QRO Technologies HF-2000 Instruction Manual - page 19
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provides a step-start when the amplifier is switched on. Components C12, R10, D10 provide the
time delay to minimize the inrush current. Resistors R11 & R12 limit the inrush current while the
delay circuit cycles. After a 1 to 2 second delay the contacts on relay K-1 close allowing the full
current to be applied to the primary of T-1.Terminal Block TB1 allows for the selection of the line
voltage to be used. See the Terminal Block Wiring Diagram for the correct wiring. Cooling Fan
B1 is provided 100/120 VAC by connection across only one of the primary windings at TB1. B1
always operates on 100/120 VAC regardless of which AC line voltage is used.The secondary
windings of T1 provides the following secondary voltages: 5.6 VAC tube filament, 12.0VAC +12
VDC Power Supply, 110 VAC RF I/O Switching Supply, 1200 VAC Plate Voltage Supply
PLATE VOLTAGE
The 1200 VAC output of T1 is connected to a full-wave voltage doubler circuit consisting of the
components on HV Capacitor PCB-105 and HV Diode PCB-110. Diodes D101 - D110 rectify the
secondary AC voltage and capacitors C201 - C208 filter this voltage. Capacitors C101 - C110 are
connected across the diodes to protect them against transients. Resistors R201 - R208 are
connected across the filter capacitors to equalize the voltage drop across each capacitor. These
resistors also act as bleeder resistors for the filter capacitors to discharge them when the amplifier
is turned off. One of the red 1200 VAC secondary lead is connected to the junction of C204 &
C205, and the other red 1200 VAC secondary lead is connected to the junction of D105 & D106.
During the AC line cycle when the lead at C204 & C205 junction is positive, diodes D101 - D105
conduct and capacitors C201 - C204 are charged. During the other half of the AC line cycle, the
red lead connected to the junction of D105 & D106 becomes positive. Capacitors C205 - C208
charge and Diodes D106 - D110 conduct. The two capacitor strings are in series across the load,
and the voltages of each capacitor group add together. The capacitor bank charges to twice the
peak voltage of the 1200 VAC secondary voltage (1200 VAC x 1.414 x 2 =3,393 VDC).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 3-500 triode. Resistors R301 - R304 are
discussed under Metering Circuits.
TUBE FILAMENT AND BIAS
The 5.2 VAC secondary windings on T1 secondary supplies 15 amperes for the amplifier's tube
filament. The filament voltage is fed to tube V1 through RFC2 which is a bifilar wound coil on a
ferrite rod core. This coil forms a choke to raise the tube filaments above RF ground potential so
that the driving voltage is not short circuited. Capacitors C59 & C60 keep RF out of the filament
supply circuit. In the transmit mode, the center tap of the filament winding (5.2 VAC) is connected
to ground through the bias circuit located on the LV & Bias PCB-260. This circuit consists of bias
transistor Q302, transient suppressor TSD301, bias resistor 301, bypass capacitor C302,
resistors R304 & R311, relay K301A, diodes D303 - D310. This bias circuit develops +11 VDC
operating bias for tube V1 and limits the idling plate current. In Standby (receive) Mode, a +24
VDC voltage is formed across resistor R307. This voltage is applied to the tube filaments, in
addition to the AC voltage, to bias the tube beyond cut off so that no plate current flows.
12 VAC SUPPLY