HP 400D Operating And Servicing Manual page 23

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If
the response
is
down
excessively,
and known good
tubes in
V1 through
VSrdo not
bring
it up. It is
sug-
gested that the instrument
be returned to
the
factory
for repait.
(g)
Repeat
step (d) at
2
mc, 20 cps,
and
10 cps.
The
400D reading
should be
within
216
at 2
mc,
$s
at
I
mc and
20
cps,
and
370
at
10
cps.
Weak
tubes,
particularly V3,
V4 and Vb
wilt
affect
10 cps
re-
sponse and should be
checked
if
response
is
out of
specification.
The 10 and 20 cps
response can be correctedby
adjustment
of either
R80
or
R81.
The
minimum
value
for
either of these resistors is 2.2
megohms. Many
instnrments
will
have neither
one
of these
resistors.
Either
one but not both may
be added
to correct
fre-
quency
response. Adding
R80
will
lower the
response
at
10
and 20 cps while
adding
R81
will
raise the
re-
sponse at tlese
two frequencies.
Set
the instrument being tested to the .01-volt
or
lO-volt
range and repeat steps (c)
and
(d).
Adjust
C16
to
obtain
the
same reading
on the
in-
strument under test as established in step
(h).
Set
the instrument being tested to the 1.0 volt
range
and
repeat steps (c) and
(d).
Adjust
C4
to
obtain the same reading
on
the
in-
strument under test as established in step fi).
NOTE
The
proper setting of
C4 should be extremely close
to the setting
previously
made
at
500
kc in the
cali-
bration procedure.
If
the setting
is
changed
ap-
preciably,
the basic calibration
wiII
be destroyed
on
the 1.0 volt
range and above.
(l)
Set the instrument
being tested to the 3-volt
range and repeat steps (c) and
(d).
(m) Adjust
C14
to
obtain the same reading
on the
instrument as
estabLished
in
step (t).
(n)
Recheck
calibration.
The tested instrument
should
now be within published specifications.
Resistor
R82 along
with capacitors
C26 and
C40
are
factory
adjusted
for
best instrument response.
These
factory
adjusted vafues
should not be changed. Capa-
citor
C40
and
resistor
R82
will
not
be
found
in
all
instruments.
Sect.IV
Page
7
4.8
TnOUSLE SHOOTING
To
assist in trouble shooting,
a chart has
been
pre-
pared
which
lists
various trouble symptoms, possible
causes and/or
remedies. ff
the
cause
of
the
trouble
is
of a more obscure nature than
can be covered by
t}te
chart, trouble
can be Located by
tesh
first
bry
sec-
tionalizing,
and second
by localization within the
section by
means
of voltage
and
resistance measure-
ments.
The
circuit
sections
are
indicated in the block
diagram
(Figure
3-1).
a.
The
first
step
in sectionalizing is to
check the
output of the regulated high-voltage supply:
the
d-c
voltage
between
pin
3 of V7
and
ground
should
measure
250
volts, tb
volts.
If
the regulator output
is
not
within these limits,
see
paragraph 4-9.
If
the regulator output is within these
limits,
Ute
power supply
probably is functioning
properly,
and
the remainder of the circuit
should be
tested.
Apply a signal aIJNPUT,
pnd check the
level of the
signal voltage at
the
input
and
output
of
each
of
ttre
sections
shown
in Figure 4-3,
except
the levels at
1st
amplifier
V2.
(The
low level of signal applied
to the
grid
of
V2,
together with the large
amount
of negative
feedback applied to
its
cathode,
make
any signal-voltage measurements at
V2 impractical.
)
Typical
signal voltages at the input
and output of
each
section,
for
an
input signal of 0.3 volt at
400
cycles,
may be found
in
Figure
4-3, a
servicing
block
diagram.
b.
When
trouble has
been
localized to
a
particular
section,
it is
recommended that
tube
substitution
be
tried
before any other tests are made. If
tubes
known to
be good do not clear the trouble, voltage-
resistance measurements
should be
made. Typical
voltage
and resistance values
may be found
on Tube
Socket Voltage-Resistance
Diagram, Figure
4-4.
For the circuitry in
each section reference may
be
made
to
t}re schematic
diagram.
The
servicing block
diagram
(Figure 4-3)
shows
the signal path
in
heavy
Iine
and
directional
arrows.
Feedback paths are
shown by means of
directional arrows, but in lighter
Iine.
Localization by d-c voltage analysis is
discussed
in paragraph 4-10.
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