Kenwood VT-181, Service Manual

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VT-181/VT-181E
5
General
A Signal voltage to be measured, which is input from the
INPUT connector, is passed through the First Attenuator
and is converted to a low impedance by the Impedance
Convertor. The impedance-converted signal is normalized,
or further attenuated in proportion to 1mVrms fullscale value
through the Second and Third Attenuator. The normalized
signal is magnified 20-fold by the Main Amplifier and is fed
to the Output Amplifier and the Absolute-Mean Value Detec-
tor.
The Output Amplifier magnifies the signal 50-fold and feeds
to the OUTPUT connector. The Absolute-Mean Value
Detector converts the signal from the Main Amplifier to DC
current in proportion to the absolute mean value. The con-
verted signal activates the Meter.
The Attenuator Control encodes the signal led from the
RANGE selector to generate an Attenuator Control signal.
This signal controls the First, Second and Third attenuator to
set the sensitivity corresponding to each range.
The Power Supply feeds to the functional circuit ±5V DC
voltages stabilized by its IC regulator.
Description of Functional Circuit
1) First Attenuator
A potential divider act as a attenuator. The amount of atten-
uation is switched in tow steps by relay contacts:0dB and -
60dB.
2) Impedance Converter
A FET differential input Amplifier act as a impedance con-
verter with 0dB(10dB) gain. Which converts the First Attenu-
ator output signal to a sufficiently low impedance and feeds
of the Second Attenuator.
3) Second Attenuator
A resistance divider act as a attenuator. The amount of
attenuation is switch in two steps by relay contacts:0dB and
-30dB.
4) Third Attenuator
A resistance divider network act as a attenuator. The
amount of attenuation is switched in four steps by FET
switch:0dB,-10dB,-20dB,and -30dB.
5) Main Amplifier
A wideband,non-inverting differential amplifier act as the
main amplifier. Which has high input impedance,low output
impedance and 20-fold gain. This output signal level is
20mVrms for the fullscale read on the Meter.
6) Output Amplifier
A wideband,non-inverting differential amplifier act as a out-
put amplifier. Which has 50-fold gain and 600
Ω
output
impedance. The output signal level is 1Vrms for fullscale
read on the Meter,and works stable even for capacitive
loads.
7) Absolute-Mean Value Detector
An absolute-mean value detector comprised of a high
through-rate and high gain amplifier. Which has a very good
linearity by negative feedback from the current flowing
through the Meter load. In switching, this provides a suffi-
ciently wide frequency band so that the high frequency
phase compensation circuit is reset.
8) Attenuator Control
A logic control circuit comprised of a diode matrix and output
buffer transistors. This encodes a 12-bit signal from the
RANGE selector switch to 6-bit signals,which control the
First,Second and Third Attenuator. The remote control con-
nector is connected to this circuit.
9) Rotary Switch
A 12 contacts rotary switch which design the operation
range. For different range,different attenuation ratio are acti-
vated.
10) Gain Control
To using the dB scale to measure relative voltage, you may
get a reference reading by change the gain control. (relative
ref.) Which capable to vary 0 to 10dB.
11) Power Supply
The power source circuit supply±5V DC from the AC input.
Which contain a silicon diode bridge for full-wave rectifica-
tion,high-capacitance electrolytic capacitors for
smoothing,and an IC regulator stabilization.
CIRCUIT DESCRIPTION
The voltage or sentence in parenthes is applicable in case of the "VT-181E".
In studying the operation of each circuit in voltmeter please refer to "BLOCK DIAGRAM".
VT-181/VT-181E(K) INT 98.4.9 10:31 AM y [ W 10