Thermocouple Replacement - HP 3400A Operating And Service Manual

Section
V
Model 3400A
K
Table
5-6.
Troubleshooting
Procedure
(Cont'd)
CHECK
PROCEDURE
®
®
Measure
the ac signal at
the negative side
Of
C427.
The
reading should be approximately
32
mV
rms.
Investigate
C413 and
C415.
Investigate
Q404, Q405, and Q40 6 circuit.
Also
check thermocouples.
See Paragraph
5-34
for
thermocouple
check.
Measure
the ac signal
at
emitter
of
Q402.
Reading
should be
1.
85
mV
rms.
Investigate
Q401 and Q402
circuit."
Investigate
Q403
circuit.
Investigate
second
attenuator
circuit
A3.
Measure the ac signal
at
the input to the
impedance
converter
pin 7
on
A2
board.
The reading should
be approximately
1
mV
rms.
Investigate the
impedance converter or
power
supply.
See Table 5-4
for specifications
on
power
supply.
Investigate the input attenuator
Al.
ACTION
PASSES: Proceed
to
FAILS: Proceed
to
PASSES: Proceed
to
FAILS:
Proceed
to
@
PASSES: Proceed
to
FAILS:
Proceed
to
© ®
«
t
d.
Remove
leads
of
412A, connected
in
step
b,
and connect
412A
between
sliver
-colored lead
on
R4
(CAL
pot
on chassis) and pin 14
on
A6
socket
(this
step
checks the resistance
of
heater
in
TC402.
412A
should indicate
be-
tween 76. 5 to 103. 5
ohms.
e.
Leave
412A connected as
in
step
d (412A
is
used
to
supply 5
mA
of
current to
heater
of
TC402
in this step).
Note indication
on
DC
Voltmeter connected between pins 11 and 13
on
A6
socket.
Indication
should be negative
with respect
to
reading
in
step
c
and within
1
mV
of
that
reading.
f.
Remove
the
DC
Voltmeter and
412A from
3400
A.
Connect the 4 12
A
between pins 11
and
14
on
A6
socket
(this
steps
checks
for
thermocouples
shorted
to
ground through
cover.
Indication
of
412A should be greater
than 200
kft. If
less
than 200
kfi,
look for short
to
ground.
5-37.
thermocouple replacement.
{cautions
exercise
extreme care
when
removing
or
replacing the
amplifier printed circuit
board assembly and
when
shap-
ing
the
thermocouple leads.
5-14
5-38. Should
athermocouple be defective,
it
is
neces-
sary
to
replace both as a
matched pair (see Section
VH, Table
of
Replaceable
Parts) for part
number.
To
replace thermocouples,
perform the following steps:
a.
Turn
instrument
power
off
and
remove
right-
side
(INPUT
side)
and top covers.
b.
Remove
the four lead
connection to the
A4
amplifier board.
1.
Black
coaxial cable (two leads)
2.
Violet lead
3.
White lead/blue
lead.
c.
Remove
the
three mounting
screws on
the
amplifier board.
d.
Gently
pull
bottom
of
board
out at the
same
time
relieving stress
on thermocouple
cable
until
the
board
will
drop
down
and the top
will
clear
main frame.
Carefully
fold
board
down
to
expose
the
four nuts holding the ther-
mocouple
shield.
e.
Remove
four shield
nuts;
lift
shield
off.
Re-
move
thermocouples,
noting orientation.
f
Leads on
new
thermocouples
must
be
shaped
before
inserting
into
PC
board.
During
the
shaping process, hold leads between bending
point
and
glass with
long-nose
pliers.
01768-2
: