Harris Platinum HT EL 2000LS, Техническое руководство

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c. Choose the correct soldering equipment for the job. Use
tips that are the appropriate size for the components in-
volved. Use a grounded iron when installing static-sensi-
tive components (most semiconductors).
d. Choose a soldering temperature just hot enough to melt the
solder quickly, while as low as possible to prevent damage
to the new components. An iron with a temperature adjust-
ment is best. Typical settings are:
650°F for small chip caps
750°F for RF FET tabs
800°F for coax cables and wiring on large pads.
e. Make the new joint as mechanically sound as possible
before making the electrical solder connection. Provide
mechanical strain relief for leads on components which are
flange-mounted.
f. Clean all flux residue away from the area when finished.
When working around devices where thermal compound
is used, be sure not to allow solvents to flow between the
device and the heat sink, which can cause the heat thermal
compound to dissolve.
g. Be sure to search for and remove solder splashes, solder
bridges, loose solder wire or wire lead clippings, and
screws before replacing the cover. Loose metal inside the
module can lead to short circuits, which can cause serious
damage to the module and possible injury.
A.4.2
Quarter Module Replacement
CAUTION
DO NOT REPLACE ACTIVE QUARTER MODULES WITH PASSIVE
BIAS QUARTER MODULES, THE PCMS ARE INCOMPATIBLE.
Platinum quarter modules can be field replaced with another
quarter module FACTORY TUNED to the same channel. The
gain of each quarter module is adjusted by the value of the quarter
module input pad. The input to output phase relationship is set
by TL1, the phase setting coax. This coax must remain with the
quarter module, use the replacement cable already attached to the
REPLACEMENT quarter module for proper phasing. Replace-
ment quarter modules are furnished with the bias for PA usage,
for DRIVER usage the bias must be reset.
A.4.3
RF FET Replacement
IMPORTANT
The RF amplifier FETs are sensitive to damage from static electrical
discharge, and should be handled in an anti-static environment. A
grounded working surface, grounded iron, and electrostatic discharge
bracelet should be used.
IMPORTANT
IN ORDER TO PROTECT THE NEW FETS FROM ACCIDENTAL
DAMAGE TO OVERCURRENT, BE SURE TO TURN OFF THE BIAS
(FULLY COUNTER-CLOCKWISE) TO ALL FOUR FET POSITIONS
ON THE QUARTER MODULE BEFORE INSTALLING THE NEW
FETS.
IMPORTANT
WHEN CLEANING THE OLD THERMAL COMPOUND FROM UN-
DERNEATH THE FET AFTER REMOVAL, USE A SWAB WITH JUST
ENOUGH SOLVENT TO CLEAN THE SURFACE. DO NOT USE TOO
MUCH SOLVENT, AND DO NOT USE AN AEROSOL SPRAY
CLEANER, AS EITHER MAY SEEP UNDERNEATH NEARBY FETS
AND DISSOLVE THE THERMAL COMPOUND FROM UNDER
THEM, CAUSING PREMATURE FAILURE.
WARNING
RF TRANSISTORS, ISOLATION RESISTORS, AND INPUT ATTENU-
ATORS CONTAIN BERYLLIUM OXIDE (BeO) CERAMIC, A HAZ-
ARDOUS MATERIAL.THE LIDS ARE MADE FROM Al2O3 AND ARE
HARMLESS. THE BeO IS HARMLESS WHILE INTACT, BUT THE
DUST IS TOXIC. AVOID CRUSHING OR BREAKING THE BeO CE-
RAMIC, AND DISPOSE OF FAILED DEVICES PROPERLY.
The Philips FET (ON4402H) is used for both low band and high
band modules. Each FET is marked with a gain code and a
threshold code. For replacement the gain code is the most impor-
tant. The quarter module has been assembled in the factory with
FET’s that have the same gain code. When the quarter module is
aligned the gain is set with an attenuator on the input. Therefore
the FET being replaced must have the same gain code as the other
FET’s on the quarter module for proper performance. The gain
code is a number (3 through 7) located above and to the left of
the ON4402H marking on the cap.
Each gain code has a part number assigned to it. These are shown
in the following table:
Gain Code Harris Part Number
3 380-0737-003
4 380-0737-004
5 380-0737-005
6 380-0737-006
7 380-0737-007
Once a failed FET is isolated, remove it from the board using the
following procedure:
a. Turn off the bias to all four FETs by rotating the bias
control pots counter-clockwise.
b. Remove the clamp holding down the transistors.
c. Using a 45 Watt soldering iron with a wide blunt tip,
desolder the leads lifting them with a small knife. It is
important to use enough heat to quickly flow the solder and
work quickly so as not to damage the foil.
d. Remove the old heat sink compound. Use a small amount
of solvent, such as Isopropyl Alcohol, on a swab, being
careful not to allow it to run. Do not use sprays of any kind,
as this may dissolve heat sink compound from underneath
nearby FETs.
e. Re-flow the solder left on the foil where the tabs will seat.
Be sure the surface is smooth and that no solder bridges
remain.
To install the new FET:
a. Tin the bottom of the FET leads lightly with solder.
Appendix A - RF Amplifier Modules
888-2457-001 A-13
WARNING: Disconnect primary power prior to servcing.