Leader Electronics Corp. LMV-181A

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Summary

General Information and Features

General Instrument Description

Describes the millivoltmeters/voltmeters' capabilities, sensitivity, precision, and use as amplifiers.

Key Features

Lists features like wide bandwidth, high accuracy, stable design, decibel scales, and pre-amplifier use.

Technical Specifications

Voltmeter Specifications

Details voltage ranges, accuracy, frequency characteristics, input impedance, and max input voltage.

Amplifier Specifications

Details output voltage, frequency characteristics, output impedance, distortion factor, and operating temperature.

System and Panel Overview

System Block Diagram

Illustrates the internal functional blocks of the AC Millivoltmeter.

Front and Rear Panel Controls

Explains the various controls and indicators on the instrument's panels.

Front Panel Elements

Meter Display and Input;Output

Details the meter scales and the INPUT/OUTPUT terminals.

Operational Controls

Covers the POWER switch, pilot lamp, and RANGE button.

Adjustment and Protection Features

Explains zero adjustment, FUSE, power cord, and cord winder.

Preliminary Operational Notes

Line Voltage Requirements

Specifies the allowable tolerance for the line voltage used by the instrument.

Indicator Zero Adjustment

Provides instructions for zeroing the indicator when the power is off.

Operational Guidelines

Input Voltage and Noise Considerations

Discusses excessive input voltage, waveform distortion, and induction noise.

Measurement Preparation

Steps for preparing the instrument before connecting to power and performing measurements.

AC Voltage Measurement Procedure

Details the steps for measuring AC voltage, including range selection and reading.

Advanced Operation and Specifications

LMV-181 B Specific Ranges

Details the voltage ranges, scales, and multipliers for the LMV-181B model.

Decibel Range Usage

Explains decibel calibrations and how to read decibel values.

Output Terminal Functionality

Explains using the OUTPUT terminal for monitoring or as a pre-amplifier.

Alternating Current Calculation

Provides the formula relating AC voltage, current, and resistance.

Internal Circuitry Description

Circuitry Components

Describes the attenuators, protective circuit, impedance conversion, and amplifiers.

Meter Amplifier and Circuit

Explains how the meter amplifier and rectifier circuits drive the meter indication.

Monitor and Stabilization Circuits

Details the monitor amplifier's output and the stabilization power circuit.

Maintenance Procedures

Cover Removal

Instructions for removing the instrument's cover.

Power Transformer Voltage Adjustment

Guidance on changing wiring for different power source voltages.

Calibration and Adjustment

Stabilization Power Adjustment

Procedure to adjust VR4 to achieve +18V on the stabilization power line.

Correcting Indicated Values

Steps for calibrating voltage readings using VR2, VR1, and VC1.

Output Voltage Terminal Adjustment

Procedure to adjust VR3 for correct output voltage at the terminals.

Schematic Diagram and Components

Circuit Diagram

The detailed electronic schematic diagram of the LMV-181A.

Component Markings

Explanation of markings for capacitors, resistors, and other components.

The Leader LMV-181A(B) is an AC Millivoltmeter, a highly sensitive and precise instrument designed for measuring sine wave alternating current voltages. It functions as both a millivoltmeter and a voltmeter, utilizing mean value indicating devices calibrated for effective value readings.

Function Description

The primary function of the LMV-181A(B) is to measure AC voltages across a wide range of frequencies and magnitudes. It can also be used as a low-noise, high-gain pre-amplifier or amplifier for other electronic apparatus, thanks to its output terminals.

Internally, the instrument's functional block diagram illustrates its operational flow. The input signal first passes through a 60 dB attenuator, which controls the input voltage to ensure the impedance conversion circuit operates normally. This attenuator provides 0 dB attenuation for lower voltage ranges (1 mV – 300 mV for 181A, 1.5 mV – 500 mV for 181B) and 60 dB attenuation for higher ranges. Following this, an input protective circuit, consisting of transistors Q1 and Q2, safeguards the subsequent FET (Q3) from excessively large input voltages.

The signal then enters an impedance conversion circuit, which includes FET Q3 and Q4. This circuit converts the high input impedance to a low impedance and provides a +10 dB amplification factor. A 10 dB step attenuator works in conjunction with the 60 dB attenuator to select appropriate values for measurement.

Next, the signal is fed into a meter pre-amplifier (Q5 and Q7), which amplifies small signals. The amplified signal then proceeds to the meter amplifier and meter circuit, comprising Q9, Q10, Q11, diodes D2, D3, and the meter itself. The rectifier circuits (D2 and D3) and the meter are integrated into the feedback loop of the amplifier, ensuring that the meter's indications are directly proportional to the input voltage.

A monitor amplifier is also included, which takes a signal from a part of the meter pre-amplifier and amplifies it. This allows for a 1 Vrms output at the "OUTPUT" terminal when the meter indicates full scale, enabling the instrument to act as a pre-amplifier or for monitoring waveforms with an oscilloscope.

Finally, a stabilization power circuit, consisting of rectifier circuits (D4, D5, D6, D7) and stabilization circuits (D8, Q12, Q13), provides a stable +18V supply. D8, a Zener diode, serves as the reference voltage for this stabilized supply, which powers the impedance conversion circuit, meter pre-amplifier, meter amplifier, and monitor amplifier.

