HP 435B - P Ower S Upply

S ERV IC E S HEE T 3 (Cont'd)
so until it i s the same l evel but opposite polarity as the p ower
sensing d evice output . The input to the P ower Meter circuits goes
to z ero which means the D C Amplifier o utput i s also z ero . W hen
t he ZERO switch is r eleased, r elay Α 4 Α 1 Κ 1 opens and t he final
Servo Amplifier e rror voltage i s stored on A4A1C1 at the high
impedance input to A 4A1Q1 . The correction voltage across the
p ower sensing d evice r emains constant as l ong as the error vol-
tage r emains on C1 .
Diodes A 4CR4and A 4A1CR1 r educe v oltage spikes caused by
s witching the r elays . A 4R69 also
r educes switching transients in t he f eedback path .
The v oltage which a ppears at the source of A4A1Q1 i s coupled to
A 4U6 pin 2 through A4R44, C20andC19 . This v oltage tends to
k eep t he Servo
Amplifier
o utput constant
when t he
ZERO
switch
is first pressed
. I
t d ampens the
violent change which t ries to occur
because of t he high gain o f t he Servo
Amplifier . The
initial
change thus occurs slowly .
A 4A1R1 establishes an RC time constant (1s)
with
A4A1C1
which averages out the thermal noise
during
the
z
eroing
operation .
The special
construction o f t
he
Α 4 Α
1
assembly
a
nd t he high
gate
impedance of A4A1Q1 r educe l eakage
from
A4A1C1 and t hus
increases the correction voltage storage time
.
A 4A1R2, R 3, R 4, C2,
C3
and
C4
are p art of α frequency response
n etwork which keeps the auto z ero f eedback
loop
from oscillating
during the z eroing sequence .
A 4R46, R
45
and A4A1R4 f orm
α
v oltage
divider
stick . The
Bal-
ance
control A4R46 r emoves
the d o
offset introduced by t he Auto
Z ero
circuit
thus centering i ts effective r ange at
0
Vdc .
T R
O
UBLE SH OOTI N
G
General
B efore attempting to troubleshoot these circuits, v erify t hat the
p ower s upply is operating properly . The v oltage on ΤΡ 9 should
be
+12 Vdc ; on ΤΡ 10, -12 Vdc .
If t he d o offset controls A 4R32, R 42 or R46 are incorrectly
a d j usted, the Auto Z ero circuits m ay not r espond properly . R efer
to the ad j ustment procedures in Section V .
N oise problems m ay be due to d efective components in t he L ow
Pass Filter (especially t he three m ost sensitive r anges) or t he
Servo Amplifier w hich is an active l ow pass filter . Α n oise prob-
lem in t he Servo Amplifier will be evident only during the z eroing
sequence .