Service manual

Page 13
raw input voltage on the audio output amplifiers in order to allow them to handle the very high
instantaneous currents (up to an amp!) needed for good speech reproduction.
Antenna switch, SWR meter
The DCD board also has an antenna A/B switch and an SWR meter on it (see sheet 2 of 3 on
the DCD/Tuner schematic). Relay K17 and K18 switch between main antennas A and B. The
use of two relays instead of one provides better port-to-port isolation. A portion of the transmit-
ted signal, both forward and reflected signals, is picked up by transformer T1 and fed into
buffer U5. The outputs are clamped to keep the levels from exceeding 5V, and are fed to the
main controller’s A/D converters for measurement. C17, RV1 and RV2 are used to calibrate the
meter so that the measured voltages correspond to correct power levels as measured against an
external wattmeter. If no wattmeter is available, a 50 ohm dummy load allows the meter cali-
bration to come close.
In order to handle both 10W transmitters and 100W amplifiers, the output of U5 is scaled by
resistive dividers R5/R32 and R6/R33. A signal (/NOPA) from the amplifier goes high if the
amplifier is present and enabled. If it is not present, R31 keeps the voltage low, forcing Q6 and
Q7 to be off and removing the scaling factor.
Control circuits are discussed in the Controller theory section.