QRO Technologies HF-2500DX Instruction Manual page 20

Resistor R209 is a protection resistor connected in series with the Plate Voltage Circuit. If a short
develops, this resistor prevents the B- from rising to the B+ potential. Thus providing protection
for all the components within the B+ circuit including the 4CX800A tetrodes. Resistors R110 -
R113 & R212 - R215 are discussed under Metering Circuits.
The 12.6 VAC secondary windings on T2 secondary supplies 8 amperes for the amplifier's tube
filaments. The 4CX800A tetrodes are indirectly heated cathodes, and the tube filaments must
heat them for an initial two minute warmup period.
The 12 VAC secondary winding of T2 supplies the voltage for the +12 VDC supply circuit located
on the LV & Bias PCB. +24 VDC is also produced on the LV & Bias PCB using a voltage doubler
circuit using this 12 VAC supplied by T2.
The 120 VAC secondary winding of T2 provides an isolated supply voltage for the +120 VDC
power supply located on the LV & Bias PCB. This supply uses a bridge rectifier diode to rectify
the voltage along with a filter capacitor. The +120 VDC power supply provides the voltages for
the internal T/R switching plus the tube grid bias.
A cutoff bias voltage of -160 V is supplied by circuit consisting of bridge rectifier D307, capacitor
C311. The transmit -70V bias is supply by these components along with zener diodes ZD301,
resistor R304 when relay K4 has been engaged by the T/R circuit.
The Screen Supply PCB develops the screen grid bias of +495 volts. The secondary of T1
supplies 200 VAC. Diodes D401 & D403 along with capacitors C401 & C402 form a voltage
double circuit. These capacitors charge to 2.828 times the supplied rms175 VAC (175 X 2.828
= 495). Zener diodes ZD402 - ZD404 regulate the output voltage. Resistors R402 & R403 are
the bleeder resistors for Capacitors C401 & C402.
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 the antenna. 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
TUBE FILAMENTS
12 VAC SUPPLY
120 VAC ISOLATION SUPPLY
CONTROL GRID BIAS
SCREEN GRID BIAS
RF INPUT CIRCUITS
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