The operation of the Driver Supply Regulator is also dependent on
the operation of the AUTO module from the Driver Encoder/Temp
Sense Board. If the AUTO Driver D7 is ON, the Driver Supply
Regulator will decrease the voltage to D8A and D8B.
If the DC supply goes DOWN, the Driver Supply Regulator will
INCREASE the voltage to D8A and D8B.
If the DC supply goes UP, the Driver Supply Regulator will
DECREASE the voltage to D8A and D8B.
Proper operation of the Driver Supply Regulator can be deter-
mined by changing the overall drive level and noting the opera-
tion of the regulator circuit.
To change the overall drive level, move S1 on the Driver En-
coder/Temp Sense Board to the ON position. This will enable the
spare driver D6. If the Regulated D8A and D8B readings on the
RF MULTIMETER decrease, the board is working properly.
E.5.1
Symptom: Driver Sect D8A +VDC and Sect
D8B +VDC Both High
E.5.1.1
Possible Cause: No +15 VDC
Check the Reg15 VDC position on the RF MULTIME-
TER. This voltage should be present any time primary power is
applied to the transmitter, even if the transmitter is “OFF.” If this
voltage is zero (or very low), possible causes include no +22
VDC input or defective U1E. Check for +22 VDC on the front
panel multimeter; check connector to J2 on the Driver Supply
Regulator. To check U1 operation, you can remove the Driver
Supply Regulator assembly and check on the bench with an input
of +22 VDC at J2-1.
E.5.1.2
Possible Cause: Defective U2
If U2 output remains LOW, both Driver Supply Regulator output
voltages will be high. Check U2 operation by removing the
Driver Supply Regulator assembly, and checking on the bench
by applying an input of about +22 Volts at J2-1. When OPEN
LOOP ADJUST control R2 is adjusted over its range, U2 output
should vary from about +1.5 to +10 Volts.
E.5.1.3
Possible Cause: Defective S1
If the Gate Voltage at Q2 remains low (no input voltage from S1),
both Driver Supply Regulator outputs will remain high. Refer to
the “bench check” under “Defective U2,” above.
E.5.2
Symptom: One Output Voltage Is +100 To
+110 Volts, Other Can Be Adjusted.
E.5.2.1
Possible Cause: Shorted MOSFET in a series regula-
tor section (Q3, Q4, Q5, Q6, Q7, or Q8)
Remove the regulator assembly and check MOSFETs. If a MOS-
FET is shorted, its gate-to-source zener diode should also be
checked as a precaution.
E.5.3
Symptom: Both Driver Supply Regulator Out-
put Voltages Are Zero.
E.5.3.1
Possible Cause: No +115 VDC
Remove all primary power and check F10 on Fuse Board A24 in
the high voltage power supply compartment. If F10 is open,
check for possible short circuits to ground in the +115 VDC
supply components, cabling, and on the Driver Supply Regulator.
If F10 is good, check for loose connector or other open circuit
between the +115 VDC supply output and the Driver Supply
Regulator.
E.5.3.2
Possible Cause: Driver Supply Regulator component
The fault is probably in the Driver Supply Regulator, and could
be any of the following:
a. Defective U2. If the input to Q2 remains HIGH, both
Driver Supply Regulator output voltages will remain
LOW. Check U2 operation by removing the Driver Supply
Regulator assembly, and checking on the bench by apply-
ing an input of about +22 Volts at J2-1. When OPEN LOOP
ADJUST control R2 is adjusted over its range, U2 output
should vary from about +1.5 to +10 Volts.
b. Shorted Q2. Check Q2, using the out-of-circuit MOSFET
ohmmeter check in Section 5, Maintenance (The ohmme-
ter check used for bipolar transistors will NOT check
MOSFET’s).
E.5.4
Symptom: One Driver Supply Output Voltage
is Zero, the Other Can Be Adjusted.
E.5.4.1
Possible Cause: Shorted Gate-to-Source Zener Diode
(CR8, CR11, CR12, CR14, CR16, or CR17)
Use an ohmmeter to check the zener diodes in the faulty section
(Q3-Q4-Q7) circuit or Q5-Q6 -Q8 circuit). You should read a
high resistance in one direction and a low resistance in the other
direction. One shorted zener diode will cause both MOSFETs to
remain cut off, so that output voltage for that section is zero.
E.5.5
Symptom: Section D8B Voltage Increases Be-
fore Section D8A Voltage R100 Volts.
E.5.5.1
Possible Causes: Voltage Offset is Too Low
A low offset voltage in the regulator section could be caused by
a leaky or shorted transistor Q1, or a zener diode that is shorted
or is conducting at a low voltage. Remove all primary power from
the transmitter, remove the driver regulator assembly from the
transmitter, and check these components.
E.5.6
Symptom: Open Loop Operation is Correct,
Closed Loop Operation is Faulty.
E.5.6.1
Possible Cause: No RF Sample Voltage
If there is no RF sample voltage, Driver Supply Regulator output
voltages will be high, because the regulator will attempt to
increase Driver output. Adjusting CLOSED LOOP ADJUST
control (R12) to minimum may reduce Driver output. Check the
coaxial cable and connectors between RF Drive Splitter A15 and
Driver Supply Regulator A22 for continuity.
E.5.6.2
Possible Cause: Shorted Diode in Bridge Rectifier
CR1-CR4
Check bridge rectifier diodes CR1 through CR4 for shorted
diodes (the diodes in this bridge can be checked in-circuit, using
an ohmmeter).
E.5.6.3
Possible Cause: Defective U2
Operation of U2 can be checked with the Driver Supply Regula-
tor assembly on the bench, using a +22 VDC DC supply at J2-1.
DX-25U
E-4
888-2297-002
WARNING: Disconnect primary power prior to servicing.
