The fuse blows right away. How do I isolate this problem?

Well, hope this helps you.

Fuse Requirements:
USA @ 120 Volts /60 HZ - 10 Amps, 5 x 20 mm GMC/T/slow blow
Europe @ 240 Volts/50 Hz - 5 Amps, 5 x 20 mm GMC/T/slow blow
Japan @ 100 Volts/60 Hz - 12 Amps, 5 x 20 mm GMC/T/slow blow

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http://blog.newyorkbrass.com/2010/07/07/what-does-a-neve-1073-and-a-behringer-ada-8000-have-in-common/

This speaker at full power draws average 1.3A at 230V and 2.5A at 120V. Mackie's recommended fuses are in line with this so your fuse is actually too high a rating. It is possible you are using cheap fast blow fuses or are over-driving the speaker (which should cause the fuse to blow) These speakers are only capable of 118db sustained (@1M) so cannot be expected to cope with anything but small venues or gentle music.

It is likely that the O/P transistors have blown, especially if the fuse keeps blowing and the O/L led comes on. It's a repair shop jobbie

Most likely one of the diodes CR6 through CR9 is shorted... Test them with an ohmmeter with the standby switch OFF and no power plugged in either. Slight chance one of the power tubes is shorted but that is rare. A shorted tube socket is another possibility but it too is rare. If one of the diodes is shorted, replace them all as the others are likely degraded. Put a 150 watt lamp in series with the power cord to test to avoid blowing fuses. If the lamp lights and stays on, there i still a short. the filter capacitors may also be degraded if a diode has shorted.

The power supply has likely failed. Schematics are hard to find and working on these takes some skill. The supply is a switching type and usually the main switching transistor and sometimes the driver and the regulator chip fail. There OFTEN is NO visiblke evidence of the failure. To avoid blowing fuses use a 40 watt lamp in series with the power cord. For personal safety use an isolation transformner when troubleshooting.

Likely the filter caps or the rectifier diodes have shorted. Power supplies are easy to work on unless the are switching type.

To work on this, replace the fuse and put a 25 watt light bulb in series with the hot side of the inout power... saves fuses and will send enough current in to troubleshoot with a meter. Use SAFE procedures when working on electrical equipment. For switching power supplies you need to use a line isolation transformer for personal safety.

This is either a shorted component in the power supply section or in the power amp electronics itself. Continuing to replace a fuse MAY damage additional components. Take unit in for servicing.

Replace the fuse only once. If it blows right away, then servicing is required as continued replacement will likely damage more inside.

Most of thes have a fuse holder that is a part of the input power socket. There is a little rectangular panel adjacent where the cord connects with a tab that you gently pry out and carries a small fuse. Replace the fuse and re-install the carrier.

These units are very hard to work on due to packaging.