1.1. Theory of Operation:
The oscillator and series regulator for the FA 390A are one assembly (See drawing M-2259-C). The
regulator consists of Q1, Q2, Q3, Q4, and Q5, and associated components. The oscillator consists of Q6,
Q7, Q8 and associated components.
The series regulator is keyed on by placing a negative going signal from the timer on terminal 5 through a
470W resistor R2. This negative signal biases Q1 to a full-on condition. When Q1 turns on it biases Q5
on, and applies a short pulse to the base of Q4 through C2, a 10 MFD capacitor. With Q5 biased on and
Q4 pulsed on, Q3 is biased on. Q3 is connected to Q2, a high power transistor mounted on one end of the
mounting bracket in a Darlington configuration. When the Q3-Q2 circuit is driven on it causes Q2 to pass
current through its collector to the oscillator circuit. Capacitor C2 charges rapidly to full potential as
supplied by Q1, therefore Q4 must obtain base drive through R10 and divider network R17, R18, and R19.
Voltage regulation is obtained by proper adjustment of adjustable component R18.
At the end of the on signal, Q1 is turned off which turns Q5 off, which removes drive form the remainder of
the circuit, thus turning the regulator off.
Form 6310 0238