LDM-177/178
AUTOMATIC DISTORTION METER
自动失真 测量仪
SERVICE MANUAL
服务手册
LDM-177: Single-channel version
LDM-178: Dual-channel version
LDM-177: 单通道版本
LDM-177：双通道版本
NOTE
These servicing instruc ons are for use
by qualified personnel only. To avoid electrical
shock, do not perform any servicing other than
that contained in the service manual unless
you are qualified to do so.
这些维护说明仅供合格人员使用。
为避免触电，请勿进行服务手册以外的任何维护，
除非您具备相关资格。
目录
1.
SPECIFICATIONS（规格） ........................................................................................................................................................ - 2 2.
TEST EQUIPMENT REQUIRED（所需测试设备）..................................................................................................................... - 4 -
3.
CALIBRATION PROCEDURE（校准程序） ................................................................................................................................ - 5 3.1.
General（一般情况） ................................................................................................................................................... - 5 -
3.2.
Adjustment of Meter Mechanical Zero（调整仪表机械零位） ................................................................................... - 5 -
3.3.
Initial Control Settings（初始控制设置） .................................................................................................................... - 5 -
3.4.
Power Supply （电源） ................................................................................................................................................ - 6 -
3.5.
Level Meter Section（电平表部分） ............................................................................................................................ - 6 -
(1)
Sensitivity Adjustment 灵敏度调整 ......................................................................................................................... - 6 -
(2)
Automatic Level Controller Adjustment 控制器自动调节 ....................................................................................... - 6 -
(3)
Frequency Response Adjustment 频率响应调整..................................................................................................... - 7 -
3.6.
Distortion Meter Section（失真表部分） .................................................................................................................... - 7 -
(1)
4.
Sensitivity Adjustment 灵敏度调整 ......................................................................................................................... - 7 -
TROUBLESHOOTING PROCEDURE（故障排除程序） ............................................................................................................. - 8 4.1.
Troubleshooting Aid-1（故障排除 Aid-1） .................................................................................................................. - 8 -
4.2.
Troubleshooting Aid-2 （故障排除 Aid-2）.................................................................................................................. - 9 -
4.3.
Troubleshooting Aid-3（故障排除 Aid-3） ................................................................................................................ - 10 -
(1)
Overfall operation is not satisfactory, or unit is dead. 整体运行不理想，或设备死机。 .................................... - 10 -
(2)
Level Meter Section 电平表部分 ........................................................................................................................... - 11 -
(3)
Distortion Meter Section 失真表部分 ................................................................................................................... - 12 -
5.
ADJUSTMENT LOCATIONS（调整位置） ............................................................................................................................... - 14 -
6.
PRINTED CIRCUIT BOARD（印刷电路板） ............................................................................................................................ - 15 T-2398
Main Board(主板) ........................................................................................................................................ - 15 -
T-2399
Input Level Switch Board（输入电平开关板） ............................................................................................... - 16 -
T-2399
Distortion Switch Board（失真开关板） ................................................................................................... - 16 -
T-2415
Frequency Switch Board（频率开关板） ........................................................................................................ - 16 -
7.
BLOCK DIAGRAM（方框图） ................................................................................................................................................. - 17 -
8.
SCHEMATIC DIAGRAM（原理图） ........................................................................................................................................ - 17 -
9.
10.
(1/2)
Level Meter Section（电位表部分） ............................................................................................................... - 17 -
(2/2)
Power Supply/Distortion Section（电源/失真部分） ..................................................................................... - 18 -
PARTS LIST（部件清单） ....................................................................................................................................................... - 19 CABINET REMOVAL（机壳拆除） .................................................................................................................................... - 20 -
-1-
1. SPECIFICATIONS（规格）
Distor on Measurement 失真测量
Measurement Frequency 测试频率
Range 范围
315Hz±5%,1kHz±5%
Op onal 选项
1 Point of 333Hz, 400Hz, and 3kHz
Measurement Distor on 失真测量
Rate range 百分率范围
0.03% to 30%
Measurement Range 测量范围
0.1, 0.3, 1, 3, 10 and 30%,
6 Ranges
Input Level Range 输入电平范围
10mV to 30V
Automa c Level Control 自动电平控制
Range 范围
10 dB
Input Impedance 输入阻抗
100 kΩ unbalanced（非平衡）.
Input Capacitance 输入电容
＜70 pF
Measurement Accuracy 精度
Within ±5% of full scale (However, ±10% of full
scale for 0.1% range)
在满量程的±5%范围内(然而，0.1%范围内为
满量程的±10%)
Fundamental Frequency 基频
Suppression 抑制
More than -76 dB
Level Measurement 电平测量
Input Impedance 输入阻抗
100 kΩ unbalanced（非平衡）.
