Harris Platinum HT EL 2000LS, Technical Manual

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b. Use the following procedure for filling a syringe with
thermal compound.
1. Required Equipment:
a) A 5mL syringe
b) Zinc oxide (Wakefield™ ) thermal compound
c) A stirrer (clean, no lint)
d) A clean cloth
2. Procedure:
a) On a clean, dry surface open the heat sink compound
jar and stir the compound thoroughly with a clean
stirrer. Make sure there is no settling in the com-
pound before proceeding.
b) Assemble the syringe if necessary.
c) Put syringe tip in the compound up to the beginning
of the barrel of the syringe.
d) Push and pull plunger several times while the tip is
in the compound (2 to 4 times) to make sure there
are no air gaps when filling the syringe.
e) With the tip of the syringe still in the compound,
begin swirling the tip around in the compound while
drawing back the plunger to fill the syringe to 5mL.
f) Remove syringe from compound and clean off care-
fully with the clean cloth.
c. Apply heat sink compound on the RF FET.
1. Required Equipment:
a) Xacto™ knife (blade #11). Use only a fresh blade
for this procedure (no nicks or mars, has not been
used for anything else. When in doubt, change the
blade)
b) Cleaning solvent
c) Q-tip
d) Wakefield compound in new 5mL syringe
e) A clean cloth
f) ESD equipment
2. Procedure:
a) Make sure you are ESD safe through the entire
procedure.
b) Take the FET to be installed and make sure the back
side is clean. Make sure that the heat sink mounting
surface is clean as well. If the surfaces are not clean,
clean them with a Q-tip dipped in cleaning solvent.
Make sure solvent is dry before proceeding.
c) Get the Xacto knife (blade #11). Only use a clean,
fresh blade (this blade should only be used for this
procedure). Measure out a small amount (1-2mm
from the tip of the syringe) of compound from the
dispensing syringe onto the Xacto blade.
d) Apply the compound evenly on the FET by moving
the flat side of the blade in a circular motion on the
back side of the FET. Clean excess compound off the
blade.
e) Holding the Xacto blade at a 45 degree angle or less
from the FET’s surface, gently press down with the
blade edge.
f) Continuing to hold the blade at 45 degrees or less,
and starting at one end of the FET, sweep slowly
across the FET. Made sure the blade does not lift up.
There should be a thin opaque film left on the surface
after sweeping. The gold flashed back of the FET is
slightly concave, the heat sink compound should be
thickest in the center. There should be excess heat
sink compound on the blade. Carefully wipe the
excess compound off on a clean cloth (do NOT try
to re-use this compound).
g) Place FET firmly into the holes of the PC board. Try
to pull the FET up, applying moderate force. If the
FET resists being pulled up, it is well seated. If it is
easily pulled up, clean both surfaces, inspect for
surface irregularities, and try again.
d. Install spacer, levelers and leaf spring. Insure that leaf
spring and levelers are centered over the FET packages and
that the spacer is resting flush with the heatsink. Tighten
the screw securely. The leaf spring should bottom out on
the spacer and the split washer should be fully compressed.
e. Solder the leads using low-temperature solder. Inspect for
solder bridges. Scrape away any flux using a small knife.
Do not use any sprays or liquids that may run under the
transistor and dissolve the heatsink compound. Inspect for
proper flow of solder between the FET leads and the board
foil.
f. Check to see that all bias pots of the quarter module have
been turned fully counter-clockwise before applying any
power.
Refer to the section on Idle Current Testing to set bias controls.
A.4.4
Testing and Replacing Isolation Resistors
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.
In order to test ISO resistors, it is necessary to desolder one of
the leads before testing the resistor with an ohmmeter.
When replacing a flange-mounted ISO resistor, bend the resistor
leads curving upward slightly to provide mechanical strain relief
to allow for differing expansion between the circuit board and
the heat sink. Be sure to clean away the old thermal compound
from the heat sink surface, and apply just enough compound to
the flange of the new device in order to assure a good thermal
interface. After applying reasonable torque to the flange screws,
solder the leads quickly using a hot iron.
A.4.5
Pass FET Replacement
If pass FET replacement is necessary, replace both FETs with the
matching parts. If this is not done there may be a tendency for
one FET to carry more of the current and lead to a repeated
failure.
Platinum™ Series
A-14 888-2457-001
WARNING: Disconnect primary power prior to servcing.