QRO Technologies HF-2500DX Instruction Manual page 23

across R602 and RY1. 12 VDC is then dropped across RY2 and R603. The remaining +62 VDC
is dropped across resistor R604. Capacitor C601 and resistors R601 & R602 plus diode D602
form a break make delay circuit for RY1 & RY2. This helps to make the proper latching and
unlatching sequence for relays K301, RY1, and RY2. Transistor Q601 plus associated
components form a electronic switch to key the Amplifier utilizing +15 VDC on transmit supplied
by the transceiver through connector J4. The Amplifier can also be keyed by sinking the 80
milliamperes to ground through connector J3 and the transceiver's keying relay. The
Operate/Standby Switch SW4 located on the front panel must be closed for the keying circuit to
work. Otherwise the amplifier will be bypassed.
SCREEN GRID PROTECTION CIRCUIT
The screen grid protection circuit consists of the following components located on the LV & Bias
PCB: R305, C312, K1, R310, C317, C318, D310, D311, Q2 K3. When screen current flows, a
voltage drop develops across R305. If the current flow is large enough to cause a voltage drop
sufficient enough to latch relay K1, a +24 volts is delivered to the base of Q2 through resistor
R310. This causes Q1 to conduct causing relay K5 to latch. This breaks the VOX line causing the
amplifier to go into a bypass state. Cutoff bias is supplied to the tubes, and the input rf signal is
removed from the grids of the tubes. K5 also supplies +24 volts to the grid trip light located in the
Fault reset switch (SW6) located on the front panel. When switch SW6 is pressed, a break in the
+ 24 volt circuit occurs. This causes K5 to reset which reconnects the VOX line.
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