Mackie SRM450 Service Manual page 18

SRM450 Bias Adjustment, sheet 1 of 2
Model: SRM450 Active Speaker
Problem: The Amplifier Bias of every SRM450 must be carefully adjusted as follows.
Procedure:
1. Place the SRM450 on its side so that the heatsink fins are horizontal.
2. Connect the power cord and turn it on, with no input signal applied.
Make sure that the TIMED TURNOFF switch is OFF.
3. Let it sit this way for an hour or more to allow the
heatsink temperature to rise. You can play music
through to speed things up.
If you have a thermometer, the bias adjustment
procedure is best done when the heatsink temperature is 45 to 50 deg C.
4. Turn off the SRM450 and remove the power cord.
5. Remove the complete amplifier/heatsink assembly from the plastic
speaker case but leave all wires connected, including the speaker wires.
Support the amplifier so it can be safely turned on again.
6. Replace the power cord and turn on the amplifier and take care not to touch
any exposed live AC components.
7. No input signal is applied.
8. Low frequency amplifier bias adjustment:
While the heatsink is still warm, measure the DC voltage across the 2 pins of J21.
Adjust R202 until a reading of 4mV to 4.5mV is reached.
See sheet 2 for the location of J21 and R202.
Important: When finished adjusting, secure R202 with a small drop of paint, such
as nail varnish.
9. High frequency amplifier bias adjustment:
Measure the DC voltage across the 2 pins of J20. Adjust R212 until a reading of
4mV to 4.5mV is reached.
Important: Secure R212 with a small drop of paint.
10. This completes the adjustment procedure. Reassemble and perform a full test.
Serial Number: ALL Effective: June 1999
SRM450 Bias SSE June 1999