Service manual

Page 31
in the secondary is thus about 3 Amps rms.
Stability filter and T/R switch
RF appearing at K3 is routed to a high pass filter consisting of toroids L14 and L15 and capaci-
tor C70. with a breakpoint at about 1.5MHz. Thus the amplifier cannot be used at full power
below this frequency. If the PTT signal is active, the TXPVCC voltage is enabled, and current
flows through R23, RFC5, D3, RFC6 and R24, biasing D3 at about 6V and allowing the AC
waveform to pass through it via PIN diode action. D3 is large to be able to handle the high AC
current.
Low pass filters
The Xcv output signal is then routed to the low pass filters consisting of L1-L12, K4-K13, and
the silver mica capacitors. Control signals from U1 and U2 select one bank of these filters de-
pending on band. The output of the filters is fed to the antenna output.
Receive switching
When the amplifier is not transmitting, the signal appearing at the antenna input is routed
through the low-pass filters to the receiver (C21, D5, D13 and C22 on Sheet 4). Since this path
must be disabled during transmit, and since high voltage appears at the Xcv point, a circuit con-
sisting of U4 and surrounding parts creates a low current, 100V backbias voltage that keeps D5
and D13 turned off. When PTT is removed, Q1 turns off the backbias circuit and Q2 quickly
shunts the voltage to ground, which also allows the normal diode biasing to work, turning on
PIN diodes D5 and D13 and passing the signal to the receiver. In order to enable high speed
QSK, this solid state T/R switching occurs within 1ms of removal of the PTT signal.
Amp enable
When the rig is placed in “power save” mode, it is undesirable for any accidental transmitter
enable signal to turn on the transmitter and amp. The amplifier uses one control bit to drive
Q12, which keeps the TXPVCC signal from going high.