Furuno FS-1570, Service Manual

Page: 149 / 322

5.5 Power Amplifier (PA)
5-32
5.5 Power Amplifier (PA)
5.5.1 FS-1570T PA board (05P0735)
The amplifier circuit on PA board consists of drivers, Q1 and Q2 and push-pull
amplifiers Q3 and Q4. The gain is about 27 dB. 0.3 W (about 10 Vp-p) RF signal from
TX/RX board is boosted up to about 150 W.
PA bias current is adjusted to 200 mA by R25. When measuring the current, the
ammeter is placed in series to L4 in PA Vc circuit.
The driver bias current is about 30 to 40 mA. No adjuster is provided. To check the bias
current, measure the voltage across R27 (0.22
Ω
) and use formula, I
BIAS
=V
R27
/0.22.
The ground of PA board is isolated by C39, C40 and C50 from chassis ground. When
measuring voltage, use the ground on PA board.
Fig.5.5.1 Block diagram of FS-1570 PA board
VC and IC signals
PA collector voltage (VC) is adjusted by R37. The VC detector detects the voltage and
sends it to TX FIL board. VC is displayed numerically in Power SET UP menu. PA
collector current (IC) is adjusted by R33. The current is detected by R47 and R48 and
displayed in Power SETUP menu as “IC”.