HP 435B - Page 98

Se rvi ce
S ERV IC E
S
HEE T
4 (Cont'd)
Wh e n t h e lin e vo l tage i s re m ov e d , CR 7 i s f
o rw a rd

.-
bi ase
d
by t h e curre n t flo win g to t h e P owe r
M ete r
b eg in s to t urn o ff . Th
e co ll ecto r vo l tage b egin s to

c ir
c ui ts fr o m t h e b atte ry . CR 6 i s t urn e d o ff andno
go p os i t iv ea nd t h e c h a n ge i
s tr a n s mi tte d t hr ou gh

curr
e
n
t flo w s t hrou gh t h e c h ar g in g cir c ui t .
R 51 a nd VR 5 to Q18 . A s
Q18 b eg in s to t urn off, i ts
co ll ecto r goes m o r e n egat iv
e . Α n egat iv e go ing

Current
L ί m ί ter

.
t
r
a n s i e n t i scouple d t hro ugh
R 55 to t h e b aseo f Q 14
whi c h sp ee d s up t h e t urn -o
ff t im e . Th e p os i t iv e

If t h e curr
e
n
t
flo
w t hr ou gh t h e24 V r eg ul ato r w e r
e
s upply v o l tage i s r e m o v
e
d
fr o m t h e co ll ecto r of

to s udd
e
nly in
c
r
ease to a ppr o xim ate ly 90 mA,Q15
Q18 a nd al so t h e DC
Amplifi e r . A s t h e b atte ry

w o uld
t
urn
ona nd dr aw t h e d r iv e curre n t a
w ay
v o l tage i s fur t h e r r e du
ce d, t h e se ri es r eg ul ate d

fr o m
Q16 . Co n se qu e n t ly , t h e c u r r e n t flo w
to Q13
ou t pu t b eg in s to d ec r
ease f aste r t han t h e b atte ry

w o uld
di sa ppea r and t h e r eg ul ato r o u t pu t
w o
uld
v o l tage and, e v e n t u a lly,
t
h
e 3 vo l t t hr es h o ld vo l t-

b e r e
du ce d .
age i s e x ceed e d . Q14 i
s t h e n bi ase d on, bu t,
because t h eb atte ry v o l
tage
i
s less than 20 Vd c,t h e

T
R
O
UBLE
S H OOTI N G
kn
ee v o l tage o f VR 5 ca nn ot
b e
r
eac h e d . Th er e f o r e,
VR 5 d oes n ot co ndu ct and Q18 r e m
a
in
s bi ased off .

Set t h e LINE swi
tc
h
to OFF and r e m o v e Α 4 Ρ 1(red
wir e) fr o m A 4 J 1 a nd Α 4 Ρ
2 (blu e wir e) fr o m A 4 J 2 .
Thi s di sco nn ects t h e l oa d fro
m t h e po w e r supply .
If t h e s upply vo l tages
a
r
e now cor rect, t h e m a l-
If α b atte ry
h as b ee n pl ace d in t h e P owe r
M ete r as

fun ct i o n i s n ot in t h e p owe r
supply .
α seco nd
a
ry p
o w e r so ur ce, i t i s a l
wa y s b e in g
c h a r ge d wh
e n e ve r t h e lin e v o l tage i
s coupl e d to

If, afte r r e m oving t h
e l oad , t h e ou t pu t vo l tages
t h e in
st rum e n t a nd t h e LINE swi
tc
h i
s ON . Wi t h

m eas ur e d are l ess t h
an n o rm a l bu t of equ a l and
ac lin
e (Mains) p o w e r supplyin g
e
n
e r gy VR 3 i s

o pp os i te po l a ri ty, t h
e m a lfun ct i o n is probably in
t urn e d
on, w hi c h bi ases Q12 f o r
α ch a r g ing c ur-

t h e se ri es r eg ul ator cir
c ui ts .
r e n t of appr o xim
ate ly 90 mA . Thi scurr
e
n
t i s supp-
lied t hr ou gh
CR 6 to t h e b atte ry
ΒΤ 1 . C R 7 i s

V o l tages s h o wn in p a r e n
t
h
es i s a r e f o r b atte ry
r e ve r se bi ase d
whil e t h e b atte ry i s b e in g
c ha r ged .

o pe r at i o n only .
B
attery C h arger
M o d e l 435 Β