51-0181-0100 EP06-06 MPA4-150R Anglès.doc 3 of 6
28. The unit's gain level is now controlled only by channel three's input potentiometer. Turn it up
and leave it at its maximum position.
29. Check the amplifier’s frequency response. Using a 0’5V input signal, verify that the output
level remains equal between 20Hz and 20kHz, and the waveform suffers no distortion at any
frequency. Also check that with a 50kHz input signal, the output level drops only 1 or 2dB,
and no visible distortion is detected.
30. Apply a 1kHz square wave signal. Moreover, shunt a 1µ5 capacitor to channel 3's 4
Ω
output
load. While monitoring the output signal on the oscilloscope, vary the input signal level in
order to see a two- or three-cycled ringing on the flat sections of the square output
waveform. Repeat this same process with channel four.
31. Select again a sine wave output on the signal generator.
32. At this point, all clip indicators should be lighting. If necessary, add a little bit of level to the
input signal.
33. Set the IN1/IN1+IN2 selector to IN1+IN2, and verify that the output signal level drops 6dB on
all outputs.
34. Return the switch back to IN1.
35. To verify the thermal protection circuits, short the thermal probe pins +Vs and
Vout, and verify that the relay releases and opens up, while the THERMAL
LED indicator lights on and the cooling fan increases its speed until it reaches
maximum airflow.
