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2 RMX0001 Rev. B
5. If the amplifier exhibits the symptoms described in this bulletin, continue with the removal and replacement of transistor
Q205. If it doesn’t exhibit any symptoms, reconnect J204 and check that the amplifier operates correctly, and then
procede to
“Finishing the repair.”
Procedure: Removing Q205
1. Unscrew Q205 from the heat sink.
2. Using diagonal cutters, clip Q205’s leads and discard the transistor.
3. On each remnant of Q205’s three leads, melt the solder and use your long-nose pliers to pull the remnant from the circuit board.
4. Using desoldering equipment or solder braid, remove excess solder from the three circuit board holes.
Procedure: Replacing Q205
1. Insert the strain-relief-prepped MJE15032 transistor into Q205’s location, oriented so the metal tab can be affixed to the heat sink.
2. Make sure the transistor’s leads are correctly inserted into the solder pad holes in the circuit board, but do not solder them yet.
3. Use the same screw to firmly attach the new transistor to the heat sink. Do not over-torque the screw.
4. Solder the three leads of the new transistor to the circuit board. Soldering from the top side should suffice. Be careful to
avoid solder bridges.
There is plenty of clearance under the circuit board, so you don’t need to trim the leads on the underside of the board.
6. Reattach the connector at J204.
Procedure: Testing and calibrating
Note: During this test, the amplifier will draw approximately 22 amperes at 120 volts AC, or 11 amperes at 230 volts. Be thorough,
but do not take too long to perform this test so you don’t trip the amplifier’s circuit breaker.
1. Locate trimpot R239 on the circuit board; you may need to adjust it later in the procedure to set the symmetry of the current limiting.
2. Turn down the gain controls of both amp channels fully counterclockwise. Connect the output of channel 2 to the 2-ohm load.
Connect the oscillocope probe across the output terminals of channel 2.
3. Set the audio generator to 1 kHz at 1 V RMS and connect it to the input of channel 2.
4. Turn the variable transformer to 0 volts and plug the amplifier’s AC cord into it.
5. Turn the amplifier on.
6. Gradually turn up the transformer to the amplifier’s normal operating voltage and make sure the amplifier is functioning correctly.
7. If all is well, turn up channel 2’s gain control. On the oscilloscope you should see the amplitude of the sine wave increase.
8. Turn up the gain control until you see the sine wave clip, which should be at about 51 volts RMS or 72 volts peak. If it
clips asymmetrically—on either the positive or negative side first—adjust trimpot R239 until the clipping is symmetrical.
9. Turn channel 2’s gain control all the way down. Apply a short circuit across the output of channel 2.
10. Turn channel 2’s gain control all the way up. The AC current draw should be from 4 to 5.5 amperes for 120V models and
2 to 2.75 amperes for 230V models.
Procedure: Finishing the repair
1. Turn the amplifier off and disconnect all cables.
2. Re-install the top cover. The amplifier can be returned to use.
Telephone:
800-772-2834 (within USA only)
+1 (714) 957-7150
Fax:
+1 (714) 754-6173
E-mail:
tech_support@qscaudio.com
Postal and parcel address:
QSC Audio Products, Inc.
Technical Services Group
1665 MacArthur Blvd.
Costa Mesa, CA 92626 USA
Contact information
If you need any further information regarding this service procedure, please contact QSC Technical Services at the addresses
or numbers below.