resistor and the 30K resistor and diode CR11 or CR12 on the RF
amplifier.
The voltage level at U62-7 will drop below the voltage at U62-6
and force U62-1 to logic LOW. This will force U66-8 to a logic
HIGH and illuminate the red Blown Fuse segment of DS1 on the
Modulation Encoder and the red “RF AMP” led on the Color-
Stat™ panel.
Under normal conditions, the +230 VDC PA Power Supply
provides current flow through the fuse and not through the 56K
resistors and LED’s on the RF amplifier. Diodes CR11 and CR12
are reverse biased and no current will flow from the +22 VDC
supply through the 30K resistor on the RF amplifier. Instead,
current flows through the resistors on the Modulation Encoder.
Under this condition, the voltage at U62-7 is greater than the
voltage at U62-6 which causes U62-1 to be logic HIGH.
When the transmitter is OFF and the Low Voltage Supply is ON,
the PA Power Supply is not energized and there is no +230 VDC
supply to F1 and F2 on the PA Modules. This applies a virtual
ground to F1 & F2 and provides a current path for the +22 VDC
supply. Therefore, the circuit would indicate a “Blown Fuse”
fault. However, the Blown Fuse segment on DS1 will not illumi-
nate since the +5 VDC supply to the Modulation Encoder is
muted by the DC Regulator until the transmitter is turned ON.
NOTE
The LED supply voltages are muted by the DC Regulator when
the transmitter is OFF. In order to activate the DS1 display when
the transmitter is OFF, P1 on the DC Regulator must be placed
in the TEST position. It is normal for the BLOWN FUSE LED
and the PA OFF LED to be red under this condition. Return P1
to the NORMAL position after troubleshooting.
L.3.7
Clip Circuit
L.3.7.1
Function
The Clip Circuit keeps the “Binary Steps” from switching OFF
if the audio input level exceeds the maximum positive peak
modulation capabilities of the transmitter.
L.3.7.2
Circuit Description
As the analog input signal to the A/D Converter increases, the
“Binary Steps” begin turning on in a binary sequence to increase
the RF output in small increments. When all “Binary Steps” are
ON and the audio level is still increasing, the next “Big Step”
turns ON and all “Binary Steps” turn OFF. This sequence con-
tinues as long as the audio input level continues to increase. If
all “Big Steps” are ON and all “Binary Steps” are ON, a further
increase in the audio input would cause the A/D converter to
provide a turn-on signal for a “Big Step” that does not exist. This
would turn OFF all “Binary Steps” and the RF output would drop
by almost one “Big Step” (actually by 63/64 of a “Big Step”).
The “Binary Steps” would then turn ON again. This would result
in a “Sawtooth” peak instead of a “Flat Topped” peak.
Eliminating the “Sawtooth” and providing a “Flat Topped” posi-
tive peak is accomplished by simply holding all “Binary Steps”
ON if excessive audio input causes the A/D converter to attempt
to turn ON a “Big Step” that does not exist.
A “CLIP” patch cable is used to jumper P15 to the first unused
“Big Step” latch output. If the A/D converter attempts to turn this
unavailable step ON, the line will go HIGH and drive inverter
U57-9. The inverted signal at U57-8 is buffered by U59, to
become the “CLIP-L” signal at TP5. This signal is inverted at
U56-8 and fed to one input of U60 and U61 in the “Binary Step”
data lines. This logic HIGH input holds all “Binary Steps” ON,
as long as the CLIP signal is present.
NOTE
Clip patch (P15) should be moved to the first unused output if the
module is used for FlexPatch™ operation.
L.4
FlexPatch™
P1 through P8 are 16-pin sockets with U-shaped jumper plugs.
If a PA Module fails, a high step module (starting at RF90 and
working down) can be substituted for the failed module without
turning the transmitter off by using the FlexPatch™ function.
Refer to FlexPatch™ operation in SECTION VI, Troubleshoot-
ing, for additional information.
L.4.1
Example
Assume that the step six (RF38) PA Module has failed. Because 23
PA Modules are turned ON for 25 kW carrier, the output power will
decrease. Step six will be ON except when negative modulation
peaks exceed -70%, and the failed step will also increase distortion
slightly. PA Module RF90 is only used for extremely high positive
modulation peaks at 30 kW carrier, and can be substituted for step
six with (in the worst case) only a slight reduction in positive peak
capability. This module substitution can be done on the Modulation
Encoder without turning the transmitter OFF, or physically ex-
changing modules, as follows:
•
Remove the U-shaped jumpers for RF38 and RF90 at P5-6
(11,12) and P8C-2 (3,4). With the jumpers removed the
modules remain OFF.
•
Connect a jumper from the control signal (latch output) for
RF38 at P5-6 (11) to the turn-on/turn-off inverter input for
RF90 at P8C-2 (4). In this configuration whenever RF38 is
required to be ON, RF90 will turn ON and substitute for
the faulty RF38.
•
The DX-25U can now be operated at normal performance,
until the next regular maintenance period. Up to three PA
Modules may be substituted in this manner.
NOTE
FlexPatch™ can be used for “Big Step” RF amplifier substitu-
tions ONLY! If a “Binary Step” fails it is necessary to turn the
transmitter OFF and replace the RF amplifier. The failure of a
“Binary Step” amplifier will result only in a slight power reduc-
tion and minimal increase in distortion.
L.4.2
Single RF Amp Momentary Test
Sheet 3 of the Mod Encoder Schematic diagram shows this circuit.
The circuit consists of switch (S2), which connects pins P9-1 and 2
to +5 Volts through R28, to provide a logic HIGH signal.
This circuit is useful for troubleshooting. When any U-shaped
jumper is removed for a “Big Step”, and a patch cable is then
DX-25U
L-4
888-2297-002
Rev. T2: 06-27-97
WARNING: Disconnect primary power prior to servicing.
