QSC III Series, Technical & Service Manual

33

Test Procedures: 

Model 3500 

1. Inspection 

a.  Carefully inspect the amplifier module for burned or damaged components. 

2. Hook-up 

a.  Connect load resistors, oscilloscope channel A (20V/Div.), and distortion analyzer (1000 

Watt/0.1% input/distortion) to the amplifier output. Note: the output may be measured before 
the relay at the center tap (pins 7 and 8 of the 8-pin connector) 

b.  Connect the analyzer’s distortion output to channel B of the oscilloscope (0.1V/Div.) 
c.  Set the analyzer signal output to 2kHz, 1.25V RMS. Make all signal level changes with the 

amplifier gain controls after this adjustment is completed. 

d.  Connect the Variac (set to 0) to the amplifier’s AC inlet receptacle. 

Note:   Do not connect the telephone mod signal connector at this point. 

The following instructions are for 120 V models. For 240 V models, expect half the AC current draw 
at twice the AC voltage with respect to the AC measurements given. 

 
3. Initial 

start-up 

a.  Disconnect the load resistors from the amplifier. Use a 5A AC meter. 
b.  Turn the amplifier on and slowly increase the Variac setting to 120VAC. Watch the 5 amp 

current meter for excessive current draw (more than 0.8 amps). The power/protect LED should 
come on red at about 20VAC. At approximately 100VAC the speaker mute relay should turn on 
and the power/protect LED should turn green. 

c.  Install the input mod connector. Observe the output for symmetrical waveform; the distortion 

analyzer should indicate less than 0.02% distortion. The signal level on the oscilloscope should 
read 150V

P-P

 (300W) when set in accordance with the above procedure (step 2c.). This verifies 

correct overall channel gain. Characteristic step spikes should be visible on the positive and 
negative halves of the waveform (still into no load) on channel B of the oscilloscope. 

d.  Move the wiring harness and tap the circuit boards with a non-conductive implement such as 

the handle of a small screwdriver and watch the scope output and distortion trace for 
intermittency. Remove the 8-ohm load. 

4. DC 

protect 

a.  Check that there is no load on the output of the amplifier. Use a jumper to short pin 8 (+) of the 

IC to ground; the relay must turn off and the power/protect LED must turn red immediately 
(within 0.1 second). The amp should return to normal operation 2 to 3 seconds after removing 
the short. 

b.  Short pin 4 (-) of the IC to ground; the relay must turn off and the power/protect LED must turn 

red immediately (within 0.1 second). The amp should return to normal operation 2 to 3 seconds 
after removing the short. 

5. Signal 

indicators 

a.  With a 2kHz input signal, adjust the amp gain to obtain 300W (-6 on dB scale) of output power. 

The two yellow LEDs should be on at this point. 

b.  Attenuate the signal by 10dB. The –6dB LED should be extremely dim or off. 
c.  Switch the volts/power setting to the 100V/30W range. Reduce the amplifier gain for a –20dB 

meter reading. Attenuate the signal by 20dB. The –6dB LED should be off and the –30dB LED 
should be dimly lit. 

6. Current 

limit 

adjustment 

a.  Adjust the input signal for 160W (-9dB) of output power. 
b.  Select a load resistance of 2 ohms. Vary the Gain to display a small amount of ordinary voltage 

clipping (indicated by the presence of ripple at peaks of the output signal). Note: Check that the 
red clip LED is working. 

c.  Adjust the current limit trimpots (T2 & T3) until the amp begins current limiting (ripple and 

amplitude of peaks flatten) and then back off the pot slightly to the point where the amp just 
shows full ripple. 

d.  Check the short circuit current (3.0 to 5.5 amps). 

7.  Crossover (bias) adjustment