Input Capacitance 输入电 容
Less than 70 pF
Measurement Level 测量电平
Range 范围
3mV to 30V
Measurement Range 量程
0. 03 ,0. 1, 0. 3, 1, 3, 10 and 30V, 7 Ranges
-2-
Measurement Frequency 频率测量
Range 范围
20 Hz to 30 kHz, within 土 0.5dB
20 Hz to 20 kHz, within 土 0.5dB
(0.03V Range)
Measurement Accuracy 精度
Within ±5% of full scale（在满量程±5%范围
内）
Output Terminals 输出端子
Level Monitor 电平监视器
1v ±5% (at full scale)
Filter Output 滤波器输出
1v ±5% (at full scale)
Power Supply 电源
AC 100, 120, 200, 220 and 240V, 50/60 Hz
Power Consump on 耗电量
LDM 一 177;10VA, LDM-178; 20 VA
Size 尺寸
LDM-177; approx. 7 kg（约 7 公斤）
Weight 质量
LDM-178; approx. 8 kg
Accessories 配件
Pair Plug to Clip cable (LC-2021)
对插夹电缆
一一一 1 ea. (LDM-177),
一一一 2 ea. (LDM-178)
-3-
2. TEST EQUIPMENT REQUIRED（所需测试设备）
The following test equipment is required for Calibra on and servicing of the Model LDM177/178. The suggested specifica ons are the minimum necessary for proper calibra on of this
instrument.
校准和维修 LDH-177/178 型仪器需要以下测试设备。 建议的具体要求是正确校 准该仪
器所需的最低要求。
Test Equipment 测试设备
Minimum Spec 测试设备
 Mul meter 万用表
0 to 20V
Accuracy 精度 <1%
*LEADER Model LDM-853A
 Oscilloscope 示波器
10mV sensi vity 灵敏度
10MHz-bandwidth 带宽
Low capacitance probe 低电容探头
*LEADER Model LB0-323
 AC Voltmeter Calibrator 交流电压表校准器
30mV to 30V 400Hz
 Audio Generator 音频发生器
40Hz to 40kHz
3V to 30Vrms
 AC Millivoltmeter 交流毫伏表
1Vrms full scale 满刻度
100kHz band width 波段宽度
*LEADER Model LMV-181A
-4-
3. CALIBRATION PROCEDURE（校准程序）
3.1. General（一般情况）
Calibra on should be performed ed a er a 30-minute warm-up period. It should also be
confirmed that the unit is connected to the rated power line voltage.
校准应在预热 30 分钟后进行。 还应确认设备已连接到额定的电源线电压。
All adjustments should be completed in the Given order, because some adjustments interact
with others.
所有调整都应按指定顺序完成，因为有些调整会与其他调整相互影响。
During the adjustment procedure, remove the case only when necessary and replace
immediately a er making an adjustment. This will maintain all circuits at constant opera ng
temperature.
在调整过程中，仅在必要时拆下外壳，并在调整后立即更换。这将使所有电路保 持恒定的
工作温度。
3.2. Adjustment of Meter Mechanical Zero（调整仪表机械零位）
Turn the instrument off and allow 30 second for all capacitors discharge.
关闭仪器，让所有电容器放电 30 秒钟。
Rotate zero adjustment screw clockwise or counterclockwise so that the pointer indicates exactly
zero.
顺时针或逆时针旋转调零螺丝，使指针精确地指向零。
A er pointer is exactly at zero, rotate the screw slightly opposite direc on to release tension of
meter suspension.
当指针精确地处于零位后，稍微朝相反方向旋转螺杆，以释放仪表悬架的张力。
3.3. Ini al Control Se ngs（初始控制设置）
The ini al control se ngs to be used for each check and adjustment are listed below. Any
varia ons from these se ngs are stated in the applicable procedure.
每次检查和调整所使用的初始控制设置如下。任何与这些设置不同的设置都将在适用的程
序中说明。
FREQUENCY 频率
1kHz
-5-
INPUT LEVEL 输入电平
30V
DISTORTION 失真
OFF 关
Rear panel 后面板
CIRCUIT GROUND 电路接地
GROUND 接地
MEASURE-CAL 测量-校准
MEASURE 测量
3.4. Power Supply （电源）
 Connect the DC voltmeter between test point and chassis.
 在测试点和机壳之间连接直流电压表。
 Check that the DC voltage is within tolerance according to Table 3-1.
