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them properly. The higher power one is
FORWARD, orientation is per the mark-
ing on the coupler support bracket.
h. Return the APC ON-OFF switch to the
APC position.
5.7.2.6. Reject Load - Balance Detector
Fault Calibration
a.
Tum the Transmitter OFF. Tum off the
circuit breaker(s) on the Amplifier(s).
b.
Unplug the reject load detector connec-
tor, J8, at the rear
of
the Controller.
c.
Using a small DC power supply, apply
6.0 volts to the J8 connector pins. J8-2 is
positive.
d.
Adjust the
BAL
SET control, R16, for a
REJECT LOAD fault LED indication.
The TX ON command resets the fault.
There is considerable hysteresis in this
control.
e.
Tum the amplifier circuit breakers back
on.
5.7.2.7. Setting Foldback Power Level
When the reject balance or the
PA
fault
occurs, the power
level is folded back to a safe level to pre
vent damage to the Transmitter.
a.
Adjust the Transmitter for normal TPO
at 100%.
b.
Tum the Transmitter OFF. Tum off the
circuit breaker(s) on the Amplifier(s).
c.
Unplug the reject load detector connec-
tor, J8, at the rear
of
the Controller.
d.
Using a small DC power supply, apply
8 volts to the J8 connector pins. J8-2 is
positive. This should cause the REJECT
LOAD fault LED on the front panel to
trip.
e. Tum the amplifier circuit breakers back
on.
f.
Turn
the
TX
ON.
Adjust
the
FOLDBACK control, R63, for 20%
TPO.
5.7.2.8.
Low
Power Trip Set
a.
Place an ohmmeter across the POWER
OUT FAULT line, TB
1-5
and common.
b. Tum the Transmitter ON and adjust the
output power to 85% TPO
(or
any de-
sired alarm setting).
5-4
c. While observing the ohmmeter, adjust
the PO TRIP control, R9, for a low resis-
tance reading, <500 ohms.
5.8. Procedure for Removing an
RF
Power
Transistor
from
a
PA
Module
CAUTION
BEFORE
THE
FOUOWING
PROCEDURE
IS
INITIATED,
THE
PA
MODULE
MUST
BE
RE
-
MOVED
FROM
THE
TRANSMITTER
AND
PLACED IN A SUITABLE WORKPLACE.
NONE
OFTHETRANSISTOR REPLACEMENI
PROCEDURES
SHOULD
BE
PERFORMED
WHILE
THE
MODULE
IS
CONNECTED
TO
THE
TRANSMITTER.
NOTE
If
a failure has occurred
in
an
PA
mod-
ule, the transistors must be replaced
in
pairs. QI and Q2, or
Q3
and Q4, or
Q5
and
Q6,
or Q7 and
Q8
(
Ql
-Q2
and Q3-
Q4
in HT 250FM).
The
RF
devices used in this module are
sold as Matched Pairs.
NOTE
Note the placement
of
the resistors
lo-
cated close to each matched pair
of
RF
transistors. Remove resistors
as
neces-
sary to gain access
to
transistors to be
removed. Also, one end
of
inductors L3,
L4, L5, and L6, can be lifted to gain eas-
ier access.
a.
Before removing transist
or,
note orien-
tation
of
leads and body to ensure you
can install the new transistor properly.
b.
With a modified cutting tool (Harris P
/N
929 9326 001) cut the leads
of
the tran-
sistor adjacent to the ceramic cap
of
the
transistor.
c. Remove the screws holding the transis-
tor
to
the heat sink and remove the tran-
sistor.
d.
Hold the transistor leads remaining on
the pc board with needle nose pliers and
remove the leads with a soldering iron.
NOTE
Use
a soldering iron
of
sufficient watt-
age
to
remove
the
leads,
but
do
not
overheat or damage to the pc board will
result.
888-2312-002
5.9.
Procedure
to
Mount
New
Tran-
sistors
to
PA
Module
a.
Clean and inspect transistor position on
the heat sink to ensure heat sink area
where transistor mounts is flat and even.
b. Brush
on
a
thin,
even film
of
heat sink
compound (Harris
PIN
555
0100
005)
where the transistoris to be placed on the
heat sink.
Do
not put compound
on
the
screw holes in heat sink.
c. Wipe
on
a thin, even layer
of
heat sink
compound on the bottom
of
the transis-
tor flange that mounts to the heat sink.
d.
Place the transistor on the heat sink.
Ensure that body and leads are orien-
tated properly as noted previously in
transistor removal procedure.
e. Insert two screws into transistor mount-
ing holes and into heat sink. Alternately
turn the two screws in the transistor
flange clockwise; using only minimal
pressure until both screws are snug.
Final torque for the two screws should
be 6-inch-lbs.
f. Use a torque screwdriver for this opera-
tion to ensure proper transistor to heat
sink heat transfer.
g.
Using a constant temperature 65-watt
soldering iron, solder the tabs
of
the
transistor to the proper positions on the
pc board. Use only enough heat to flow
the solder over the transistor leads.
h.
Replace any resistors or inductors that
were removed.
i.
Remove excess solder flux from the
transistors and resistors by brushing
with a suitable flux remover available in
electronic supply houses.
j. Reinstall module in transmitter. Bring
up drive level slowly and check for
proper operation
of
the PA.
WARNING:
Disconnect primary
power
prior
to
servicing.
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