Imd Improvement; Bias Adjustment; Vc Reading Adjustment; Ic Reading Adjustment - Furuno FS-1575 Service Manual

2. IMD improvement

As DRV board output power increases, IMD (intermodulation distortion) gets
increasingly worse. To avoid this problem, the CR14 limiter detects output power from
the Q4 and Q5 to reduce the bias of these amplifiers.

3. Bias adjustment

For the adjustment procedure, refer to information on pages 5-35 and 5-39. To make
check or adjustment of bias current, be sure to disconnect the [TX OUT] connector from
the TX board.
Adjustment
TR/FET
Point
Q4 and Q5 R79
Q3 and Q4 R81
Q1 R80

4. VC reading adjustment

For the adjustment procedure, refer to information on page 5-36.
Use "R85" to make adjustment so that VC reading displayed on the RT window or on
the TONE TEST window will come to the voltage (24V) between the collector and
emitter of the PA transistor when selecting [MENU] -> 1.TEST -> 3.TONE TEST.

5. IC reading adjustment

For the adjustment procedure, refer to information on page 5-37. Use "R82" to make
adjustment so that IC reading displayed on the RT window or on the TONE TEST
window will come to the collector current of the PA transistor when selecting [MENU]
-> 1.TEST -> 3.TONE TEST.
Concretely speaking, insert a DC ammeter in the 24V power supply line of the PA board,
and then make measurement of PA IC.
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It is strictly prohibited to reproduce the document in whole or in part without prior written permission of FURUNO.
Table 7.2.8 Bias Currents of PA Board
Bias
Measurement Point
Current
400 mA Make measurement of current of the 24V line.
450 mA TP3 - TP4: 40 mV
150 mA TP1 - TP2: 75 mV
7-27
7.2 Transmission Circuit