根据表 3-1 检查直流电压是否在公差范围内。
Table 表 31
Test point 测试点
Voltage 电压
Tolerance 公差
TP101
+5V
+4.75V to +5 .25V
TP102
+15V
+14.25V to +15.75V
TP103
-15V
-14.25V to -15.75V
TP104
-5V
-4. 75V to -5. 2.5V
3.5. Level Meter Sec on（电平表部分）
(1) Sensi vity Adjustment 灵敏度调整
 Set 设置:
FREQUENCY 频率
315Hz
INPUT LEVEL 输入电平
1V
DISTORTION 失真
OFF 关
 Connect the voltmeter calibrator to INPUT terminal and set the frequency to 400Hz,
output voltage to 1Vrms.
 将电压表校准器连接至 INPDT 端子，将频率设为 400Hz，输出电压设为 1Vrms 。
 Adjust VR201 for a level meter reading of 1Vrms.
 调整 VR201，使电位表读数为 1Vrms
(2) Automa c Level Controller Adjustment 控制器自动调节
 Set 设置:
FREQUENCY 频率
315Hz
INPUT LEVEL 输入电平
1V
DISTORTION 失真
OFF
-6-
 Connect the voltmeter calibrator to INPUT terminal and set the frequency to 400Hz
output voltage to 1Vrms.
 将电压表校准器连接至 INPUT 端子，将频率设为 400Hz，输出电压设为 1Vrms。
 The level meter should indicate 1V.
 电平表应显示 1V。
 Connect the AC millivoltmeter to TP105.
 将交流毫伏表连接至 TP105。
 Adjust VR101 for AC millivoltmeter reading of 0.5Vrms.
 调节 VR101，使交流毫伏转换器读数为 0.5Vrms。
 Connect the DC voltmeter to TP106.
 将直流电压表连接至 TP106。
 Decrease the calibrator output voltage to 0.18Vrms.
 将校准器输出电压降至 0.18Vrms。
Do no touch the controls on -177/178.
请勿触碰 -177/178 上的控制器。
 Adjust VR102 for DC voltmeter reading of -1.5V
 调节 VR102，使直流电压表读数为 -1.5V。
(3) Frequency Response Adjustment 频率响应调整
 Set 设置: INPUT LEVEL 输入电平
3V
 Connect the audio generator to INPUT terminal and set the frequency to 400Hz, output
level for full scale indica on.
 将音频发生器连接到输入端，设置频率为 400Hz，输出电平为满量程指示。
 Change the frequency to 40kHz.
 将频率改为 4 千赫。




Adjust VC101 so that the level meter indicates full scale.
调节 VC101，使电平表显示满刻度。
Apply the same procedure for other INPUT LEVEL se ngs according to Table 3-2.
根据表 3-2 对其他 INPUT LEVEL 设置采用相同步骤。
Table 表
INPUT LEVEL 输入电平
3V
10V
30V
3.6. Distor on Meter Sec on（失真表部分）
(1) Sensi vity Adjustment 灵敏度调整
-7-
3-2
Adjustment 调整
VC101
VC103
VC102
 Set 设置:
FREQUENCY 频率
315Hz
INPUT LEVEL 输入电平
1V
MEASURE-CAL 测量-校准
CAL 校准
 Set VR301, 315Hz adjustment on the rear panel, to the midrange.
 设置 VR301，后面板 315Hz 调节为中档。
 Connect the audio generator to INPUT terminal and set the frequency to 790Hz, output
level for 1Vrms.
 将音频发生器连接到 INPUT 端子，将频率设置为 790Hz，输出电平为 1Vrms。
 Adjust VR305 so that the distor on meter indicates full scale.
 调节 VR305，使失真表显示满刻度。
 Connect the AC millivoltmeter to OUTPUT terminal.
 将交流毫伏表连接至输出端子。
 Adjust VR304 for an AC millivoltmeter reading of 1Vrms.
 调整 VR304 使交流毫伏表读数为 1Vrms。
 Set 设置:
FREQUENCY 频率
1kHz
 Change the generator frequency to 2.5kHz.
 将信号发生器频率改为 2.5kHz。
 Adjust VR302, 1kHz adjustment. on the rear panel, so that the distor on meter
indicates full scale.
 调节后面板上的 VR302（1kHz 调节器），使失真表显示满刻度。
4. TROUBLESHOOTING PROCEDURE（故障排除程序）
4.1. Troubleshoo ng Aid-1（故障排除 Aid-1）
Confirm that the any equipment used with the LDM-177/178 is opera ng correctly.
