Crown FM150, User Manual

Page: 62 / 110

5.3.2 Modulation Compensator
Refer to section 2.3.1
5.3.3 Frequency Measurement and Adjustment
Next to the 10.24–MHz VCXO in the RF Exciter circuit on the motherboard, is a 50K potenti-
ometer (R101). Use R101 to set the frequency of the 10.24–MHz VCXO while observing the
output frequency of the synthesizer.
Use one of two methods for checking frequency:
1. Use an FM frequency monitor.
2. Couple a frequency counter of known accuracy to the output of the synthesizer and ob-
serve the operating frequency.
5.3.4 FSK Frequency Offset Control
An FSK signal (used for automatic identification of FM repeaters) shifts the frequencies of
the 10.24–MHz VCXO reference oscillator and the VCO.
Ground pin 4 on the DB25 connector located on the back panel of the transmitter. This will
shift the operating frequency. Adjust R224 for an offset of the operating frequency of about
6 kHz. Un-grounding pin 4 will cause the operating frequency to return to normal.
Use one of two methods for checking frequency:
1. Use an FM frequency monitor.
2. Couple a frequency counter of known accuracy to the output of the synthesizer and ob-
serve the operating frequency.
5.4 Metering Adjustments
5.4.1 Power Calibrate
While looking at RF Power on the digital panel meter, set the Power Calibrate trim potenti-
ometer (R215) to agree with an external RF power meter.
5.4.2 Power Set
With the front panel RF Output control fully clockwise, adjust the Power Set trim pot (R55)
to 10% more than the rated power (33 W for FM30, 165 W for FM150, 330 W for FM300) as
indicated on an accurate external watt meter. If the authorized power is less than the maxi-
mum watts, you may use the Power Set to limit the range of the RF Output control.
5-4 FM30/FM150/FM300 User’s Manual