4 RMX0002
Bias adjustment (perform only with unit at ambient temperature)
Note: This calibration must be done with the amplifier circuitry at ambient temperature. If the amp has begun to warm up,
you must turn it off and wait until it cools down.
6. Measure the DC voltage across R146. Adjust bias trimpot R131 to reach the correct voltage shown in Table 1.
7. Next, measure the DC voltage across R246. Adjust bias trimpot R231 to reach the correct voltage shown in Table 1.
Current limit adjustment
8. Set the audio sine generator to 1 kHz at 1 volt RMS and connect it to channel 1's input. Connect channel 1's output to a
2-ohm load, and connect the oscilloscope probe across channel 1's output.
9. Turn up channel 1's gain control partway. On the oscilloscope you should see the amplitude of the sine wave increase
accordingly.
10. Turn the gain control back down and apply a short circuit across the output terminals of channel 1. Clamp a current
probe either onto one of the brown wires running to the AC switch or onto the gray output wire from channel 1's module.
11. Turn the gain control all the way up. Adjust trimpots R139 and R140 equally until the current measured falls within the
range shown in Table 1.
12. Turn the gain control all the way down and remove the short circuit so the channel drives the 2-ohm load. Turn the gain
control back up until the output clips. The voltage at which the signal starts to clip should fall within the range shown in
Table 1. If the clipping is asymmetrical, that is, the signal clips on either the positive or negative side first, adjust R139
to make it symmetrical.
13. Turn the gain control down. If the amp has begun to warm up shut it off and let it cool a few minutes before proceeding
with channel 2.
14. Repeat steps 8 through 12 for channel 2. Use trimpots R239 and R240 to adjust the current limiting in steps 11 and 12.
15. Turn both channels’ gain controls all the way down. Clamp the ammeter onto one of the amp’s AC wires and check the
amp’s idle current. If the amplifier is still at about room temperature, the idle current should match the value shown in
Table 1.
Procedure: finishing the repair
1. Turn the amp off and disconnect all cables.
2. Re-install the top cover. The amplifier may now be returned to use.
Contact information
If you need any further information regarding this service procedure, please contact QSC Technical Services at the addresses
or numbers below.
Telephone:
800-772-2834 (within USA only)
714-957-7150
Fax:
714-754-6173
E-mail:
tech_support@qscaudio.com
Web site:
www.qscaudio.com (product info and support)
www.qscstore.com (on-line technical services and accessory sales)
Postal and parcel address:
QSC Audio Products, Inc.
Technical Services Group
1665 MacArthur Blvd.
Costa Mesa, CA 92626
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