Electro-Voice CPS 1 Service Manual Download Page 4

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4. Settings / adjustments

4.1. IDLING CURRENT ADJUSTMENT
Connect the DC-volt meter at the BIAS measuring points (see service connector) and adjust the idling current
via the trim potentiometer VR101/VR301 (on the main PCB printed board assembly). Adjust both channels of
the power amplifier A&B to a value of U(DC) = 7.5 mV. Adjusting the idling current has to be performed at
normal room temperature. In case the power amplifier was previously in operation, it needs several hours for
cooling off.

4.2. VCA - OFFSET
Rhythmically open and short-circuit the CNS2.1 and CNS2.2 on the main PCB printed board assembly for
channel A; respectively CNS2.3 and CNS2.2 for channel B. Use VR100 respectively VR300 for adjusting the
power amplifier outputs to their minimum offset (with oscilloscope to minimal peak value or to the audible
minimal volume of the interfering pulse).

5. Testing of functions

5.1. OUTPUT - offset voltage
DC-voltage measuring at the SPEAKER A/B outputs with U(DC)

≤

±10 mV.

5.2. LIMITER

5.2.1. attenuation test
Channels separately driven with a 1 kHz signal up to U(A) = 42.4 volts, respectively U(A) = 49 volts (without
load). Increase the input voltage by 10 dB. The LIMITER LED lights and the output voltage increases by about
2 dB to approx. 58 volts – respectively 64 volts; with slight clipping. The distortion rate of the limited signal is
at THD = 1.0 … 1.5 %. Increasing the input signal up to a value of +20 dBu should not result in remarkably
higher clipping.

5.2.2. attack and release times

separately test both power amplifier channels:

testing without load resistors

1.)

Drive the power amplifier input with a burst signal (f = 1 kHz, 10 cycles, rate:

≈

0.5 sec.)

2.)

and U(E) = +10 dBu.

2.)

Monitor the output signal via oscilloscope. After 4 - 5 signal periods, the limiter controlled the major
distortion down to a minor residual distortion (THD = 1.0 … 1.5 %).
attack time: 4 - 5 mS

release time: 40 - 50 mS

5.3 POWER-ON DELAY
Signal present at the power amplifier input. Use the power-on switch to switch the power amplifier's power on.
Approximately 2 seconds after switching the power on, the signal is present at the amplifier's output.

5.4 FAN CONTROL
Upon powering-on the power amplifier, the fans will run for about 2 seconds and then stop; presuming that the
amplifier's temperature is not too high. In idling condition (power-on, no signal present) the fans are switched
between the SLOW and OFF (0 V) mode, depending on the heat sink's temperature. When CNS1.8 is
connected via a 5k1 ohms resistor to +15 V, the fans will run in SLOW mode. Fan-voltage of 16.8 V DC
measured between Pins CNS2.4 and CNS1.4. Disconnecting the plug CN5 results in the fans running at FAST
speed. Fan-voltage of 28 V DC measured between Pins CNS2.4 and CNS1.4

5.5. SOAR-PROTECTION CIRCUIT TEST
Channels separately driven on 4

Ω

up to 49 volts; respectively 42.2 volts. Parallel connect a 1

Ω

resistor. The

protection circuit reacts and tries continuously to re-start! The protect-LED lights. Repeat the test with a 2

Ω

resistor; the power amplifier should not switch off.

5.6. SHORT-CIRCUIT CURRENT-LIMITING TEST

separately test both power amplifier channels without load:

drive the channel with a burst signal (f = 1 kHz, 1 - 3 cycles, rate

≈