Checking Thermocouples; Front Panel Symptoms Table; Rms - HP 3400A Operating And Service Manual

Model 3400A
Section
V
»
circuit
parameters, rather only
to localize the
mal-
function. Therefore,
it
is
quite possible
that
add-
itional
measurements
will
be required
to
completely
isolate the
problem.
Amplifier
gain
and
biasing
may
vary
slightly
from instrument
to
instrument;
therefore,
it
should
not be
necessary
to
precisely
duplicate
waveforms
or values described.
NOTE
Do
not
use an extender board on
the
Chopper
Amplifier Board
(A6).
5-34.
CHECKING THERMOCOUPLES
TC401
AND
TC402.
One
DC
Voltmeter
with 10
mV
F.
S.
capability
such
as another -hp-
412A
or
3440A/3443A
Digital
Voltmeter.
If
the
thermocouples
fail
any
of the following
checks
replace the thermocouples according
to
procedures
outlined
in
Paragraph
5-37.
Turn
the
instrument
off
and
remove
the
A6
board.
a.
NOTE
5-35.
The
following procedure
will
allow you
to
check
the
thermocouples
for
proper
operation
in
the -hp-
Model 3400A
RMS
Voltmeter.
NOTE
b.
This check
will not yield
any informa-
tion
concerning thermocouple operation
for either sluggish or
over
shoot
-under-
shoot
response on
3400A
meter.
If
a 3400A
has
the
above
symptoms
re-
place
thermocouples.
5-36.
To
perform
these checks, supply
5
mA
of.
cur-
rent
to the heaters
of
the thermocouples. The
-hp-
Model 412A
DC
Vacuum
Tube
Voltmeter
in
ohms
function
on X100
range
is
ideal for this application.
Instruments required
for these checks are as
follows:
One
-hp-
Model
4 12
A DC
Vacuum Tube
Voltmeter.
See Figure 6-3
for
component
locations.
Connect
test leads
of
412A,
in
ohms
function
on X100
range,
between ground and
junction
of
C413
and
C415 on
A4
board
(this step
checks the resistance
of
heater
in
TC401).
412A
should
indicate
between
76. 5 to 103. 5
ohms.
If
not within limits, replace
the
matched
set
of
thermocouples
(-hp- Part No.
0853-0003).
Leave
the
412A
connected as
in
Step b (412A
is
used
to
supply
5
mA
of
current
to
heater
of
TC401
in
this step). Connect
a
DC
Volt-
meter
as
follows:
Negative lead
to pin 11 on
A6
socket.
Positive lead
to
pin 13 on
A6
socket.
Voltmeter
should
indicate
between
+6. 5
mV
and
1
9
m
V.
Note
the indication for future
re-
ference.
If
within the
test limits,
leave
DC
Volt-
meter
connected and proceed
to step d page 5-14.
Table 5-5. Front Panel Symptoms
SYMPTOMS
POSSIBLE
CAUSE
1/2 scale readings
on
all
RANGE
switch settings
and
input voltages.
Chopper
Amplifier
(A6),
Check
C612.
3 to
5% meter
offset
on
all
ranges
with shorted
input.
R7
(1/10
SCALE
ADJ)
misadjusted.
C405
(10
MHz
ADJ)
misadjusted.
400
Hz
calibration low and frequency response
falls off
above
50 kHz.
Q401
or
Q402
shorted.
Switching transients exceed 5%
of full
scale with
shorted
input.
Check
collector voltage
of
Q201
(should not ex-
ceed 9.0
V).
Instrument has been overloaded.
Check
Q201, Q401,
Q402 and TC401.
See
Paragraph
5-34
for details on TC401.
Meter
jitter
exceeds
0.
5%
of
full scale.
Check
Q601, Photochopper
Assembly
(neons)
see Figure
5-9. Verify
the
value
of
R606
to
be
3. 3
kJ2
(-hp- Part No. 0683-3325).
Full-scale difference from range
to range.
Check resistors
in
second
attenuator.
Meter
pegs
full scale.
Check
thermocouples.
See
Paragraph 5-34
for
details.
Overshoot, undershoot or sluggish response on
meter.
Replace Thermocouples.
See Paragraph 5-37.
5-7