确认与 LDM-177/178 一起使用的任何设备都运行正常。
Check all control se ngs, because an incorrect se ng can make a good unit appear
defec ve. If there is any ques on about the func on, refer to the Instruc on Manual for a
correct opera on.
检查所有控制设置，因为不正确的设置会使好的设备出现故障。如果对功能有任何疑
问，请参阅《使用手册》以正确操作。
Check all circuit for visual defects such as broken component, loose connec on, open wire,
poor soldering etc.
检查所有电路是否有可视故障，如元件断裂、连接松动、电线开路、焊接不良等 。
Some troubles can be solved with proper adjustment.
某些故障可以通过适当调整解决。
Check the voltage and waveform as shown in the Schema c Diagram to locate the defec ve
circuit. Start with the power supply.
检查电压和波形，如电路图所示，找出故障电路。 从电源开始。
Voltage and waveform are nominal value, and may vary 10% or more from instrument to
instrument.
电压和波形为标称值，可能会有 10% 或更大的变化。 设备之间的差异更大。
-8-
4.2. Troubleshoo ng Aid-2 （故障排除 Aid-2）
Refer to the simplified block diagram shown in Figure 4-1.
请参考图 4-1 所示的简化框图。
The distor on meter consists of a level meter sec on and a fundamental rejec on
filter/distor on meter sec on. Therefore, troubleshoot the level meter func on first to
confirm that it works properly, then proceed to the distor on meter func on.
失真表由电平表部分和基波抑制滤波器/失真表部分组成。 因此，首先要排除电平表
功能的故障，以便 确认其工作正常，然后继续使用失真仪功能。
The input amplifier has two outputs. One of them VL drives the level meter, and the other
output VD is applied to the high-pass filter and metering circuit for measuring distor on.
The VD becomes constant amplitude with the ALC circuit regardless of input signal level
changes.
输入放大器有两个输出端。其中一个输出端 VL 驱动电平表，另一个输出端 VD 则连接
到高通滤波器和测量失真度的计量电路。无论输入信号电平如何变化 ，VD 在 ALC 电
路中都保持恒定的振幅。
Figure 图 4-1
The distor on factor represented by a percentage between the input signal level and its
harmonic components as follows.
输入信号电平与其谐波分量之间的分辨系数用百分比表示，如下所示。
Harmonic components
Distortion factor(%) =
× 100
Input signal level
谐波分量
失真系数(%) =
× 100
输入信号电平
The input signal becomes constant level at the AGC circuit. Therefore, the distor on factor
can be obtained by the level measurement of the harmonic components which removed
from the input signal at the high-pass filter.
输入信号在 AGC 电路中变成恒定电平。因此，可以通过测量高通滤波器从输入 信号
中去除的谐波成分的电平来获得失真系数。
Figure 4-2 shown the rela on between the fundamental frequency(f0) and harmonic
components (f2, f3, ---). Correct distor on measurement is possible, even the signal
includes frequency varia on such as an output signal of tape recorder, because of high-pass
filter system is used.
图 4-2 显示了基频（f0）与谐波分量（f2、f3、 ）之间的关系。由于使用 了高通滤波
器系统，即使信号包含频率变化（如磁带录音机的输出信号）
，也能 进行正确的失真
测量。
-9-
Figure 4-2
4.3. Troubleshoo ng Aid-3（故障排除 Aid-3）
(1) Overfall opera on is not sa sfactory, or unit is dead. 整体运行不理想，或设备死机。
a. Check that the power supply voltage is within tolerance according to Table 3-1. 根据表
3-1 检查电源电压是否在公差范围内。
Yes 是: Proceed to step (2). 进入步骤 (2)。
No 否: Check and troubleshoot on following points. 检查以下几点并排除故障。
Line fuse:
0.315A me lag for 90V to 132V opera on
0~2A me lag for 180V to 264V opera on
线路熔断器:
运行 90V 至 132V 时滞后 0.315 a
运行 180V 至 264V 时滞后 0~2A
Secondary voltage of the power transformer.
电源变压器的二次电压。
+15V:
IC402 and associated circuit.
-15V:
IC403 and associated circuit.
-5V:
-15V line, D410 and associated and associated circuit.
+5V:
IC401 and associated circuit.
