QSC PowerLight3 Series Technical & Service Manual page 24

2.3 PL380 Test Procedure (continued)
9. Plug the amplifier's power cord into a variable autoformer (set to
0 V) and set up an ammeter to monitor AC line current.
Power On and Mute Delay Test
1. Turn the amplifier on and slowly increase the AC voltage to
120 V (120 V model) or 240 V (230 V model) while monitoring the
AC line current. At full voltage, the idle AC current should be
about 2.5 A (120 V model) or 1.25 A (230V model).
2. Verify that the fan is operating at low speed.
3. Turn the power switch off and on a few times to verify the 3-
second power-on muting delay and the instant power-off mute;
listen to how the power relays engage about 3 seconds after
turn-on but disengage instantly at turn-off.
NOTE: When AC power is turned off, the amplifier mutes the
audio immediately, but the power supply switches continue
operating until the primary and secondary DC voltages have
discharged to about 20% of normal. Below this point, the
auxiliary supply itself stops switching and the amplifier becomes
fully quiescent. On amplifiers made in January 3008 or earlier, it
is normal for the Clip and Power LEDs to remain lit for about 15
to 20 seconds after the amplifier shuts off. On later amplifiers,
the Clip LED will flash at the moment of turn-off, and they and
the Power LED will shut off.
Channel Output Test
1. Look for amplified signal on scope channel 1. Check for a noisy
or contaminated gain potentiometer by looking for general
instability on the distortion waveform (scope channel 2) while
you rotate the gain control.
2. Set the amp gain control to maximum (fully clockwise) and verify
the output level of 32 dBV (40 V rms), with a 1 V input.
3. Select 8-ohm load and set the analyzer signal output at
2.7 V rms, 1 kHz. Confirm that this amplifier is producing 1500
watts (109.5 V rms), with less than 1% THD. Repeat steps 1
through 3 with the other channel.
Signal Indicators Test
1. Disconnect the load resistors.
2. With a 2.7 V rms, 1 kHz input signal into the channel, turn the
gain control to maximum. The three signal LEDs (Signal, -20 dB,
and -10 dB) should be lit. Set the analyzer's 0 dB reference to
this level.
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3. Turn down the signal by 10 dB. The -10 dB LED should be dim.
4. Turn down the signal another 10 dB (-20 dB to the reference
level). Both the -10 dB and -20 dB LEDs should be off.
5. Turn the amp gain control to minimum. All three signal LEDs
should be off. Repeat this test with the other channel.
Bridge Mode Test
1. Turn off the power switch.
2. Set the mode switch to the bridge position. The bridge LEDs on
the front and rear panels should light when you turn the
amplifier back on.
3. Connect the load to the two red output binding posts (channel 1
positive and channel 2 negative).
4. Select an 8 ohm load resistance and apply a 2.7 V rms, 1 kHz
sine wave signal to amp channel 1's XLR input. Adjust the amp
gain control to obtain 5000 watts output (200 V rms).
Quickly verify that the THD is below 1%, and then turn the
gain control down all the way to prevent excessive
current stress on the AC line and power stress on the test
load.
5. Turn the amplifier off and set its mode switch back to stereo.
Connect a separate load resistance to each channel's output.
Frequency Response Test
1. Connect an 8-ohm load resistance to channel 1's output. Apply a
2.7 V rms, 1 kHz sine wave signal to amp channel 1's XLR input
and turn its gain control all the way up. The output voltage
should be 108 V rms.
2. Reduce the signal by 10 dB to 34.1 V rms (150 watts output). Set
the analyzer's 0 dB reference to this point.
3. Check the frequency response from 20 Hz to 20 kHz by sweeping
or spot-checking frequencies between these extremes. Verify
that the output voltage's amplitude at each frequency is 0 dB,
±0.20 dB.
4. Repeat steps 1 through 3 for channel 2.
4 Ohm Power vs. Distortion Test
1. Connect a 4-ohm load resistance to channel 1's output. Apply a
2.7 V rms, 1 kHz sine wave signal to amp channel 1's XLR input .
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