Service manual

Page 11
Theory of Operation
The Sienna HF transceiver is organized into five major blocks:
• DC Power conditioning (DCD board)
• Control (Controller, Front Panel and VFD boards)
• Receiver (Receiver, BPF and IF Filter boards)
• Transmitter (Transmitter, TXBPF, Tuner and 100W amplifier boards)
• PC (PCPS and PC boards)
Refer to the Block Diagram on page 43 and the Schematics starting on page 44.
DC Power Conditioning: DCD board
Over-voltage. Under-voltage, reverse polarity and overcurrent protection
Kit rigs sometimes fail to do some basic power conditioning on the input, making them failure-
prone. The Sienna includes over- and under-voltage protection, over-current protection (fuses),
and reverse polarity protection. In addition, the internal DC voltages of +5V, +9V and –9V are
derived from regulators which have additional input and output protection circuitry.
Refer to the DCD board, sheet 3. The DC input from Anderson Powerpole connectors is pro-
tected from reverse polarity by diode D6. Capacitor C21 adds some protection against static dis-
charge and shunts any high frequency energy to ground.
R26 and zener diode D9 tap a small amount of current from the input even when power is off in
order to provide a 2.7V DC reference for comparator U7, an LM393. Capacitor C22 assures
that any rapid changes in the input voltage will be swamped out, keeping the reference voltage
stable until the input voltage drops well below normal.
R9, R10, D7 and C20 provide a voltage divider with long attack and very fast decay, setting the
low trip point at about 1/3 of the input voltage. When the voltage rises above 2.7*3 (8.4V), the
output of comparator U7 (pin 1) goes high. As long as the other comparator output (pin 7) is not
low, the DC voltage will be enabled. Diode D7 assures that when the voltage drops, capacitor
C20 is discharged quickly.
R24, R25 and C23 form a similar divider and stabilizer to detect when the voltage is above
15V.
Resistors R11 and R12 on one comparator, and R13 and R23 on the other, add a couple of Volts
of hysteresis to keep the output of the comparators from oscillating at the trip points.
If the output of either comparator goes low, transistor Q2 turns off , causing K19, a 25A auto-