CAUTION
ENSURE THAT THERMAL COMPOUND IS
APPLIED TO PLATES OF THE PDM AMPLI-
FIER BOARDS BEFORE THEY ARE IN-
STALLED ON HEAT SINKS AND THAT HARD-
WARE SECURING BOARD TO HEAT SINK IS
PROPERLY TORQUED TO 6-INCH POUNDS.
ALSO INSURE NO BURRS OR DIRT PARTI-
CLES ARE ON THE MATING SURFACES, AS
THESE WILL IMPAIR THE HEAT SINKING.
G.2.3. In-Circuit Ohmmetering the
PDM Amp Module (A6 thru A9)
MOSFET’s
WARNING
REMOVE ALL PRIMARY POWER AND
DISCHARGE ALL HIGH VOLTAGE COM-
PONENTS WITH GROUNDING STICK BE-
FORE PERFORMING THE FOLLOWING
PROCEDURE.
Remove all primary power and discharge
all high voltage components with ground-
ing stick.
Check fuses F1-F4 on PDM Pull-Up
boards A6A1-A9A1. If open fuses are dis-
covered replace as necessary but complete
this troubleshooting procedure prior to re-
activating the transmitter.
With the positive lead toward the heat
sink, measure across R21, R22, R41, and
R42 using ohmmeter. Each should read 47k
ohms. If the ohmmeter shows a short, the
transistor connected with that resistor has
failed.ThisisonlyacheckoftheMOSFET
gate circuit to see if it is shorted, but it is
usually the only test needed to identify de-
fective transistors.
If the ohmmeter checks across R21, R22,
R41 and R42 appear normal, but a PDM
Amplifier module is strongly suspected of
having a shorted transistor, ohmmeter
check the drain circuit.
Remove the PDM pull-up board fuses
from A6 thru A9. Check the resistance from
E8, E9, E10, and E11 of each board to
ground. Each should measure greater than
700k ohms. A low reading indicates that a
circuit element attached to the output side
of E8, E9, E10 or E11 is probably short
circuited. Next measure from the case to
ground respectively on Q19, Q20, Q39 &
Q40ofmodulesA6-A9.Allshouldmeasure
at least 2 Meg ohms. A low reading is in-
dicative of a short circuited transistor.
If it is necessary to replace one of the
powerMOSFET’s(Q19,Q20,Q39, or Q40)
refer to the paragraphs on Handling MOS-
FET’sandTestingMOSFETS beforebegin-
ning.
1
2
3
4
Figure G-1. PDM Pull-Up Board, Controls and Indicators
1,2,3,4 A6A1F1-F4 (A6A1 located at rearof
transmitter).
Disconnects the PDM
Amplifier/Damper Circuitry from
PA Amplifier during an excessive
supply current condition.
1,2,3,4 A7A1F1-F4 (A7A1 located at rearof
transmitter).
Disconnects the PDM
Amplifier/Damper Circuitry from
PA Amplifier during an excessive
supply current condition.
1,2,3,4 A8A1F1-4 (A8A1 located atfront of
transmitter).
Disconnects the PDM
Amplifier/Damper Circuitry from
PA Amplifier during an excessive
supply current condition.
1,2,3,4 A9A1F1-4 (A9A1 located atfront of
transmitter).
Disconnects the PDM
Amplifier/Damper Circuitry from
PA Amplifier during an excessive
supply current condition.
Table G-1. PDM Pull-Up Board, Controls and Indicators
G-2 888-2314-001 Rev. AE: 03-05-2002
WARNING: Disconnect primary power prior to servicing.
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