C10, L3. Refer to the DX-25U Overall Schematic in the Draw-
ing Package for circuit details.
6.14.1.4
No Drive To The Driver Stage
The Driver Modules require a minimum of 20Vp-p of drive to
each module. This drive is generated on the Oscillator, and
amplified by the Buffer and Predriver. If any of these stages is
inoperative an Underdrive fault will occur. With only the Low
Voltage on, the Oscillator, Buffer, and Predriver Indicator LED’s
should all be GREEN. Depress S1 on the LED Board to turn the
Oscillator ON and activate the RF Sense circuits. The Predriver
LED should now change to RED until the PA Power Supply is
energized. At that time, the Predriver LED will change back to
GREEN when all drive levels are normal in all stages. If the
Oscillator or Buffer LED’s are RED, troubleshoot that particular
stage. To measure the RF drive to the Driver Modules, refer to
the “Measuring RF Drive” procedure in SECTION V, Mainte-
nance.
6.14.1.5
Driver Module Failure
The transmitter has a gain controlled Driver stage with redun-
dant Driver Modules to maintain drive level if a module fails. If
additional modules fail, it may not be possible for the control
circuits to keep the drive level within limits. Check for a Driver
Module failure by viewing the LED’s on each of the Driver
Modules. Depress the LOW power button and note if any of the
LED’s illuminate RED before the transmitter shuts OFF. If any
LED’s illuminate RED, remove all power from the transmitter
and replace or repair the modules. If a spare module is not
available, exchange the defective module with a PA Module,
starting with RF90.
6.14.1.6
Excessive RF Amplifier Failures
The failure of a large number of PA Modules could load the drive
level down enough to cause an Underdrive Overload. To check
for this type of failure, observe the PA Module LED’s through
the interlocked inner doors. Depress the LOW power button and
note any LED’s that illuminate during the step-start sequence.
If more than five LED’s are RED on any combination of PA
Modules, repair the modules before proceeding with further
attempts to troubleshoot an Underdrive problem.
6.14.1.7
Driver Supply Regulator Failure
If the Driver voltage is present, (VDC deflecting to-
wards +110 VDC during the step-start sequence), the Driver
Supply regulator can be checked. Operate the RF Multimeter
switch to the DRIVER D8A position. Depress the LOW power
button. The meter indication should deflect upward before the
transmitter shuts back down. If the reading does not deflect upward,
the Driver Supply regulator could be defective. Refer to SECTION
E for more information on Driver Supply Regulator.
6.14.1.8
Driver Supply Regulator Loop Select
If the Driver Supply Regulator is the problem, it may be possible
to get the transmitter operational by switching the regulator loop
select switch S1 to the OPEN LOOP position. This switch is set
through the cover over the Driver Supply Regulator in the Driver
Compartment. If the transmitter will operates, the problem is in
the Closed Loop regulator section of the Driver Supply Regu-
lator. The transmitter will operate normally with the loop set in
the OPEN position, but it will not have the Automatic gain
control feature enabled.
6.14.1.9
Defective Driver Encoder Signals
Check the PA Module control signals from the Driver En-
coder/Temp Sensor Board, A19. Most of the GREEN LED’s on
the Driver Modules should illuminate when the PA Power
Supply is energized. Under normal conditions D6 and D7 are
held off in reserve capacity depending where S1 and S2 are
positioned. Another module in the D5, D9, or D10 position may
be factory selected to be off. Refer to SECTION S, Driver
Encoder/Temp Sense Board, for additional information.
6.14.1.10
Severe Driver Mistuning
The control circuits will normally compensate for slight tuning
problems. If the mistuning is severe the regulator will not be
able to keep the drive level in range. To check the Driver stage
tuning, refer to “Driver Adjustments” in SECTION V, Mainte-
nance. It is possible for one of the Driver tuning capacitors to
have failed and change the tuning. Failed capacitors can some-
times be identified by checking for abnormal temperature after
operating for a short time.
6.15
ColorStat™ panel Overdrive Fault
6.15.1
Possible Causes
6.15.1.1
High AC Line Voltage
The RF drive regulation should compensate for most line vari-
ations. However, if the AC line increases are greater than the
regulation capabilities of the Driver, an OVERDRIVE overload
can occur.
6.15.1.2
Defective Driver Encoder Signals
Some of the Driver Amplifiers are selected to be off under
normal conditions. If circuitry fails on the Driver Encoder, some
or all of these modules could turn on which could create an
OVERDRIVE fault. Refer to SECTION S, Driver En-
coder/Temp Sense Board, for additional information.
6.15.1.3
RF Amplifier ON/OFF Circuitry
The on/off circuitry on the amplifiers could fail in such a way
as to permanently activate the RF amplifier. The most probable
cause would be a shorted Q5 or Q6. See SECTION C, RF
Amplifier, for more information.
6.15.1.4
Driver Supply Regulator Failure
If the transmitter incurs an Overdrive fault as soon as the PA
Power Supply is energized, it is still possible to see if the
regulator has a problem before the transmitter turns off. Operate
the RF MULTIMETER to the DRIVER D8A position. Monitor
the meter reading while depressing the LOW power button and
note the meter deflection during the step-start sequence. It will
probably deflect near the +115 VDC mark. Now operate the RF
MULTIMETER switch to the DRIVER D8B position. Again
depress the LOW power button and note the meter deflection.
If the meter deflects upward toward the +115 VDC mark, the
Driver Supply regulator is turning on both sections due to some
Section VI - Troubleshooting
Rev. R: 11-11-96
888-2297-002
6-11
WARNING: Disconnect primary power prior to servicing.
