Voltage Regulator Circuit Board - Crown FM30 User Manual

4.8 Voltage Regulator Circuit Board

The voltage regulator board is the longer of two boards mounted under the chassis toward
the front of the unit. It has switch-mode voltage regulators to provide +12, –12, and 20 volts.
It also contains the program detection and automatic carrier control circuits.
Illustration 6–10 and accompanying schematic complement this discussion.
U3E and U3F convert a 38–kHz sine wave from the stereo generator into a synchronization
pulse. In the transmitter, synchronization is not used, thus D9 is omitted.
U4 and U5 form a 20–volt switching regulator running at about 35 kHz. U4 is used as a
pulse-width modulator; U5 is a high-side driver for MOSFET switch Q1. Supply voltage for
the two IC's (approximately 15.5 volts) comes from linear regulator DZ2/Q5. Bootstrap volt-
age, provided by D10 and C14, allows the gate voltage of Q1 to swing about 15 volts above
the source when Q1 is turned on. Current through the FET is sensed by R38 and R38A. If
the voltage between pin 5 and 6 of U05 exceeds 0.23 volts on a current fault, drive to Q1 is
turned off. Turn-off happens cycle by cycle. The speed of the turn-off is set by C13.
U6 is a switching regulator for both +12 volts and –12 volts. It runs at about 52 kHz. Energy
for –12 volts is taken from inductor L2 during the off portion of the switching cycle. The –12
volts tracks the +12 volts within a few tenths of a volt. There will be no –12 volts until current
is drawn from the +12 volts.
Q2, Q3, and Q4 form an active filter and switch, supplying DC voltage to the RF driver,
when the Carrier switch is on.
The program detection circuit is made up of U1 and U2. U1A and U1D and associated cir-
cuitry discriminate between normal program material and white noise (such as might be pre-
sent from a studio-transmitter link during program failure) or silence. U1A and surrounding
components form a band-pass filter with a Q of 3 tuned to about 5 kHz. U1D is a first-order
low-pass filter. Red and green LEDs on the board indicate the presence or absence of pro-
gram determined by the balance of the detected signals from the two filters. U2 and U1C
form a count-down timer. The time between a program fault and shutdown is selected by
jumpering pins on header JP1. For times, see section 5.7. The times are proportional to the
value of R21 (that is, times can be doubled by doubling the value of R21) and are listed in
minutes.
Principles of Operation 4-11