HP Lab Series - Regulating Loop Troubles

•
•
•
~-+--rt-AC
ACC
~-~~GNO
I
I
I
~
~
:
I
EG
R
ls
I
L--E}-,,J.._
~
- - -
--
- -
--
_J
A.
INCORRECT
METHOD
-
GROUNO
CURRENT
ls
PRODUCES
60
CYCLE
DROP
IN
NEGATIVE LEAD
WHICH ADOS
TO
THE
POllllER
SUPPLY RIPPLE DISPLAYED
ON
SCOPE.
~-.._.!TWISTED
PA~-----'
ii
LENGTH
OF
LEAD
BEl"llrEEN
Rs
ANO
OUTPUT
TERMINAL
OF
POWER
SUPPLY
MUST
8E
HELD
TO
A850<.UTE
MINIMUM.
B.
A
CORRECT
METHOD
USH•G
A
SINGLE
-ENDED
SCOPE.
OUTPUT FLOATED
TO
-SllEAK
GROUND
CURRENT
LOOP, TWISTED
PA
IR
REDUCES
STRAY
PICKUP or.
SCOPE
LEADS.
*
*
LENGTH
OF
LEAD BETWEEN
Rs
AND
GROU".COED
OUTPUT'
TERMINAL
OF
POWER' SUPPLY MUST
BE
HELD
TO
AllSOLUTE
MINIMUM.
C.
A
CORRECT
METHOD
USING
A
DIFFERENTIAL
SCOPE
WITH
FLOATING
INPUT.
GROUND.
CURRENT
PATH IS 8ROICEN;
COMMON
MODE
REJECTION
OF
DIFFERENTIAL INPUT
SCOPE
IGNORES
DIFFEREhCE IN
GROUND
POTENTIAL
OF
POWER
SUPPLY B
SCOPE,
SHIELDED TWO-
WIRE
FURTHER
REDUCES
STRAY
PICKUP
ON
SCOPE
LEAD.
Figure
5-11.
CC
Ripple
and
Noise,
Test
Setup
d,
The
ripple
and
noise
indication
should
be
less
than
l.SmVrms
for
Model
6207B
and
3mVrms
for
Model
6209B.
5-51
TROUBLESHOOTING
5-52
Components
within
Hewlett-Packard
power
supplies
are
conservatively
operated
to
provide
maximum
reliability.
In
spite
of
this,
parts
within
a
supply
may
fail.
Usually
the
instrument
must
be
immediately
repaired
with
a
minimum
of"
down
time"
and
a
systematic
approach
as
outlined
in
succeed-
ing
paragraphs
can
greatly
simplify
and
speed
up
the
repair,
5-53
Before
attempting
to
troubleshoot
this
instru-
ment,
ensure
that
the
fault
is
with
the
instrument
5-9
and
not
with
an
associated
circuit.
The
performance
test
(Paragraph
5-5)
enables
this
to
be
determined
without
having
to
remove
the
instrument
from
the
cabinet.
S-54
A
good
understanding
of
the
principles
of
operation
is
a
helpful
aid
in
troubleshooting,
and
it
.ts recommended
that
the
reader
review
Section
IV
of
the
manual
before
attempting
to
.troubleshoot
the
unit
in
detail.
Once
the
principles
of
operation
are
understood,
refer
to
the
overall
troubleshooting
procedures
in
Paragraph
5-57
to
locate
the
symptom
and
probable
cause.
5-55
The
schematic
diagram
at
the
rear
of
the
manual
(Figure
7-1)
contains
normal
voltage
read-
ings
taken
at
various
points
within
the
circuits.
These
voltages
are
positioned
adjacent
to
the
appli-
cable
test
points
(identified
by
encircled
numbers).
Component
and
test
point
designations
are
marked
directly
on
the
main
printed
wiring
board.
5-56
If
a
defective
component
is
located,
replace
it
and
re-conduct
the
performance
test.
When
a
component
is
replaced,
refer
to
the
repair
and
re-
placements
and
adjustment
and
calibration
para-
graphs
in
this
section.
5-57
OVERALL
TROUBLESHOOTING PROCEDURE
5-58
To
locate
the
cause
of
trouble,
follow
Steps
1,
2,
and
3
in
sequence
.
(1)
Check
for
obvious
troubles
such
as
open
fuse,
defective
power
cord,
input
power
failure,
or
defective
voltage
or
current
meter,
Next
remove
the
top
and
bottom
covers
(each
held
by
four
retaining
screws)
and
inspect
for
open
connections,
charred
components,
etc.
If
the
trouble
source
cannot
be
detected
by
visual
inspection,
proceed
with
Step
2.
(2)
In
almost
all
cases,
the
trouble
can
be
caused
by
improper
de
bias
or
reference
voltages;
thus.
it
is
a good
practive
to
check
voltages
in
Table
5-2
before
proceeding
with
Step
3.
(3)
Disconnect
the
load
and
examine
Table
5-2
to
determine
your
symptom
and
probable
cause.
5-59
Reference
Circuit.
a.
Make
an
ohmmeter
check
to
be
certain
that
neither
the
positive
nor
negative
output
termi-
nal
is
grounded.
b.
Turn front
panel
VOLTAGE
and
CURRENT
controls
fully
clockwise
(maximum).
c.
Turn
on
power
supply
(no
load
connected).
d.
Proceed
as
instructed
in
Table
5-2.
5-60
To
check
the
Zener
diodes
in
the
reference
circuit,
proceed
as
follows:
a.
Connect
differential
voltmeter
across
Zener
diode
(VRl
or
VR2)
•
b.
Connect
appropriate
load
resistor,
given
in
Figure
5-10,
across
(+)
and
(-)
output
terminals.