Trouble; Power-Supply - HP 400D Operating And Servicing Manual

Table
4-2.
Trouble
Shooting
Chart (Cont'd.)
SYMPTOM
POSSIBLE
CAUSE
AND/ON
REMEDY
Motorboating.
Motorboating may be caused by low voltage from the
regulated
power supply, or
from
low
a-c line
con-
ditions.
Weak
V2, V3, V4, V5
(see paragraph 4-10 to locate
trouble).
Trouble in power supply
(see paragraph 4-9).
High
plate
or
screen voltage.for
V2,
V3,
V4,
and/or Y5.
Weak
or
defective
tube. Refer to
paragraph 4-10.
Open
cathode
resistor.
Low plate
or
screen voltage
for V2,
VS,
V4, and/or
V5.
Defective
grid
coupling, cathode, or
screen
bypass
capacitor. Refer to
paragraph 4-10.
Defective
tube.
Voltmeter
reads low or high on
l-volt
and
higher
raurges,
error
increasing
as 800-volt
range
is
approached.
No
error
0.001-volt to 0.3-volt
ranges.
Noticeable mostly at high frequencies.
Black jumper
wire
(on
front
wafer of range
switch)
which
goes to
an empty
lug, is lmproperly
positioned;
this wire
must
be
touching the
rear
shietd
plate. This
Iead
is part
of the
capa,city voltage
divider
and moving
it will
destroy calibration of
the
l-volt
range
and
above.
E the
leads
are
moved
to proporly position them
it
will
be
necessary to reset
C4. All
ranges
1
volt to
300
volts
should now
track.
If
ttre
100qrolt and 300-volt ranges are
still
not
ac-
curate, but the
l-volt,
3-volt
and lO-volt
ranges
are, replace
C4
(dielectric
constant
is
changing
with
voltage),
and
recalibrate the instrument.
Divider trimmer capacitor
C4
defective.
Replace
trim-
mer
and
recalibrate instmment.
Voltmeter
reads high
on
two voltage r:urges
that
are multipes of
1000
apart.
Defective range switch
resistor.
Correct by reptacing
entire
range
switch assembly.
Sect.IV
Page
10
4.9
POWER.SUPPLY
tOCAttrAflON
CHECKS
If trouble is
localized to the power-supply section,
the
following
checks
will
be
helpful in trouble shoot-
ing.
a.
Set
line
voltage
to exactly
115
volts.
b.
The
d-c
voltage
at plate pin 9 of 1284A series
regulator
tube V?
or at
cathode
pin
I
of
6AX5
rectifier
tube V6
shor.rld
be between 400 and 420 votts.
Low
voltage at
this point
may be
due
to rectifier
tube
V6, power transformer T1, or
input capacitor
C30C being defective.
To
check
V6,
connect a
d-c voltmeter
between
pin
8
of V6
and
chassis.
Reduce
line voltage
from
115
to
103
volts.
The d-c voltmeter reading should drop
immediately
when the
a-c line
voltage
is
reduced
and then
remain
steady. If
the
d-c voltage
continues
to drop at
a slow
rate, rectifier
V6
is probably
weak
and should be replaced.
c.
The
d-c
voltage
between
pin
3
of
tube V?
and
chassis should be
250
volts, r5
volts.
L