Service manual

Page 29
T/R switch and low pass filters
The 10W output from T5 is fed into one of five low pass filters shown on Sheet 7. If the 100W
amplifier is not installed, the bandpass filters are also tapped at the 10W signal point and fed
through a transmit/receive PIN diode switch network consisting of diodes D3 and D7, chokes
RFC1 and RFC2, and output coupling capacitors C80 and C113. Resistors R40 and R42 bias
the PIN diodes at half the supply voltage when enabled by Q1. The low pass filtered receive
signal is then passed to the RXBPF board’s “Main Antenna” input. Of course, if the rig is used
in full duplex mode, this path cannot be used and the receive antenna input is automatically se-
lected to avoid feeding high power RF into the receiver.
The low pass filters are 7-element Elliptical LC filters.
Control logic is shown on Sheet 8 and consists of 8-bit registers that produce the control bits
already discussed.
100W amplifier
If the 100W amplifier is installed, the T/R switching is done on this board instead of the trans-
mitter, so the RF cable going to the BPF board is removed from the transmitter (J8) and recon-
nected to the 100W amp (J6). Similarly, the transmitter output is re-routed from J5 on the DCD/
Tuner board to the 10W RF input (J5) on the 100W amp, and the antenna output of the 100W
amp (J2) is routed to the DCD/Tuner board’s antenna jack (J4).
Fan control
Sheet 1 shows decoding logic, fan control and the high power supply switching. A voltage di-
vider consisting of thermistor TH1, which touches the heatsink, and R28 produces a nominal
(cool heatsink) voltage of about 1.09V. The reference input to the comparator is set to 2.88V.
As the heatsink gets hot, the thermistor’s resistance goes down, increasing the voltage. Capaci-
tor C69 stabilizes the voltage. Once the voltage rises to 2.88V, the comparator’s output turns off
(open), and current then flows through R31 and the base-emitter junction of Q10, turning on
Q10. This shorts out R32 and causes maximum current to flow through the two fans. Resistors
R29 and R30 provide hysteresis, which keeps the comparator from oscillating at the trip point.
When the fans and/or lack of transmitting reduces the temperature enough, the comparator trips
the other direction and restores the fans to idle (low speed).
T/R switching
Unregulated DC input voltage at J5 pins 3, 4 and 5 provides the high current to the final ampli-
fier transistors, and this voltage is not switched. It is present at all times. The amplifier’s T/R
switch is switched via signal TXPVCC