Introductory; Case Removal; Capacitor Replacement Ry; Replacement Of Crystal Diodes Ff - HP 400D Operating And Servicing Manual

4.I
INTRODUCTORY
This section contains
instructions
for
maintaining
and
trouble
shooting
the Model
400D as
well
as
procedures
for
replacing tubes and making
internal
adjustments. A tube replacement chart
lists
the
checks and
adjustments required aJter tube
replace-
ment,
and a
trouble
shooting
chart assists in local-
izing most types
or
troubles which might occur.
Photographs showing
tlre
physical location of
compo-
nents, a
voltage and resistance diagram,
and a
schematic
diagram are also provided
for
conveni-
ence
in trouble shooting.
The
more intricate adjustment
procedures described
in
tlis
section are provided for
those
who have t}te
necessary
skill
and
test equipment.
When
qualified
personnel
and
test
facilities are not available,
it
is
suggested
that adjustments
not
be
made
in the
field.
Instructions are given
on
the Warranty page at
tle
back of
t}is
instruction
manual
for
the
procedure to
be
followed
should
repair service
be
required.
The
following information
appears in
this
section:
4-2
Meter
Zero Adjustment
4-3
Case Removal
4-4
Capacitor
Replacement
4-5
Replacement
of Crystal
Diodes
4-6
Tube
Replacement
4-7
Adjustments
4-8
Trouble
Shooting
4-9
Power-SupplyLocalizationChecks
4-10
Voltmeter Localization
Checks
4.2
METER
ZERO
ADJUSTMENT
Whenever
the
mder pointer
does
not
indicate exactly
on
zero,
the
pointer
should be
reset to
zero.
For
the
most accurate positioning of tfie meter pointer,
the
instrument
should be allowed
to
reach operating
temperature before the adjustment
is made.
With
the instrument
in its
case, the instrument
should
be
aLlowed
to
heat
for
at least
15
minutes.
Shut
off
Sect.fV
Page
1
sEcTl0l{
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1llAI1{TEI{41{CE
the
power
and
wait at least
2
minutes
for all
capaci-
tors to
completely discharge,
then
proceed
as
follows:
The adjust
screw
is
in the meter frame at a midpoint
immediately below
the meter
face.
The adjustment
is
made
properly
only
when
the
pointer is
traveling
in
the opposite
direction to
the
turn
of t}te adjust
screw.
Although
the adjust screw
may
be
turned
in
either direction,
a
practical
procedure is
to turn
the screw in a clockwise direction until the pointer
starts to swing back toward
zero.
Then,
still
turn-
ing the scre'tr
clockwise, bring the pointer
(now
traveling counterclockwise) back to zero.
4.3
CASE
REMOVAL
The instrument case
is
fastened
to t}le rear of
the
chassis with two
screws. To remove ttre case,
re-
move
the screws,
and
slide the
case
to the rear
and
off the instmment.
When
replacrng
the case, pr.ll
tlre
powercablethrough
the opening at
the rear of
the
case.
Be sure
the cable
is free
of the case along the
entire
length of
the cable
so
tfiat
it
carurot
get
caught between
chassis
and case
as
tie
case
is slid
onto
the
instrtment.
4.4
CAPACITOR,
REPLACEMENT
Electrolytic capacitors in the Model
400D
are
high-
quahf
capacitors that have a
useful
life
of
from five
to
ten
years.
Do not replace these
capacitors unless
proved defective by accurate
tests.
4.5
REPLACEMENT
OF
CRYSTAL
DIODES
Meter tracking
is
affeeted
by the characteristics of
the diodes in the
rectifier-meter
circuit.
When a
diode
is replaced, therefore,
it
is
important
1)
that
the replacement diodes
have t}te
proper character-
istics
and 2)
that the meter calibration be
checked
and,
if
necessary,
readjusted.