in different supplies. Also, the +15 VDC supplies do not have the
“Start Up” circuit used in the +5 VDC supplies.
M.2.4
Modulated B- Supplies
The Modulated B- supplies provide a negative voltage to the
Modulation Encoder, which varies with the transmitter audio
input and power level. The effect of the modulated B-voltage is
to control PA Module turn-on/turn-off times. Turn-on/turn-off
times depend on loading on each module, which in turn depends
on the total number of modules which are operating.
At low power levels (including negative modulation peaks), only
a few “Big Steps” are ON, and each PA Module is lightly loaded.
As additional “Big Steps” turn ON, the loading changes consid-
erably and the required turn-on/turn-off times also change. At
higher power levels (more modules turned ON), the loading on
each module does not change nearly as rapidly when additional
modules turn ON or OFF.
To compensate, the B- voltage must be more negative on positive
peaks, but must change more slowly as the transmitter’s instan-
taneous output becomes greater (and more modules are turned
ON). Therefore, the B- voltage must vary in a non-linear manner
as the -(Audio + DC) sample changes. A non-linearity circuit
between the -(Audio + DC) input and the regulator IC reference
voltage input purposely distorts the audio input.
M.2.4.1
Approximate Modulated B- Supply Output Voltages
At an operating power of 25 kilowatts and with 100% modula-
tion, the instantaneous Modulated B- voltage should vary be-
tween roughly -1 and -3 Volts. With no power out, the DC level
should be approximately -1.25 VDC. At negative 100% modula-
tion peaks, the instantaneous voltage should be approximately
-1.35 Volts, and at positive 100% peaks, the instantaneous volt-
age should be approximately -2.85 Volts. This voltage range will
be less at lower operating powers.
There are three adjustments for the Modulated B- supplies: one
on the Analog Input Board (A35) and two on the DC Regulator.
These adjustments are described in SECTION V, Maintenance.
M.2.5
Modulated B- Supplies: Circuit Description
There are two identical Modulated B- supply circuits on the
DX-25U DC Regulator, only the “A” supply associated with
Modulation Encoder (A37) will be described here. The modu-
lated B- supply uses regulator U6, and series pass transistor Q2.
M.2.5.1
-(Audio + DC) Input
The “reference voltage” for the Modulated B- supply is a nega-
tive (inverted) sample of the analog audio signal and DC power
control signal from the output of the Analog Input Board (A35).
The pre-distortion circuit consists of diode CR16, Diode CR17,
and resistors R65, R56, and R53. Zener diode CR20 and resistor
R46 provide a regulated -1.22 Volt reference for this circuit.
Schottky diode CR15 provides protection, preventing accidental
positive reference voltages.
M.2.5.2
Modulated B- Supply Regulator Circuit
The output voltage from the regulator circuit depends on the
reference voltage and the setting of adjustable resistor R51 in the
output voltage sample divider. The error amplifier in U6 com-
pares the pre-distorted reference voltage, at pin 9, and a sample
of the supply output voltage at pin 8. The regulator IC controls
the base current into series pass transistor Q2 to adjust the output
voltage, so that the differential voltage between pin 9 and pin 8
is essentially zero.
The unregulated input is -8 VDC, fused by F3, from the Low
Voltage power supply. Capacitors C23 and C22 provide high and
low frequency bypassing. The unregulated input voltage can be
measured at TP5. The negative output voltage, measured at TP6,
depends on transmitter power and instantaneous modulation
level. Supply voltages for the internal circuitry in the regulator
IC are +8 volts, through R40 to pin 1 (V+) and -8 Volts, through
R47 to pin 5 (V-).
The compensation network is R43 and C30, from pins U6-14/15
to V-. The “Fault Delay” is determined by capacitor C33, from
pin 11 to the unregulated input. Diode CR10, across series pass
transistor Q2, protects the regulator circuit if reverse voltage is
accidentally applied at the unregulated input. Zener diode CR14
provides transient protection and clamping should the regulator
fail. Capacitors C27 and C34 are bypass capacitors for transients.
M.2.6
Other Negative Regulated Supplies in the
Transmitter
Operation of all negative regulated supplies using the UC3834
linear regulator IC is similar to operation of the modulated B-
supply on the DC Regulator except the reference voltage is not
modulated. Other negative supplies return pin 1, “V+ IN” to
ground. The -5 VDC supplies include a start-up circuit like the
one used for the +5 VDC supply; -15 VDC supplies do not have
the start-up circuit. Voltage sample divider resistances, compen-
sation components, and IC driver emitter resistances will be
different in different supplies.
M.2.7
Regulator Output Inhibit Circuit (U2)
CMOS analog multiplexer U2 and associated circuitry will shut
down the B+ and B-regulators until the +230 VDC supply is
energized. This prevents over dissipation of the regulator pass
transistors until air flow is available. Integrated circuit U2 is used
to multiplex 2 different control requirements from one command.
The B+ (+5.75 VDC) regulators are shut down by tying pins 14
and 15 of U3 and U13 to ground (GND). The shut down signals
for these two IC’s are gated together through CR8 and CR35 into
pin 13 of U2. Pin 14 of U2 provides the switched connection to
GND.
The B- regulators are shut down by tying pins 14 and 15 of U6
and U15 to -8 VDC. The U3 and U4 shut down signals are gated
together through CR9 and CR40 into U2-1. U2-15 provides the
switched connection to -8 VDC.
When the PA Power Supply is energized, a “TXONEN” signal
enters the DC Regulator at J3-19 and is routed to pins 9, 10, and
11 of U2 to control the 2 analog switches. The “TXONEN” signal
will be logic LOW when the transmitter is ON. Jumper plug P1
is used for initial setup and troubleshooting of the DC Regulator.
Section M - DC Regulator (A30)
888-2297-002
M-3
WARNING: Disconnect primary power prior to servicing.
