Rf Amplifier Troubleshooting; Power Amplifier Transistor Replacement - ConMed Sabre 2400 Operators & Service Manual

4.7.7 RF Amplifier Troubleshooting

The first step in troubleshooting a Power
Amplifier (PA) is to remove all PA collector and
base fuses (check the fuses as you remove them
and note if any of them are blown). This will pre-
vent possible secondary failures in the PA due to
overload.
Use a DVM in the diode check position to check
for shorted bipolar power transistors, power
Mosfets, and snubbing networks. Check to see
that diodes are functional. Check RFSUP TP-
8 for approximately +140 Vdc and U2-6 for
+8Vdc with power on.
Check the Base Voltage Generator and Waveform
Generator for proper function before proceeding.
If everything checks this far, enter dIA 2 (See sec-
tion 4.6.1) and select Monopolar Pure Cut from
the front panel. Confirm that A1TP1 looks like
the waveform of Fig. 3.2 Photo 2.
Turn off the power to the unit, confirm that
RFSUP has dropped below 10Vdc and replace
the base fuses. Reenter dIA 2 and select
Monopolar Pure Cut from the front panel. Use
an oscilloscope on the drain of A1Q5 (TP6) to
confirm that the power Mosfets are switching
properly.
If the unit has passed all tests to this point, it will
be necessary to replace the fuses to troubleshoot
further. Proceed with caution!
4.7.8. Power Amplifier Transistor
Replacement
All bipolar power transistors A1Q1-Q4 may be
replaced singly without replacing the entire set.
Calibration should be checked after transistor
replacement. When replacing the power transis-
tors:
1. Remove the entire Power Board/Heatsink
Assembly for easy access to the bottom of the
PWB. See Section 4.2.1 for detailed instructions.
2. Remove the screw and compression washer
from the transistor being removed. Slightly bend
the transistor away from the heatsink to make
desoldering easier.
3. Desolder and remove transistor.
4. Bend leads of the new transistor to fit in their
appropriate holes, and to allow the transistor
body to lie flat against the heatsink. Be sure to
check positioning with the screw securing the
transistor in place before soldering transistor.
5. Be sure to install the insulator pad. No thermal
compound is necessary, but the mating surfaces
of the transistor, insulator and heatsink should be
clean. Replace any insulator which is torn, punc-
tured or dirty.
6. Ensure that the body of the transistor is flat
against the heatsink before tightening screws.
This ensures a maximum heat transfer and longer
transistor life, and minimizes cutting of the insu-
lator pad.
7. Install the compression washer and tighten
the screws to 4-6 pound inches. Excessive torque
may cause the insulator pad to be cut, destroy-
ing its dielectric strength and shorting RFSUP to
ground.
8. Clean solder connections and replace the
assembly into the unit, tighten all fasteners and
reconnect all harnesses.
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