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HARRIS
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WARNING: Disconnect and lockout AC primary power prior to servicing
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9.2.4.1 Controller Power Supply
The rectified V+ flows through steering diode CR58 to filter capacitor C22. D6, a three
terminal reference diode develops a 6.06 reference voltage, REF#2, that can be
monitored at TP2. These circuits are active whenever AC power is present.
When AC power is first applied to the supply, the V+ biases FET Q7 on, the bias
voltage clamped by steering diode CR30 to REF#2. The low ohmic state of Q7 removes
the bias from the series regulator transistor Q8, effectively turning off the 24 volt
supply. When the transmitter PA cabinet control system logic has a satisfied air switch
closure, it will send current through the opto-isolator Q4. The transistor in Q4 now
conducts and removes the bias from FET Q7, allowing the Q8 to generate 24 volts,
effectively turning the 50V supply on.
The turn on sequence starts with 6 volts of bias on Q 16, 17 and 18, this will turn those
transistors on, resetting the SCRs to off. During turn-on, all three LEDs will flash
momentarily.
9.2.4.2 50 V Supply Regulation Circuitry
U5A is the differential amplifier that compares the supply output voltage with reference
REF#1. This reference develops voltage when the 24 volts is turned on and charges C9.
This time constant was chosen to ramp up the output voltage at a rate that does not
cause a over-current condition as the large 1.44 farad capacitor bank charges. U5B is a
differential amplifier that has the + input tied to +12 volts and the inverting input is the
output of U5A. This feedback control voltage is a input to the inverting terminals of the
six opamps that control the SCR firing angle. The non inverting inputs are connected to
the proper AC line voltage sample from transformer T2. When the AC sample exceeds
the feedback control voltage, the transition is capacity coupled to the bipolar and FET
transistors that generate the drive for the trigger transformers. The pulses gate the SCRs
into conduction.
As the 50V supply load increases the circuit will fire the SCRs earlier in the AC line
cycle, sending more energy to the choke input filter network. During an AC power
outage, the drop in V+ as compared to REF#2 will turn on transistor Q20, this
discharges the ramp timing capacitor, C9, to limit the charging current when AC voltage
returns. The voltage adjust, R41 adjusts the over a range of 46 to 56 volts. The delay
balance adjustments R62 to R66 adjust five of the SCRs to balance for even firing
angles. This adjustment is for minimum ripple. The output voltage regulation is 0.5% no
load to full load and the ripple is less than 150 mV p-p.
