Test Procedures - Mackie HR824 Service Manual

TEST PROCEDURES

REQUIRED TESTS
The following pages contain the test procedures for the amplifier section disconnected
from the woofer and tweeter. For minor repairs, there is no need to run through all tests
but you MUST at least do the following:
• Adjust the bias, and check the rails and current draw as shown below.
• Verify that both amplifiers meet full power into resistive loads, as shown on page 19.
• Operate all the switches and verify their effect.
• Manually (and gently) check the woofer for any sticking or rubbing in its travel.
NOTE:
The four adjustment pots VR2, VR3, VR5 and VR6 are set and glued at the Mackie Factory
for optimum performance and should not be touched. The factory settings are made
when the speaker is all assembled and its output measured in an anechoic chamber
with a calibrated microphone. The setting of these pots takes into account
manufacturing tolerances of the complete assembly, not just the amplifier on its own.
TEST EQUIPMENT
The amplifer performance must be tested into resistive loads:
4 ohm, 200 watt resistor for the low frequency amplifier
8 ohm, 200 watt resistor for the high frequency amplifier
Other equipment required:
Audio range sinewave generator, oscilloscope, dc millivolt/volt meter, Vrms meter,
THD meter.
NOTE: Take care as the amplifier will turn on using the rear panel power switch even
when the front panel switch is not connected.
INITIAL SIGNS OF HAPPINESS
• The amplifier is on and cold, NO loads, NO input signal
• See the next page for the location of the test points
BIAS:
Measure the dc voltage across the two pins of J1, and adjust VR1 for a reading of 2.5 mV
Measure the dc voltage across the two pins of J6, and adjust VR4 for a reading of 2.5 mV
Don't stop, there is more:
IMPORTANT NOTE: After the amplifier has been on for 30 minutes and if the rails and
current draw seem good (as shown below), set both bias voltages to 9 mV (with no
signal, no load).
RAILS:
Verify the various dc voltages are present, relative to ground (see the diagram on the
next page):
-56V, +56V, -49V, +49V, -15V, +15V.
CURRENT DRAW
The current should be less than 200mA with no loads attached.
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