Service manual

Page 28
is identical to the ones used on the RXBPF board for the receiver discussed earlier. The only
difference is that the TXBPF board selects the band based on 4 bits of band data from U1
(Sheet 8). This band data is compatible with that used by Yaesu’s linear amplifiers.
Voltage Controlled Amplifier, ALC
The bandpass filtered signal from J13 is next fed into another NXP SA603 voltage controlled
amplifier (U8). The BUFALC control voltage (Sheet 9) provides the 2.5V to 3.5V required to
change the gain of this amplifier over a 30dB range.
K13 (Sheet 9) selects either the internal 0-4.1V ALC control voltage from output A of DAC U5
(Sheet 6) or an external ALC voltage from an external amplifier. The 0 to –4.1V voltage pro-
vided by most amplifiers is inverted first by U11 (Pins 5,6,7). The selected ALC voltage is then
passed through differential amplifier U11 (pins 1,2,3) where it is conditioned, resulting in the
2.5V to 3.5V control voltage for U8. This amplifier (U8) along with the drive amplifier (U17)
and the RF Proc amplifier (U20) are the sole sources of adjustable transmitter power gain.
Transverter output
The 1mW output of U8 is buffered by Darlington transistor Q31 to provide a transverter output.
Driver
The 1mW output of U8 is amplified by Q14 by about 10dB and then transformer coupled to the
power amps. Sheet 6 shows the power amplifier stages. Q3 amplifies the signal up to 1 Watt,
which is then coupled into the paralleled final amplifiers consisting of Q4 and Q5. Q4 and Q5’s
bias point is changed from Class B (for CW and FM modes) to Class AB for SSB and AM
modes. This is done by Q6 and Q12, “Class” (U2 pin 12) is the control bit that turns Q8 on and
off. When the transmitter is off (output of Q16, HTxEn, low), Q13 is off, so TXPVCC is at a
low voltage that is insufficient to bias Q3, keeping any low level RF from feeding through.
When Q13 turns on (TXPVCC = 8.4V), Q3 receives bias, allowing the RF to pass to the finals.
10Watt PA
The 1W output of the driver is fed into Q4 and Q5, which are out of phase with each other by
180 degrees thanks to toroid transformer T3. The outputs are recombined by T5 to produce the
final 10 Watt output signal. Transformer T4 serves as the DC collector inductor for both transis-
tors. A portion of the output signal is sampled by one-turn windings on the secondary of T5 and
fed back to the inputs to provide some automatic level control.
A protection circuit consisting of U13 and Q26 guarantees that if anything—static charges,
glitches, software errors, etc.— removes drive from any of the relays that connect the 10W out-
put to the low pass filters, the TXPVCC signal that enables the transmitter will be removed.
Since the relays take up to 15 ms to switch, the solid state circuitry causes the RF drive to be
removed before the relays actually open. This protects the finals from the high voltage flyback
pulse generated by T5 when its load is removed.