Important Technical Specifications

Measuring Voltage Range:
- LMV-181A: 100μV to 300V
- LMV-181B: 150μV to 500V
Measuring Range: 12 ranges (e.g., 1, 3, 10, 30, 100, 300 mV; 1, 3, 10, 30, 100, 300V)
Decibel Range: 12 ranges (-60 to +50 dB, with 0 dB = 1V or 0.775V)
Measuring Accuracy: ±2% of full scale (at 1 kHz or 400 Hz)
Frequency Characteristics:
- 5 Hz to 1 MHz: ±10%
- 10 Hz to 500 kHz: ±5%
- 20 Hz to 100 kHz: ±2% (1 kHz as base)
Input Resistance: 10 MΩ
Input Capacitance:
- Under 50 pF (1mV to 300mV for 181A, 1.5mV to 500mV for 181B)
- Under 35 pF (1V to 300V for 181A, 1.5V to 500V for 181B)
Maximum Input Voltage: AC peak + DC = 600V
Noise: Within 2% of full scale by shorting input
Amplifier Output Voltage: 1V no load (when 1.0V for 181A, 5.0V for 181B is indicated at full scale of each range)
Amplifier Frequency Characteristics: 10Hz to 500kHz -3 dB (1 kHz as base)
Amplifier Output Impedance: 600Ω±20%
Amplifier Distortion Factor: Within 1% at full scale (1 kHz)
Operating Temperature Range: 0 to 40°C
Operating Humidity Range: Less than 85%
Power Supply: 100V ±10% 50/60 Hz (changeable to 120V, 220V, 240V by switching interior trans-tap)
Power Consumption: 2.5 VA
Size and Weight: 132(W)×150(H)×250(D)mm, 2 kg (exclusive of buttons, rubber legs, and handle)

Usage Features

Meter Faces: The instrument features voltage and dB scales, with two red dB calibrations (0 dB = 0.775 V for 1 mW 600Ω and 0 dB = 1 V dBV).
Mechanical Zero Adjustment: A screw allows for mechanical zero adjustment of the pointer when the power is off.
Input Terminals:
- A metallic terminal serves as the ground terminal for voltage measurement.
- A plug (U.H.F. type) connector is used for applying voltage. Its design allows for the use of Type M, UHF, and twin-type banana plugs, as well as individual banana plugs.
RANGE Button: This button selects the full-scale value. Voltage values for full scale are indicated in black, and 0 dB absolute levels are shown in dB.
OUTPUT Terminals:
- A black terminal serves as ground when the instrument is used as an amplifier.
- A red terminal is the "Hot" output terminal. When the meter indicates full scale, 1 Vrms of voltage can be obtained from this terminal, allowing the instrument to function as a pre-amplifier or for waveform monitoring with an oscilloscope.
Power Switch: Pushing the switch to "ON" turns on the power and illuminates the pilot lamp.
Fuse: Protects against overload and short circuits. It is removable by counter-clockwise rotation.
Cord Winder: Provides convenient storage for the power cord.
Measurement Procedure:
- Always ensure the pointer is at 0 when power is off.
- Connect the power plug to the correct voltage source.
- Set the range switch to a high voltage (e.g., 300V or 500V) initially.
- Turn on the power; the pilot lamp should light. Allow about five seconds for stabilization.
- When measuring AC voltage, connect the signal to the "INPUT" terminal.
- Adjust the "RANGE" button counterclockwise to reduce the voltage range gradually until the pointer reads between 30% and 100% of full scale for accurate readings.
- Decibel values are read by adding the range dB value to the indicated value on the pointer.
Important Notes for Usage:
- Do not apply input voltages exceeding AC peak + DC = 600V to prevent damage.
- The instrument is calibrated for sine waves; distorted waveforms may result in incorrect readings.
- For very small or high-impedance measured voltages, use shielded cables to reduce external noise induction.
- When using the "OUTPUT" terminal, avoid connecting low load impedance or high load capacitance, as this can lower output voltage, deteriorate low-frequency characteristics, or degrade high-frequency characteristics.

Maintenance Features

Changing Fuses: Fuses can be removed by turning the fuse holder on the rear side in the direction of the arrow. Use 0.1A fuses. After changing, investigate the cause of the fuse blowing before reapplying power.
Removing the Cover: To remove the cover, unscrew the three screws on the top and side, slide the cover backward by 1 to 2cm, and then lift it off.
Changing Power Source Voltage: The power transformer is located on the rear side when the case is opened. The terminals (numbered 1, 2, 3* for each primary winding, with two primary windings) can be rewired to adapt the instrument to 100V, 120V, 220V, or 240V power sources.
Adjustment and Correction (if specifications are not met after repairs):
- Stabilization Power Adjustment: Connect a DC voltmeter to the +18V line on the printed circuit and adjust VR4 (Vcc18V ADJ) to achieve +18V.
- Indicated Value Correction:
  1. Connect a voltmeter calibrator to the INPUT.
  2. Set the voltmeter range to 10 mV.
  3. Set the calibrator output to 10 mV and adjust VR2 (10 mV 400 Hz ADJ) for full scale.
  4. Set the voltmeter range to 10 V.
  5. Set the calibrator output to 10V and adjust VR1 (10V 400Hz ADJ) for full scale.
  6. Set the voltmeter range to 1V.
  7. Change the signal source to an oscillator with good frequency characteristics. Set the oscillator to 500Hz and its output to 1V full scale.
  8. Change the signal frequency from 500 Hz to 50 kHz and adjust VC1 (50kHz ADJ) for flat frequency characteristics. If this fails, repeat the entire correction procedure.
  - Note for 181B: Adjust each set voltage by a multiplier of 1.5 (e.g., 10 mV becomes 15 mV).
- Output Voltage Terminal Adjustment: After the above adjustments, set the voltmeter range to 1V. Apply a 1 kHz signal to achieve full scale and adjust VR3 (OUT 1V ADJ) until the output terminal voltage is 1 Vrms.