Broadcast Electronics FX-50, Instruction Manual

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©2011 Broadcast Electronics
ACTIVE FILTER. The output of U11B is also applied to an active third-order 5 Hz low-pass filter consisting of
capacitors C29 through C31, resistors R25 through R27, and loop filter buffer U11A. The filter removes the
reference frequency component to provide a dc auto matic frequency control (AFC) voltage to the modulated
oscillator through resistor R31.
LOCK DRIVER. The output of lock/unlock sensor U4B normally applies a HIGH through resistor R39 to lock
driver U13A for a locked-loop condition. U13A is activated by a slow charge/rapid discharge circuit consisting of
resistors R39, R40, diode D2, and capacitor C42.
As long as the output of U4B is HIGH, the potential on C42 will maintain U13A output HIGH. This HIGH will: 1)
illuminate front-panel LOCK indicator DS5, 2) bias transistor switch Q3/Q4 ON to remove the RF inhibit from the
rear-panel terminal strip, and 3) en able the AFC relay.
If an unlock condition exists, the output of U4B will go LOW which rapidly discharges C42 through D2 and R40
and applies a LOW to U13A. When this occurs, the output of U13A will go LOW to extinguish the lock
indicator, disable the AFC relay, inhibit the RF, and activate a dual rate loop driver.
DUAL RATE LOOP DRIVER. The LOW output from U13A is routed to a dual rate control network consisting of
R42, R43, C44, and D3. This circuit is identical in operation to the slow charge/rapid discharge circuit previously
described. The circuit forces the output of U13B HIGH which enables light dependent resistors LDR1, LDR2, and
LDR3 in the active filter circuit to increase loop lock response.
LOOP LOCK RESPONSE. Increased loop lock response is accomplished by LDR1, LDR2, and LDR3. When enabled
during an unlocked condition, LDR1 will shunt the 5 Hz lowpass filter and route the output from U11B directly
to U11A. LDR2 will shunt resistor R31 to rapidly charge capacitor C35 through resistor R34. Modulation
coupling capacitor C37 will be rapidly charged through LDR3.