Leader Electronics Corp. LMV-181A - Internal Circuitry Description; Circuitry Components; Meter Amplifier and Circuit; Monitor and Stabilization Circuits

Current
can
be
determined
out
by
measuring
the voltages
at
both
ends of
the
resistance
based
on
the
above
relationship.
Fig
4
Note:
The “INPUT” terminal
of
this instrument is grounded
at one end.
8. Functional
description
LMV-181A
and
LMV-181B are similar
as far as
functions are
concerned.
They
consist of
a
60
dB attenuator,
input protective
circuit,
impedance
conversion circuit,
10
dB
step
attenuator,
meter pre-amplifier,
meter
amplifier, meter
circuit,
monitor
amplifier and
stabilization power
circuit.
1. 60
dB attenuator
Controls input voltage
so
that the
impedance conversion
circuit will work normally. The
attenuation
quantity
is
0
dB
in
the 1 mV
—
300
mV
[1.5
mV
—
500
mVJ
range,
and 60 dB
in
higher ranges.
2. Input
protective
circuit
It
consists of
Qi
and
Q2,
and
protects
FET
Q3
from
excessively large input voltages.
3.
Impedance
conversion circuit
+
10 dB
amplifier
It consists
of
Q3
(FET)
and
Q4
converts
high
input
impedance
into
low
impedance,
and
has
an
amplification
factor
of
+10
dB.
4.
10
dB
step
attenuator
In
conjunction
with
the
60
dB
attenuator,
it
selects
proper
values
to
be
measured.
5.
Meter
pre-amplifier
It
consists
of
Qs
and
Q7
,
and
amplifies
small
signals
into
large
signals.
6.
Meter
amplifier
and
meter
circuit
Consists
of
Q9,
Qio,
Qii,
D2,
D3
and
meter.
The
rectifier
circuits
of
D2
and
D3
and
the
meter
are
in
the
feedback
circuit
of
the
amplifier,
so,
the
indications
of
the
meter
are
in
proportion
to
the
input
voltage.
7.
Monitor
amplifier
1
Vrms
output
is
obtained
at
the
“OUTPUT”
terminal
when
the
meter
indicates
full
scale
after a
signal
is
taken
out
of a
part
of
the
meter
preamplifier
and
amplified.
8 .
Stabilization
power
circuit
This
circuit
consists
of
D4
,
D5
,
and D7
rectifier
circuits
and Dg,
Qi 2
and
Q13
stabilization
circuits.
With
Dg
Zener
diode
as
the
reference
voltage,
it
supplies
+18V
stabilized
voltage
to
the
impedance
conversion
circuit,
meter
pre-
amplifier,
meter
amplifier
and
monitor
amplifier.
9.
Maintenance
9-1
Changing
fuses
A
fuse
can
be
removed
by
turning
the
fuse
holder
on
the
rear
side
in
the
direction
of
the
arrow.
Use
0.1 A
fuses.
After
fuses
are
changed,
be
sure
to
investigate
the
cause.
Take
appropriate
steps
before
power
is
turned
back
on.
11
12