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Model 1821A
Section
Vin
Paragraphs
8-1
to
8-14
SECTION
VIII
SCHEMATICS
AND
TROUBLESHOOTING
8-1.
INTRODUCTION.
8-2.
This section
contains
schematic diagrams,
com-
ponent
identificatoin,
and
troubleshooting and repair
information
for the
Model
1821A.
Performance
Check
and
Adjustment procedures are provided
in
Section V.
8-3.
SCHEMATIC DIAGRAMS.
8-4.
Schematic diagrams appear
on right-hand
pages
that
unfold outside
the right
edge
of
the
manual.
These
"throw-clear" pages
allow viewing
the
schematics
while referring
to
other sections
in the
manual.
8-5.
Schematics are primarily
drawn
to
show
circuit
function.
A
given
schematic
may
include
all
or part
of
several assemblies.
Table
8-1 provides
informa-
tion
about
symbols
and conventions used
in
the
sche-
matics.
DC
voltages
and
waveform
test
points
are
provided on
the
schematics.
DC
voltage
measurement
conditions,
waveform measurement
conditions,
and
waveforms
applicable
to
each schematic are
shown
adjacent
to
that
schematic.
8-6.
COMPONENT
IDENTIFICATION.
8-7.
Whenever
possible, the location
of
components
appearing on a schematic
is
shown
on
the
page
opposite
that
schematic.
When
components
on an
assembly
ap-
pear
on
more
than
one schematic,
the location of
all
components
on
that
assembly
are
identified
opposite
the first
schematic showing
that
assembly.
Adjust-
ments, assemblies,
and
chassis
mounted components
are
identified
in
Figure
8-2.
8-8.
TROUBLESHOOTING.
8-9.
The
first
and
most
important
prerequisite for
successful troubleshooting
is
a thorough understanding
of
instrument operation and
function.
Often,
suspected
malfunctions are
caused
by
improper
control
settings
such
as:
intensity set too low,
display selector or
mode
switch
in
wrong
position,
trigger
level
malad-
justed,
etc.
Read
Section
III,
Operation,
and
Section
IV,
Principles
of
Operation, for
this
information.
8-10.
DC
voltages for
most
active
components
(trans-
istors,
FET's,
etc.)
are
indicated
on
the
schematics.
Waveform
test
points
are
also
shown
on
the
schematic
at
various
points
along
the
main
signal path.
The
numbers
inside the
test
point
symbols
are
keyed
to
the
proper
waveform
adjacent
to
the
schematic.
These
voltages
and
waveforms
are
invaluable for trouble-
shooting
the
instrument.
Applications
include:
check-
ing
'Stage gain,
locating
unbalance
in
differential
amplifiers, locating
faulty transistors,
etc.
Always
refer
to
the specific
measurement
conditions
before
using dc voltages or
waveforms.
Allow
the level
to
stabilize
before ntoing dc
voltages.
Small
dots
are
etched on
circuit
board assemblies
next
to
the
emitter
lead
of
transistors,
the
source
lead
of
FET's,
the
cathode
side
of
diodes,
and
the positive side
of
elec-
trolytic
capacitors as an aid
to
locating test points.
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When
taking
waveform
or dc
measure-
ments, use
extreme
care
to
avoid
shorting supply voltages or
components.
8-11.
If
a malfunction occurs. Figure 8-1
may
help
isolate the
trouble
to
a particular
circuit.
Always
begin troubleshooting with a visual
inspection.
Check
for
burned or
loose
components,
loose
wire connec-
tions, faulty
switch contacts or any similar
conditions
suggesting a
source
of
trouble.
8-12.
REPAIR
AND
REPLACEMENT.
8-13.
Almost
all
electrical
components are
accessible
for
replacement
from
the
component
side
of
the
etched
circuit
board.
Small
dots
are etched on
circuit
board
assemblies
next
to
the
emitter
lead
of
transistors, the
source
lead
of
FET's,
the
cathode side
of
diodes,
and
the positive side
of
electrolytic
capacitors as
an
aid
when
replacing
components.
Section
VI provides a
detailed parts
list to
allow ordering
replacement
parts.
Mechanical and miscellaneous
electrical parts
are
listed at
the
end
of
Table
6-2.
If
satisfactory
opera-
tion
or repair cannot be accomplished,
contact the
nearest
Hewlett-Packard
Sales/Service
Office
(ad-
dresses
at
rear
of this
manual).
If
shipment
for re-
pair
is
required,
see
Section
II
for
recommended
packaging procedure.
8-14.
Etched
circuit
boards
in this
instrument have
components mounted
on one
side
of
the board,
conduct-
ive
surfaces on both
sides,
and
plated-
through
com-
ponent mounting
holes.
Hewlett-Packard
Service Note
M-20E
contains useful information
on servicing etched
circuit
boards.
Important considerations are
as
follows
a.
Use
low heat
(37
to 47.
5
watts, less than
800°F
idling
temperature),
slightly
bent chisel
tip
(1/16
to
1/8 inch
diameter) soldering
iron;
and a small
diam-
eter high
tin
content solder.
If
a
rosin solder
is
used,
clean
the
area
thoroughly
after soldering.
b.
Components
may
be
removed
by placing
the
soldering iron
on
the
component
lead
from
either side
of
the
board,
and
pulling
up on
the
lead.
If
heat
is
applied
to
the
component
side
of
the
board,
greater
care
is
required
to
avoid
damaging
the
component
(especially true for
semiconductors).
If
heat
damage
is
likely to
occur, grip
the
lead with a pair
of
pliers
to
provide a heat
sink
between
the
soldering iron
and
component.
02591-3
8-1
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