Mackie FR Series M-1200, Руководство по обслуживанию

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VERSION 1.1
SERVICE PROCEDURES
13
BIAS AND TEST PROCEDURE
After the unit has been repaired, the following should be done to assure long term
reliable operation. If a distortion analyzer is present, distortion specifications should be
verified.
1. Adjusts bias in both channels (R1 & R51) for 8mV +/- 0.3mV at bias test points (J53 &
J54) after unit has idled for a few minutes. With Full AC line voltage applied to unit,
it will pull around 65W from the line (900mA at 120V). Measure for DC offset on both
output connectors. DC offset should be less than +/- 50mV.
2. Apply a 1KHz sine wave to the inputs and verify that the unloaded outputs have a
waveform that is symmetrical and undistorted. Drive the outputs into clipping and
verify symmetrical “flat-topping” on the waveform.
3. Reduce output levels, install the 0.1uF capacitor jumper from the output to ground
connections, and verify that clipping behavior is proper. Verify that no high
frequency oscillation occurs near and at clipping (parasitic oscillation).
4. Remove capacitive loading and minimize sine output. Verify and re-adjust bias if
required. Note that the bias will not drift appreciably in a unit that is functioning
properly.
5. Connect the amplifier directly to the AC line and connect an 8 Ω dummy load to
both channels. Bring sine wave level up on both channels and verify symmetrical
clipping. The output will clip somewhere between 120V - 140V pk/pk depending on
how stiff the line is. Clipping should be as described above. Add the 0.1uF
capacitive loading and verify clipping is still well behaved.
6. Individually load Channel 1 and Channel 2 with 2 Ω . Clipping should be
symmetrical, well behaved , and occur somewhere around 80V - 100V pk/pk. Verify
that clipping is well behaved after adding the 0.1uF capacitive loading.
7. Reduce output level to 20V pk/pk and short first channel one and then channel
two. The front panel “short” and “protect” LEDs should flash on the respective
channels.
8. Place amplifier in bridge mode and connect 4
Ω
loading to bridge outputs (across
both “+” output binding posts). Slip some card stock between the heatsink-outlets
and chassis sides. Monitor one of the outputs and adjust for a 20V pk/pk signal.
Short across both outputs and verify that all four ‘Short” and “Protect” LEDs light and
the amplifier mutes.
9. Remove short, monitor one of the outputs, and adjust for a 60V pk/pk sine output
(450W of output power bridged). After a few minutes the fan will begin running fast
(heatsink at 60
° C) and a short time later the amplifier will mute (heatsink at 80 ° C).
The “Hot” and “protect” LEDs will come on. Remove card stock and after a few
minutes the amplifier will come out of mute mode and the “cold” LED will return.
10.
Disconnect loading and remove drive. Reconnect amplifier to Variac and confirm
that the idle consumption is roughly 65W or 900mA, as before. Connect the
amplifier to speakers and verify that it sounds OK with music.