Standard Communications mt930b, Service Manual

Page: 54 / 155

Maintenance
Page 6-22 Service Manual MT930B
AGC Measurement
Measurement Conditions:
• Measurement terminal: Rear panel AGC terminal (JW05)
•
Connection resistance: Connect a 10 k ohm resistor between the
+ and – AGC terminals.
Measurement Procedure:
1. Set the AGC control to OFF (MGC).
2. Measure the voltage at the AGC terminals when the MGC control is set to
minimum.
3. Measure the voltage at the AGC terminals when the MGC control is set to
maximum.
Measurement Specification:
−−−− AGC output range: ± 0 (control set to minimum) ± 5 V (control set to
maximum)
Audio Frequency Response Measurement
Measurement Procedure:
1. Set the FM signal generator as follows, and the 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)
2. Set the audio subcarrier frequency to 6800 kHz, and connect rear panel
AUDIO #1, #2, or #2 to the audio analyzer.
3. Select the appropriate audio BW, and set the frequency deviation according
to step 1,c.
4. Set the modulation frequency to 100 Hz, and adjust the level so the rear
panel audio output is 18 ± 0.1 dB.
5. Select the appropriate audio de-emphasis for other than J17, take the out-
put level when the modulation frequency is set to 100 Hz, and find the de-
viation in Table 6-2. Calculate the difference compared to the standard de-
emphasis curve for 50
µ
sec or 75
µ
sec.