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3-17.
Resetting
the Protection Amplifier is accom-
plished by cutting off the
voltage
to it.
It
is always
ad-
visable
to
determine the
cause
of the overload before
at-
tempting to
reset
the circuit. The
voltage
may be cut-
off to
the Protection
Amplifier by two
methods.
The
MODE
switch may
be placed in the
OFF position
and
the
POWER
switch
may be operated to
any position
other
than ON.
Either
of these actions will
cause
an inter-
ruption in the battery
power
to the Protection Amplifier.
The preferred method
is resetting
the protection
cir-
cuit
with
the MODE switch.
3-1.
8 .
A push button,
momentary
open, switch called
PROTECTION
DISABLE
is
located
on the front panel
of
the Model
A54-
2
plug-in unit. This
switch,
and its
function,
constitutes
the
only difference
between
the
Model
A54-1
and the Model A54-2
plug-in
units.
De-
pression of
the PROTECTION
DISABLE switch removes
the diode
circuit
(CR701 thru CR704) in parallel
with
the thermocouple, providing
a
routine method of check-
ing for diode leakage (see
Section
4-31).
The Model
A54-2
is
calibrated
with
the diode circuit active
(PRO-
TECTION DISABLE switch in the normal, up, position).
3-19.
THERMOCOUPLE
3-20.
The
thermocouple
element has been
tested and
selected
at the factory
and placed
in an insulating
case
with a standard
octal
plug base.
The thermocouple
element
is
of the high vacuum, insulated type
with
a
nominal
output
of
7
millivolts at rated
input.
The Model
A54 unit
will
have to be rechecked for calibration accu-
racy if the thermocouple
is
replaced.
The
input
to the
thermocouple may
be
reversed by operating switch S802,
POLARITY,
located
on the front panel
of
the Model A54
plug-in unit. With this
switch
it is possible
to
check
the
turn-over
error
of the
thermocouple with
a
minimum of
effort.
MOMENTARY
INPUT
LOCK
540B
3-21.
REFERENCE
3-22. When using the Transfer Standard it is
not neces-
sary
to measure
the output
voltage of the thermocouple,
it
is necessary
only to
provide
a
means
of detecting
and
comparing the thermocouple output accurately when
making transfer
measurements.
A variable
voltage
source has been designed into
the
Model
540B which
fulfills the stability
and resolution
requirements
for
this
application. Referring
to
Figure 3-2,
the
adjustment
of
the
REFERENCE
ADJUST controls
COARSE,
MEDIUM
and FINE will
vary the voltage
at
point b, from zero to
approximately
14
millivolts.
The MOMENTARY
position
on
the
GALV
switch
makes it
possible
to
search
out and
find
an
approximate null on the
galvanometer
with
the
COARSE
control
without
applying
a steady overload
to
the galvanometer.
The LOCK position allows
a fixed
path
for use
while
adjusting
the MEDIUM
and FINE
controls to maintain the
galvanometer
balance
in suc-
cessively more sensitive ranges.
3-23. GALVANOMETER
3-
24.
Refer
to
Figure 3-3. When a
de voltage from the
thermocouple and
the reference
supply is
applied to
the
galvanometer circuit,
the
Magnetic Modulator
(T2) pro-
duces
a 5
kHz
output
containing
magnitude and
polarity
information relative
to
the input de.
This
5 kHz
signal
is then
applied to a ·Tuned
Carrier
Amplifier
where
the
signal
is amplified
and undesirable
frequencies are
at-
tenuated.
The
Tuned Carrier Amplifier output is then
fed to
the
Synchronous
Demodulator. The demodulator
is
a balanced demodulator
that
has no
output when no
signal is applied. The Push-Pull Oscillator
and
Fre-
quency Doubler
generates a
2. 5kHz signal
that is fed
to the Magnetic Modulator
and a
5
kHz
signal
that is fed
to the Synchronous
Demodulator. With
de applied
to the
input of the Magnetic Modulator, a de current
flows thru
the meter which
is proportional
in magnitude
and
like
in
polarity
to
the input de.
-+
1.l5v
2.4K
680Ω
{MEDIUM)
7.5Ω
{FINE)
Figure 3-2. SIMPLIFIED BALANCE
NETWORK
3-3
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