Standard Communications mt930b, Service Manual

Page: 55 / 155

Maintenance
MT930B
Service Manual
Page 6-23
6. For J17 audio de-emphasis, adjust the level to 8 ± 1 dB at 1420 Hz.
7. Use the result as a reference, and find the deviation from J17.
Table 6-2. De-emphasis Deviation
400 700 1 k 1.5 k 2 k 3 k 4 k 5 k 6 k 8 k 10 k 12 k 15 k
50
µ
sec
–0.08 –0.20 –0.41 –0.87 –1.44 –2.76 –4.11 –5.40 –6.58 –8.64 –10.37 –11.84 –13.64
75
µ
sec
–0.15 –0.45 –0.87 –1.76 –2.76 –4.78 –6.58 –8.16 –9.53 –11.84 –13.64 –15.19 –17.08
50 200 400 800 1.42 k 2 k 4 k 6.4 k 8 k 10 k 15 k
J17
9.40 8.76 7.18 3.80 0 –2.32 –6.20 –7.81 –8.29 –8.62 –8.99
Use 100 Hz as the reference for 50 µ sec and 75 µ sec.
Use 1.42 kHz as the reference for J17.
Audio Output Level Measurement
Measurement Procedure:
1. Set the FM signal generator as follows, and input to JB01:
a. Carrier frequency: 6.8 MHz
b. Output level: –9 ± 1 dBm
c. Frequency Deviation:
i. ± 400 kHz p-p (BW 880 kHz)
ii. ± 200 kHz p-p (BW 440 kHz)
iii. ± 100 kHz p-p (BW 330 kHz, 220 kHz)
iv.
±
50 kHz p-p (BW 150 kHz, 110 kHz)
v. ± 25 kHz p-p (BW 55 kHz)
d. Modulation frequency: 1 kHz
2. Set the audio subcarrier frequency to 6800 kHz, level to 100.0, de-emphasis
to FLAT, and connect the audio analyzer to rear panel AUDIO #1, #2, or #3
to measure the output level.
Audio Higher Harmonic Distortion Measurement
1. Set the FM signal generator as follows and input to JB01:
a. Carrier frequency: 6.8 MHz
b. Output level: –9
±
1 dBm
c. Frequency Deviation:
i. ± 400 kHz p-p (BW 880 kHz)
ii. ± 200 kHz p-p (BW 440 kHz)
iii. ± 100 kHz p-p (BW 330 kHz, 220 kHz)
iv. ± 50 kHz p-p (BW 150 kHz, 110 kHz)
v. ± 25 kHz p-p (BW 55 kHz)
d. Modulation frequency: 1 kHz
2. Set the audio subcarrier frequency to 6800 kHz and set the de-emphasis
appropriately and connect the audio analyzer to rear panel AUDIO #1, #2, or
#3, adjust the output level to 18 dBm, and measure the distortion.