QRO Technologies HF-2500DX Instruction Manual page 22

your Transceiver. Capacitors C608 & C609 form a voltage divider that couples some of the RF
driving voltage to Diodes D604 & D605. When the RF driving voltage exceeds the ALC threshold,
the diodes rectify the negative half cycles. Capacitor C610 filters and bypasses this voltage, while
resistor R608 provides isolation. The negative voltage appearing at ALC Output connector (J5)
is coupled back to the transceiver to control its gain and reduce "flat topping" voice peaks due
to overdrive. This allows you to control the output of your transceiver without reducing your
transceiver's drive.
SCREEN GRID METER CIRCUIT
Screen grid current meter M-2 provides an indication of screen grid current between 0 and 150
milliamperes. The screen grid current meter is connected in parallel with shunt resistors R07 &
R308 located on the LV & Bias PCB. The screen grid return circuit is through the cathode circuit
(B-). Diodes D308 & D309 plus capacitor C314 provides M-2 with transient protection.
PV (PLATE VOLTAGE) MULTIMETER CIRCUIT
When you select this function with switch SW501 located on the Meter Switch PCB, resistors
R110 - R113 located on the HV Diode PCB form a voltage divider to measure plate voltage.
Resistors R110 - R112 form a series multiplier resistance for meter M-1 while resistor R1131
forms a current shunt for the meter. Diodes D1 & D2 plus capacitor C75 provides M-1 with
transient protection.
IP (PLATE CURRENT) MULTIMETER CIRCUIT
When you select this function with switch SW501 located on the Meter Switch PCB, meter M-1
indicates the plate current drawn by the tubes between 0 and 2.80 amperes. It is switched in
parallel with resistors R311 located on the LV & Bias PCB. The voltage drop across this resistor
is measured. The meter scale is calibrated to indicate the plate current by using the fixed parallel
resistance value of R311 and the voltage across them.
SV (SCREEN VOLTAGE) MULTIMETER CIRCUIT
When you select this function with switch SW501 located on the Meter Switch PCB, resistors
R212 - R 215 located on the HV Capacitor PCB form a voltage divider to measure plate voltage.
Resistors R212 - R214 form a series multiplier resistance for meter M-1 while resistor R215
forms a current shunt for the meter.
RF I/O SWITCHING PCB AND BIAS RELAY SWITCHING CIRCUITS
All RF T/R relays (RY1 & RY2) and bias relay (K4) are connected in series in the amplifier keying
circuit. The switching voltage is supplied by the +120 VDC 80 milliampere supply located on the
LV & Bias PCB. This circuit includes bridge rectifier D301 & filter capacitor C301. The +160 VDC
output is fed to voltage divider resistors R305 & R306 which are surface mounted on the chassis.
+110 VDC keying voltage is dropped across resistor R306 and fed back to R301 and R302
located on the LV & Bias PCB. 12 volts is dropped across relays K1 and K2. The remaining +98
VDC is passed to RY1 and R601 located on the RF I/O Switching PCB. 24 volts is dropped
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