Harris DX 25U, Техническое руководство

Страница: 163 / 274

When the amplifier is turned ON, Q5 and Q6 and diodes CR5
and CR6 provide conduction paths for the RF drive signal.
When the amplifier is turned OFF, transistors Q7 and Q8 conduct
and the RF drive signal is clamped at ground through CR7, CR8,
CR9 and CR10. The positive voltage required to turn on the
power MOSFETs is several Volts, much larger than the junction
drop across the diodes.
Transistors Q13 and Q14 switch in capacitors C12 and C13 when
the module is OFF. This will simulate the gate capacitance of
MOSFETs Q3/Q10 and Q4/Q11 to keep the load on the drive
stage constant.
C.3.6
RF Output
The output signal for each half-quad appears at the junction of
the four MOSFETs. Section A output leaves the module through
P1-1/2/3/4; section B output leaves the module through P1-
7/8/9/10. Capacitor C8 provides dc isolation between the out-
puts.
C.4
Troubleshooting
C.4.1
Symptom: Blown Fuse Indicator Illuminated
C.4.1.1
Possible Cause: Shorted MOSFETs
An open fuse probably indicates that one or both MOSFETs in
that half-quad is defective. You can continue to operate the
transmitter until a normal shut-down period, the open fuse will
cause no further damage. FlexPatch™ can be used to substitute
for a failed module without shutting the transmitter OFF, to
restore normal transmitter performance. Refer to SECTION VI,
Troubleshooting, and SECTION L, Modulation Encoder, for
information on using FlexPatch™ to substitute PA Modules.
The MOSFETs will have to be removed from the circuit in order
to perform the following test.
C.4.1.1.1
Handling MOSFETs
Due to the fragile nature of the gate of a MOSFET, special care
in their handling is required. The gate junction may be destroyed
by static electricity if the static electricity is allowed to discharge
through the MOSFET. For example, walking across a carpet to
pick up a MOSFET that is not protected by antistatic packaging
could result in the destruction of the MOSFET. A static charge
could build up on a person walking across the carpet. This static
charge will eventually have to be discharged. Discharging to the
MOSFET could damage the MOSFET. Transistors which are in
circuit are immune to this damage. The MOSFET transistors are
shipped in antistatic packaging. The transistors should remain in
this packaging until they are to be used or tested.
C.4.1.1.2
Removing MOSFETs
NOTE
DO NOT TRY TO PRY THE HEATSINK AWAY FROM THE PC
BOARD BEFORE REMOVING TRANSISTORS FROM THE
HEATSINK. THE PC BOARD MAY BE DAMAGED AND THE
HEATSINK MAY DISTORT.
•
Remove all the screws from heatsinks and transistors. In
most cases, the transistor will stick to the heatsink because
of the seal created by the transistor pad. This seal will have
to be broken before a heatsink can be removed. Remove the
screw holding the MOSFET to the heatsink and gently pry
the transistor away from its heatsink.
•
Remove the heatsinks one at a time starting with the outer
most sink. Break seals on transistor pads as each pair is
exposed.
•
Unsolder the MOSFETs from the pc board.
C.4.1.1.3
Testing MOSFETs
The MOSFETs may be checked using an ohmmeter with a
battery voltage between 3 Volts and 18 Volts. A Simpson 260,
which uses a 9 Volt battery on the Rx10k scale, works quite well.
This test will show how a MOSFET can be switched “on” and
“off” by charging and discharging the gate of the MOSFET.
Place the transistor face up on a non-conducting surface. Connect
the positive lead of the ohmmeter to the drain (center lead) of the
transistor and connect the negative lead to source (right lead).
Touch a jumper from gate (left lead) to source to turn the
MOSFET “on” and then from gate to drain to turn the MOSFET
“off”. The ohmmeter should read towards infinity (at least 2
megohms) when the MOSFET is switched “off” and less than
90k Ohms when the MOSFET is switched“on”. Do not touch the
leads when performing this test.
When repairing an RF amplifier, it is recommended that all four
MOSFETs in the failed half of a module be replaced, even though
only one or two of the four MOSFETs are found to be shorted.
The remaining MOSFETs may have been stressed internally and
may fail when supply voltage is reapplied. A blown fuse on one
half of the amplifier does not effect the other half.
MOSFETs that appear to be undamaged after testing can be kept
as spares for use if new replacements are not available. Also keep
in mind that the amplifiers used in the Driver and PA are identical
except that the Driver amplifiers operate at half voltage. This
allows you to rotate a repaired module into the Driver position if
so desired.
C.4.1.1.4
Replacing MOSFETs
•
Inspect all the transistor pads for any damage that may have
occurred when the transistors were removed from the
heatsinks. Replace any damaged pad.
•
Replace the ferrite beads on the center leads of Q3/Q10 and
Q4/Q11. Insert the transistors into the pc board. Do not
solder leads until heatsinks are in place.
•
Reattach heatsinks in reverse order as they were removed.
Tighten heatsink and pc board screws first and then tighten
transistor screws (torque to 3 inch-lbs).
Figure C-6. MOSFET Configuration
Section C - RF Amplifier Modules
888-2297-002 C-5
WARNING: Disconnect primary power prior to servicing.