HP 435B Service Manual

S ERV IC ES HEET 4 (Cont'd)
R egulating action of t he 24V s upply is started by C R9, R 58
and
R60 . When t he LINE s witch is set to ON, c
urrent b egins
to
flow
t hrough R60 and VR4 . A s the voltage
increases across
VR4,
current begins to
flow t hrough Q11 which biases Q13 and Q16
on .
The r egulator output b
egins to increase in α n egative direction .
The output voltagebiases CR9
which, in t urn, causes the v oltage
across VR4 to
increase . The r esulting r apid increasein voltage on
the b ase of Q 11
k
eeps
i
t
ahead of t hat on theb ase of Q10 . When
t
he
Q11 b ase voltage stabilizes at -12 Vdc,
the l ower voltage
on
Q10
k eeps the output
l
evel increasing
until
it
approaches -24 Vdc . A t
this p oint the base
v
oltages of Q10
andQ11 b ecome equal, the
differential
a
mplifier's error o utput goes to z ero, and t he output i s
stabilized at -24V .
C25andR61 f orm α l ow pass filter which reduces the high gain of
t he circuit at high frequencies thus preventing unwanted oscilla-
tions . R 59 andC24 f orm α n oise filter for t he z ener diode .
The input voltageto the 24V regulator may be as high as 70 Vdc
from t he line voltage or as l ow as 26 Vdc f orm t he b attery .
12 V
S
h unt R egulator
U 7 i s connected as α voltage f ollower c ircuit . Chassis ground is
coupled to theinverting input of U7 and t he non-inverting input i s
coupled across h alf t he 24V series regulator o utput byα voltage
divider R63andR64 . If t he v oltage input to pin 3 tries to shift
toward+12 or -12 Vdc, theoutput from U7 w ould bring the vol-
tage at U 7 pin 3 b ack to ground potential .
B attery Test
NOTE
Th e batte ry test c ir c ui t i s in ope r at i o n a ny t im e t he
LINE sw i tc h i s set to ON ; h owe v e
r , t
h e only
t
im e
t h e m ete r indi cat i o n i s m ea nin gf ul
i
s
w h e n t h
e
batte ry i s s up p lyin g powe
r .
W
hen
t
he battery
is
supplying p ower for t he P ower Meter c ircuits,
and t he b attery is d efective or
discharged, the b attery test circuit
r
emoves the p ositive (+12 Vdc)
supply voltage from t he D
C
Amplifier
(A4U5) .
This causes α full downscale m eter indication .
The test circuit
m
easures αp ercentage
of t he voltage difference
b etween t he -12V o
utput and t he n egative b attery terminal . As
this v
oltage
difference d ecreases to approximately 3 Vdc, Q14
Switc h Assembly and DC Ampl/Auto Zero
Ρ / ΟΑ 1 and Ρ / Ο Α 4
S ERV IC ES HEE T

HP 435B Specifications

General

Brand	HP
Model	435B
Category	Measuring Instruments
Language	English