensate f or differ-
ences in sensitivity o f individual power sensors . The gain is
d etermined by M R24, R 21 and t he CALADJ control R1 .
Sync h ronous
Detector
The phase shift of t he220 Hz s ignal t hrough t he tuned a mplifiers
i s approximately zero . B ecause the phase shift i s minimal, error
i ntroduced into the system is also minimal . This ensures that the
d etector o utput i s proportional to the RF power input level .
The Synchronous D etector, like the sampling gate circuit in t he
p ower sensor, i s driven by t he 220 Hz Multivibrator drive signal .
W hen A4Q6 i s biased on, the equivalent sampling gate FET
(which is connected to ground) i s also on . Therefore, α
n
egative
going signal is coupled to the ac amplifiers .
B
ecause
there i
s
no
phase inversion of t he signal t hroughout the ac
amplifiers, the
output of t heThird Amplifier is also the n egative
going p ortion o f
t he distorted s inusoidal waveform . During this
h
alf c ycle
current
flows from g round t hrough A4Q6 and R26 to change C12 a
nd
C13 .
Α p ositive v oltage i s stored on thep ositive terminal o f C13 . W hen
t he 220 Hz drive signal t urns A 4Q6 off a nd Q7 on, the
Third
Amplifier o utput i s the p ositive going
p
ortion
of
t he
distorted
s inusoidal waveform . This p ositive
going signal
is
superimposed
on the v oltage across C12 a nd C13 s
uch
t hat
the
p
eak voltage i s
about twice the p eak voltage of t he
Third Amplifier o utput . This
v oltage charges MC 16 through R26
and Q7 . The do output
vol-
tage i s coupled across α
d
o
pass filter to the D C Amplifier .
T R O UBLE S H OOTI NG
General
B efore attempting to troubleshoot the
circuits r epresented by t his
schematic, verify that the p
ower s upply is operating properly .
The v oltage on ΤΡ 9
should be +12 Vdc ; on ΤΡ 10, -12 Vdc .
The important
characteristics
of
t he waveforms shown on this
schematic are the frequency
a
nd peak-to-peak voltage . If t he
shape of t he waveform varies slightly, the performance of t he

HP 435B Specifications

General

Brand	HP
Model	435B
Category	Measuring Instruments
Language	English