6.30
Symptom: Consistent Loss of RF
Amplifiers
Any type of consistent or repeated failure indicates a problem
on the RF amplifier or in the location in the transmitter.
The most common method of troubleshooting an RF amplifier
after a failure is to put the repaired amplifier in a known working
step, and to put the working amplifier where the failure first
occurred. This will tell whether the amplifier fault was caused
by the position it was in or by the amplifier itself.
6.30.1
Repeated Loss of Same RF Amplifier In Any
Position
6.30.1.1
Possible Causes
6.30.1.1.1
Defective Transistor Pad
A torn or damaged MOSFET insulator pad can short the tran-
sistor. Check for debris lodged in the pad or sharp burr or high
spot on heat sink.
6.30.1.1.2
Defective Turn On/Turn Off Circuitry
Check all low level circuitry on the RF amplifier for defective
components. If only one side of amplifier fails, then concentrate
on that circuitry. If nothing significant is found, check the drive
phasing with the amplifier fuses removed. Refer to “Measuring
Drive Phasing,” in this section.
6.30.1.1.3
Improper RF Drive
Refer to the “Measuring RF Drive Level” procedure in this
section. If RF drive is not correct and the transistors are not
defective, suspect RF drive transformers T1 and/or T2 and gate
protection transorbs.
6.30.1.1.4
Poor Solder Connection on PC Board
Inspect all solder connections, especially the drive transformer
loads of T1 and/or T2.
6.30.2
Consistent Loss Of An RF Amplifier In One
Particular Position
6.30.2.1
Possible Causes
6.30.2.1.1
Improper RF Drive
The RF drive to the PA Modules must be between 20 and
25Vp-p for proper operation. The phase of the drive must also
be within five degrees of the other modules. Refer to the
paragraphs in this section on “Measuring RF Drive Level,” and
“Measuring RF Drain Phasing” for drive amplitude and phase
measurement procedures. Causes of improper drive amplitude
and phasing are defective RF amplifier transistors, defective RF
drive cable, or poor motherboard connections.
6.30.2.1.2
Improper Drain Phasing
Just as the RF drive must be within five degrees of the other
modules, the phase of the drain switching waveforms of the RF
amplifier MOSFET’s must also be in phase within five degrees.
Even if the RF drive is correct, other problems can cause the
drain phasing to be out of tolerance. To measure the drain
phasing, refer to the paragraphs in this section on “Measuring
RF Amp Drain Phasing.” Causes of improper drain phasing are
a poor connection or wrong tapping of the efficiency coil for the
RF amplifier, incorrect MOSFET transistors, or a defective RF
amplifier output toroid. An RF amplifier that fails from phasing
problems will operate for a short time before failure. During this
condition, the module will operate hotter than the other mod-
ules. This is a good indication of a module operating out of
phase.
6.30.2.1.3
Defective Output Toroid
The output toroid for each RF amplifier must couple the RF
output to the combiner. If the toroid is defective, the amplifier
will not operate efficiently and fail. Check the toroid for cracks
or signs of arcing. Some inspection can be done through the
holes in the combiner cover, but a more through inspection
requires removal of the combiner cover. Refer to the Main
Combiner/Motherboard replacement paragraphs in SECTION
V, Maintenance.
6.30.2.1.4
Improper Control Signal
The Turn-on/Turn-off control signals from the Modulation En-
coder should be at consistent levels for all steps. Check the
Encoder outputs to compare these signals.
6.30.3
Consistent Loss Of Modules In Random Positions
6.30.3.1
Possible Causes
6.30.3.1.1
Analog to Digital Phasing Improperly Set
An improperly set analog to digital phasing circuit will cause
random failures of RF amplifiers especially at the higher steps.
See paragraphs on “Analog to Digital Phasing Check” in SEC-
TION V, Maintenance.
6.30.3.1.2
Modulated B- Improperly Set
An improperly set Modulated B- can cause random RF ampli-
fier failures. See paragraph on “Overall Modulated B- Adjust-
ment” in SECTION V, Maintenance.
6.30.3.1.3
Improper VSWR Circuit Operation
If the VSWR protection circuit, including the Oscillator Sync
circuit, in not set properly, random failures RF amplifiers could
occur during VSWR conditions. To test the VSWR circuitry,
depress the ColorStat™ panel VSWR SENSOR Manual Test
Button. At that time both the Bandpass Filter and Antenna
VSWR LED’s should momentarily illuminate RED, then return
GREEN. Refer to the paragraphs on “Output Monitor” and
“Oscillator Sync” adjustments in SECTION V, Maintenance.
6.30.3.1.4
Improper Overload Settings
If an overload is improperly set or not working, the RF amplifiers
could fail during an overload condition. Refer to the “Overload
Adjustment Procedures” in SECTION V, Maintenance.
6.30.3.1.5
Improper Air Flow
Insufficient air flow should be detected by the Air interlock
circuitry and should shut the transmitter OFF. If the circuit is
defective or defeated, overheating modules could cause prema-
ture failures. Refer to SECTION V, Maintenance, for air circuit
adjustment procedures.
6.30.3.1.6
Transmitter Mistuning
Transmitter mistuning could cause the Power Amplifier stage to
be inefficient and cause modules to run hotter. Refer to the
Section VI - Troubleshooting
Rev. R: 11-11-96
888-2297-002
6-17
WARNING: Disconnect primary power prior to servicing.
