Service manual

Page 30
Amp bypass
Sheet 2 is the heart of the 100W amplifier. A 10 Watt input signal can either be fed to the am-
plifier or it can be switched directly to the T/R switch via relays K1 and K2. If the amp is by-
passed, the BON signal (derived from the PAON bit before the relay driver on Sheet 1) will
stay low, assuring that the amplifier is biased off by U3/Q7. When the amp is enabled, PAON
goes high, switching the 10W input signal to the 100W amp and making BON go high, which
enables the bias circuit and sets the amplifier for Class B operation. This provides more effi-
ciency that a Class AB amplifier, allowing for higher power out. Class C is usually used in CW
modes, but the conservatively rated parts can easily operate in Class B while still providing
over 100W out.
Drive reduction at higher frequencies
R17, R18 and C9 form an equalization network to reduce the drive requirement on higher
bands.T1 couples the signal into the amplifier.
Final current measurement
U3, R19, R20, Q5 and R21 form a current measurement circuit. When current flows through
sense resistor R19, a small voltage is developed across it. Since it is a .005 ohm resistor, a 20A
current would result in a 100mV drop. This is small enough not to impact the voltage delivered
to the final transistors. The higher voltage side of R19 also appears at R20. Op-amp U3 sees the
lower voltage on pin 2, and since an input at the negative terminal also appears at the positive
terminal while also providing a high impedance load, the net effect is to mirror the voltage of
the sense resistor across R20. Using the previous example, 100mV across the 100 ohm R20
causes 1mA to flow through it. Since the op-amp’s + input is also high impedance, that same
current must flow through Q5, developing a voltage across R21. (There’s also base-emitter cur-
rent flowing through R21, but it is equal to the collector current divided by the beta of the tran-
sistor, and thus can be ignored). 1mA through the 3.16K R21 is 3.16V. Thus, a 20A current
flowing to the final transistors is converted to 3.16V for measurement by the microprocessor.
The output of Q5 is fed into a series resistor and 5.1V Zener diode to keep the voltage going to
the microprocessor from spiking above the microprocessor’s rating.
The amplifier
The amplifier transistors are operated in push-pull. A positive swing on the AC input signal on
the secondary of T1 is amplified by Q8 but not by Q9. When the AC waveform goes negative,
the situation is reversed and Q9 amplifies while Q8 is off. The two halves of the signal are re-
assembled by transformer T2. The center tap of T2, which is at AC ground by virtue of C6 and
C25, feeds the DC supply to the collectors. T2 has a 4:1 turns ratio, so the approximately 24Vp-
p maximum swing on the primary appears as 96Vp-p on the secondary. Since transformers do
not affect power transfer appreciably (except for some minor losses), the 100Wrms signal that
is developed by the primary (24Vp-p = 8.5Vrms; 8.5V*20A = 170W input power; the effi-
ciency is about 60%, which results in about 100Wrms) is also seen at the secondary. With a sec-
ondary voltage of 96Vp-p (34Vrms), and with a power level of 100W, the AC current flowing