QSC CX Series, Service Manual

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CX Series
5 Turn power off and place the amplifier under test back into the Stereo mode with output loads
connected to each channel. Reinsert CH2 input plug and turn both gain controls fully up.
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BIAS ADJUSTMENT
1. Let the amplifier cool down to room temperature,
2. With an input amplitude of
1
Vrms, increase the input frequency to 20kHz. Reduce the input
signal 20dB (80%) from full output. At this point, a couple of watts of output power should be
observed at the output. For the first channel to be calibrated, adjust the crossover trimpot
VR501 for about a 4
00
mVpk-pk crossover spike protruding from the noise trace on the
oscilloscope. It will be necessary to have the oscilloscope measure unfiltered distortion from
the amplifier in order to see the crossover spike. It is necessary to disable the 80kHz lowpass
filter on the analyzer for this test. Further trim VR501 so that the total distortion for that
channel is at about 0.1% THD+N. If bias is not settable, see the bias description and
troubleshooting section of this manual.
3. If a distortion analyzer is not available, a less accurate crossover distortion and bias adjustment
can be made by monitoring the driver transistor (Q501 & Q502) bias current. With the
amplifier at room temperature, and with no input signal plugged into the amplifier, measure the
DC voltage across the emitter resistors of Q501 and Q502 (R501, R502). Adjust VR501 to
obtain about 80mV d.c
.
across one of these resistors.
4. With no signal plugged into the amplifier and with an 8 ohm load, verify that the AC idle
current from the AC service is no more than
1
.O amps a
.
c.
5. Let the amplifier cool down and check/adjust channel 2.
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SHORT CIRCUIT CURRENT
1. Select a 4 ohm load and apply a 1Vrms (1 kHz) input signal to both channels of the
amplifier. Ensure that power is on and that the gain controls are fully up.
2. While the amplifier is producing power into the loads, apply a short to the output binding posts
of each channel. In other words, apply a jumper between the red and black binding posts of
each channel. Once this is done, the combined AC line current draw for both channels
should be no greater than 8.
0
A ac
.
This is with a 12
0
VAC service. Current may be lower as
AC line voltage is lower. As the amplifier heats up, current draw may increase to about 9A until
thermal protect shutdown.
3. While the amplifier is driving a short, observe the main supply rail voltages. Ideally, they will be
no more than 3 volts from each other.
4. Remove the short from each channel and verify that the channels recover in to 4 ohm loads,
The output should not experience any hang up and a full sinewave should be present just as
it was before a short was applied for this test.
5. If the amplifier does not pass any of the above steps, follow the proceeding steps. If steps 2,
3, and 4 above pass, continue to the next test “Frequency Response.
”
Setting Short Circuit Current Limits
a. Select a 4 ohm load and apply a
1
.2Vrms sinewave (1 kHz) input signal to both
channels of the amplifier. Ensure that power is on and that the gain controls are
fully up.
b. Turn gain controls on CH2 fully down. CH
1
should be producing max power in the
4 ohm load.
c
.
While CH
1
is delivering power into the load, apply a short across channel
1
’s
binding posts.
d. Observe that the A.C. current draw is 3.0 - 4.0 amps A.C.. Adjust VR502 and
VR503 to get this current reading if your initial reading is off. VR503 is the positive
current limit adjust and VR502 is the negative current limit adjust. Adjust both
channels the same for a change in current limiting. Both wipers need to adjust
evenly to achieve a balanced setting.
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