Load Pcb; Pa, Iboc Pcb - Harris ZX2500 Technical Manual

ZX Series
Jumpers JP1, JP2, and JP3 disable the alarms for fans 2, 4, and 5, respectively, allowing
these fans to be permanently removed from lower power models without causing a
persistent low fan speed alarm.
4.10 16X Load PCB
Consult drawing 801-0203-321
The 16X load board monitors the flange temperature of the output assembly ballast
loads and passes the highest reading on to the transmitter control system for fault
checking and telemetry.
Thermistor RT6, attached to the flange of a ballast load, controls the gain of amplifier
U1A. The temperature variable gain is applied to a 0.1V reference developed by
voltage divider R17-R13. The result is a temperature sample based on a natural
logarithmic scale. Similar circuits provide the same functionality for the remaining
ballast loads. A diode-or circuit formed by CR1-CR2 ensures that only the high
temperature reading is passed via RFI network R20-C17-R21 and J1-11 to the
MAX_LOAD_TEMP line.

4.11 PA, IBOC PCB

Consult drawing 843-5569-071
The RF signal is amplified in the PA modules with a nominal gain of 14 – 20 dB,
depending on frequency and operating mode. RF drive power enters via J1-C and
passes through input matching network C3-C1-T1 to FETs Q1, Q2. Q1 and Q2 operate
in push-pull to produce approximately 400-450W FM power into 50 ohm load
conditions. Their outputs are impedance transformed via output network C8-C9-L5-
T2-C10-C19-C20-C32 and passed to the PA backplane via J1-14,15. 50V DC power is
supplied to Q1 and Q2 via choke network C11-L1-C28-L2. A portion of the 50V power
is also used to derive a gate bias voltage via bias stabilization network R11-R12-R13-
R8-CR1. The gate bias can be modified via a control signal at J1-U via R5. A control
voltage of 0V places the module in a positive bias – class AB state, whereas a control
voltage below approximately -2V pinches off the amplifier and drives it further into
class C for progressively more negative voltages.
Thermistor RT1 controls the temperature sensing circuit on the PA backplane via J1-N.
12/20/11
WARNING: Disconnect primary power prior to servicing.
Section 4 Theory of Operation
888-2595-001 4-11