QRO Technologies HF-2000 Instruction Manual - page 20
20
The 12 VAC secondary winding of T1 supplies the voltage for the +12 VDC supply circuit located
on the LV & Bias PCB-260.
110 VAC ISOLATION SUPPLY
The 110 VAC secondary winding of T1 provides an isolated supply voltage for the +110 VDC
power supply located on the LV & Bias PCB-260.
RF INPUT CIRCUITS
When the amplifier is in the Stanby Mode (receive), Relay RY1 and Relay RY2 (located on the
RF I/O Switching PCB) are in their normally open state. Thus, RF energy supplied from the
transceiver passes through RF Input Connector (J2), RY2, RY1, and RF Output Connector (J1)
to theantenna. The amplifier has been bypassed. Capacitors C70 & C71 tune the mismatch that
occurs with the relay contacts located inside RY1 & RY2.When the amplifier has been keyed
(Transmit Mode). RY2 closes and the RF energy supplied by the transceiver passes though
RY2's closed contacts to the Tuned Input PCB-450. The signal then passes through the selected
input matching network and then on to coupling capacitors C57 & C58located at the tube
filaments.
TUBE
The amplifier's tubes are connected in a class AB2 grounded grid circuit. RF driving power is
applied to the filaments in the normal cathode driven configuration.As mentioned in the Filament
& Bias Section, RFC2 holds the filaments above RF ground.Pins 2, 3, and 4 of the tube are
internally connected together and are connected to ground.Parasitic chokes PC1 & PC2 are
connected to the plate lead to suppress any VHF parasitic oscillations. The positive side of the
power supply (B+) is connected to the tube's plate through RFC3 (Plate Choke). Capacitor C56
provides a low impedance path to ground for any RF energy that my get through RFC3.
Capacitors C54 & C55 are DC blocking capacitors. They allow the RF signal to enter the output
tank circuit, and they block the DC high voltage from entering the tank circuit.Fan B1 circulates
cooling air around the tubes.
RF OUTPUT CIRCUITS
The tuned output circuit of the Amplifier is a pi network in conjunction with a 4:1 transformer(L3).
The pi circuit transforms the plate load impedance down to 200 ohms. The 4:1 transformer further
transforms the 200 ohms down to 50 ohms. L3 has a secondary function to provide a broadband
L coil for harmonic suppression.Band Switch SW3A progressively shorts out the unused portions
of tank coils L1 & L2. The tank coil turns in use are tuned to resonance by Tune Capacitor C5.
Load Capacitor C50 is tuned to complete the impedance match between the tube and the 4:1
transformer L3. Transformer L3 is then connected to RF Output connector J1 through T/R relay
RY1. Capacitor C52 provides additional capacitance for C51 on 80 & 160 meters. Capacitor C53
provides additional capacitance for C51 on 160 meters. If DC plate voltage should enter the tank
circuit due to a short in either DC Blocking Capacitors C54 & C55, the DC voltage has a path to
ground through the 4:1 Xfmr (L3). This will short-circuit the high voltage supply which will blow
the AC line fuses. Also, DC plate will be prevented from entering the Antenna.