400RBIII BASS AMPLIFIER
TURN-ON PROCEDURE
SETUP:
1.
Variac on zero
2.
Top cover of amplifier removed (for easy access to the amplifier outputs)
3.
Oscilloscope probe (GND) to chassis ground and (+) to tip of ¼” output jack.
4.
Set the volts/div of the oscilloscope to 20V and the time base to 1ms
5.
Turn bias pot R21 on the power amp board all the way down (counter-clockwise)
6.
Connect a DMM, set to 20mV scale DC, to the 2-pin BIAS header on the power amp
7.
Set the VOICING FILTERS and the main VOLUME to zero, all other controls to 10, and all
switches OUT
8.
Set the function generator to output a 200 Hz, 5mV (-46 dBV) sine wave
POWER AMP BIAS AND OUTPUT TEST:
1.
With the amplifier power switch on, slowly raise the Variac voltage to 60V, the
power LED should switch from RED to GREEN. Observe the ammeter for excessive
current draw (idle current should not exceed 2A)
2.
Raise the Variac to 120V
3.
Adjust trim-pot R21 to obtain a steady, 5mV reading on the DMM
4.
Connect the amp output to a 4 ohm non-inductive dummy load
5.
Connect the function generator to the INPUT jack of the amplifier
6.
Raise the main VOLUME for slight clipping on the output, approximately 33Vrms
7.
Press the –10 dB switch IN. The output should drop to approximately 13Vrms
8.
Press the –10 dB switch OUT. Turn the BOOST to zero. Output should be 3.8Vrms
9.
Turn the WOOFER MASTER to zero, output should be 0 volts
10. Turn the main VOLUME back down to zero
TONES AND FILTERS TEST:
1.
Set the VOICING FILTERS to zero, all other controls to the center (12 o’clock) and all switches out.
2.
Disconnect the load from the amplifier output.
3.
Change the ‘scope setting to 2V/Div and 1ms time base.
4.
Set the function generator to output a 200 Hz square wave at 5mVrms (-46 dBV)
5.
Turn the CONTOUR to 10 and compare the output to Figure 3 on the waveforms
page. Turn the CONTOUR back to zero
6.
Turn the PRESENCE to 10 and compare the output to Figure 4. Turn the
PRESENCE back to zero
7.
One at a time, turn each ACTIVE EQ knob all the way up and all the way down and
compare the output to the corresponding figures on the waveforms page. Remember to reset each knob to its center (12 o’clock) position
before proceeding to the next tone control
NOISE TEST:
1.
Remove the function generator from the INPUT jack
2.
Connect a speaker to the amp output jack
3.
Turn all knobs to 10 and leave all switches out
4.
Listen for any unusual noise, there should be no pops or crackling sounds
5.
Connect an AC voltmeter, set to 1V scale. Amp output noise level <225mV
6.
Set the VOICING FILTERS to zero, the noise level should be <210mV
7.
Turn the BOOST to zero, the noise level should be <25mV
8.
Turn the VOLUME to zero, the noise level should be <1mV
END OF PROCEDURE
