Specifications

static charge will eventually have to be discharged. Discharging
to the MOSFET could damage the MOSFET.
NOTE
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.
G.6.2
Tes tin g MOSFETs
The MOSFETs will have to be removed from the circuit in order
to perform the following test.
Observe the precautions in the paragraph entitled “Handling
MOSFET’S” in this section.
The following test applies to all MOSFET’susedinthetransmit-
ter, but is not necessarily applicable to MOSFET’susedinother
equipment.
The MOSFET’s used in the transmitter may be checked with an
ohmmeter. However there is a requirement which restricts the use
of some ohmmeters. If the battery voltage is too low (under 3V)
or too high (over 20V) the ohmmeter cannot be used. A battery
voltage less than 3V will not give an operational check of the
transistor and a battery voltage greater than 20V may result in
damage to the transistor under test. A Simpson 260, which uses
a 9V 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.
Refer to Figure G-6 for the following test. Connect the positive
lead of the ohmmeter to the drain or case of the transistor.
Connect the negative lead to source. Alternately touch a jumper
from gate to source and then from gate to drain. The ohmmeter
should read towards infinity or at least 2 megohms when the
MOSFET is switched off and less than 90k ohms when the
MOSFET is switched on. (To switch the MOSFET on hard, near
zero ohms, use +5 VDC gate to source signal.) When doing this
test, lay the MOSFET on a flat surface or hold sides of the case.
The resistance of your finger tips and skin will effect the readings
when you touch the leads.
G.6.3
Replacing MOSFETs
When repairing an RF Amplifier, it is recommended that all four
MOSFET’s of 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 inter-
nally and may fail when supply voltage is reapplied. The repair
process would then have to be repeated which can be very
frustrating.
Blowing a fuse on one half of the amplifier does not effect the
otherhalfoftheamplifier.
MOSFETs that appear to be undamaged after testing should 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.
NOTE
In most cases, the transistor will stick to the heatsink because of
seal created by the transistor pad. This seal will have to be bro-
ken before a heatsink can be removed. Pry the transistor out,
away from its heatsink. DO NOT TRY TO PRY THE HEATSINK
AWAY FROM THE PC BOARD WITH TRANSISTORS STUCK
TO THE HEATSINK OR THE PC BOARD MAY BE DAMAGED
AND THE HEATSINK MAY DISTORT. Sometimes the transistor
pad will tear when the seal is broken, Remove stuck pieces and
replace pad.
a. Remove all the screws from heatsinks and transistors.
b. Remove the heatsinks one at a time starting with the outer
most sink. Break seals on transistor pads as each pair is
exposed.
c. Replace failed transistors. Save and reuse the ferrite bead
on the center leads of Q5/Q7 and Q6/Q8. Do not solder
leads until heatsinks are in place.
d. Reattach the heatsinks in the reverse order in which they
wereremovedinstepb.Tightenheatsinkandpcboard
screws first and then tighten transistor screws (torque to 3
inch-lbs).
e. Make sure the ferrite beads are on the center leads of
Q5/Q7 and Q6/Q8.
f. Solder transistor leads and trim.
g. Replace blown fuse(s).
G.7 Troubleshooting RF Amp Modules
This section deals with faults on a single module. For problems
involving multiple modules refer to the Transmitter Trou-
bleshooting Section first.
Before troubleshooting themodule, verifythat the problemexists
on the module by swapping the suspected bad module with
another working module in the transmitter. If the problem does
not follow the module refer to the Combiner Section to continue.
After it has been verified that the problem is on the module, begin
troubleshooting by observing the LEDs on the Module when the
transmitter is running.
G.7.1
DS3 Not Illuminated
If this LED is not illuminated, the module is not being turned on
or no RF drive is present.
a. Move the module to a position that has DS3 illuminated.
Figure G-6. Pin Identification of MOSFET
G-6 888-2339-002
WARNING: Disconnect primary power prior to servicing.