- 10 -
+15V:
IC402 及相关电路。
-15V:
IC403 及相关电路。
-5V:
-15V 线路，D410 及关联电路。
+5V:
IC401 及相关电路。
(2) Level Meter Sec on 电平表部分
a. Setup: FREQUENCY
1 kHz
INPUT LEVEL
30mV
DISTORTION
OFF
CIRCUIT GROUND
GROUND
MEASURE-CAL
CAL
Connect the audio generator to INPUT terminal and set the frequency to 2.5kHz,
output voltage to 30mVrms (85mVp-p).
设置:
频率 1 kHz
输入电平
30mV
失真
关闭
电路接地
接地
测量- 校准
校准
将音频发生器连接到输入端，设置频率为 2.5kHz，输出电压为 30mVrms
(85mVp-p)。
If the sensi vity is too low or no Level meter deflec on, trace the signal from the input
stage to metering circuit by using the oscilloscope to locate the defec ve circuit.
如果灵敏度太低或没有电平表偏转，则使用示波器跟踪从输入级到计量电路的信
号，找出故障电路。
Check that the sine wave is present at MONITOR terminal for 2.8Vp-p.
检查 2.8Vp-p 的正弦波是否存在于 MONITOR 终端。
Yes: Troubleshoot Level meter driver, Level meter and associated circuit. Adjust VR201 if
necessary. Refer to 3.5 (1).
是:排除电位表驱动器，电位表和相关电路。根据需要调整 VR201。参见 3.5(1)。
No: Check that the sine wave is present at emi er of Q104 for 85mVp-p.
Yes- Troubleshoot A enuator-2, Amplifier-1(Q107, 108), contact of INPUT LEVEL
switch and associated circuit.
No- Troubleshoot A enuator-1, Impedance converter and associated circuit.
- 11 -
否:检查 85mVp-p 的 Q104 发射极是否存在正弦波。
是-排除衰减器 2，放大器 1(Q107, 108)，输入电平开关触点和相关电路。
否-故障排除衰减器-1，阻抗转换器和相关电路。
If the Input level meter deflec on is unstable at high sensi vity range, check following
points to locate the defec ve circuit.
- FLOAT-GROUND switch should be set to GROUND
- Resistance between GROUND terminal and chassis for 47 ohms.
如果输入电平计的偏转在高灵敏度范围内不稳定，请检查以下几点以定位故障电
路。
—浮地开关应设置为 “接地” 。
—接地端子与机箱之间的电阻为 47 欧姆。
(3) Distor on Meter Sec on 失真表部分
Confirm that the Level meter sec on works properly before proceed to troubleshoo ng of
the Distor on meter sec on.
确认电平表部分工作正常后，再对失真表部分进行故障排除。
a. Setup: Same as (2) a.
The Distor on meter should indicate full scale.
Yes: Proceed to d.
No: Proceed to b.
设置:与(2)a 相同.
失真表应显示满量程。
是:继续 d。
否:继续 b。
b. Check that the sine wave is present at TP105 for l.4Vp-p.
Yes: Check that the sine wave is present at emi er of Q304 for 2.7Vp-p.
Yes- Proceed to c.
No- Troubleshoot A enuator-3, Amplifier-3, and associated circuit.
No:
Troubleshoot Amplifier-1, ALC and associated circuit.
Adjust VR101 and 102 if necessary. Refer to 3.5 (2).
检查 1.4Vp -p 的正弦波是否存在于 TP105。
是:检查 2.7Vp-p 的正弦波是否存在于 Q304 的发射极。
是-继续 c.
否-排除衰减器 3、放大器 3 和相关电路。
否:
故障排除放大器-1,ALC 和相关电路。
请根据需要调整 VR101 和 VR102。参见 3.5(2)。
- 12 -
c. Check that the sine wave is present at collector of Q307 for 0.14Vp-p.
Yes:
Troubleshoot Distor on meter driver, Distor on meter and associated circuit.
No:
Troubleshoot A enuator-4, Amplifier-4, FREQUENCY, DISTORTION, MEASURECAL, switch and associated circuit.
检查 0.14Vp-p 的正弦波是否存在于 Q307 的集电极。
是:
故障排除失真仪驱动，失真仪和相关电路。
否:
故障排除衰减器 4，放大器 4，频率，失真，测量，开关和相关电路。
d. Distor on measurement
Setup: Same as (2) a.
Set MEASURE-CAL switch to CAL.
If the distor on meter func on not working-correctly, troubleshoot high-pass lter,
Frequency selector, A enuator-3 and associated circuit.
失真测量
设置:与(2)相同 a.
将 MEASURE-CAL 开关设置为 CAL.
如果失真计功能不能正常工作，请排除高通滤波器、频率选择器、衰减器-3 和相
关电路的故障。
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