Mackie SRM450 trouble shoot amp section

This might be a year late, but you should start with units that power up, but have no high or low (or both) output. With the unit on it's face and unplugged, remove the amplifier module from the back, and set it across the normal position. Disconnect the four wire speaker plug from the amp board that leads into the cabinet so that you can test the high and low speakers. If you have an ohmmeter, you can use it to see if you read a low resistance (10 ohms or less) across each speaker coil. If you don't have an ohmmeter, you can use a pair of wires or test leads and a low current battery like a 9 volt cell to apply a voltage to the high and low speakers, while listening for a thump (low), or a click (high) from each driver. If your ohmmeter reads open, or if you don't hear a response from the battery voltage, then the speakers are probably open and will need to be replaced. Don't worry about meter or battery polarity, or connecting to the wrong wires, because you can't hurt anything with either of these tests.

If you get good readings or response, but still have no output from one or both drivers, then the problem is most likely in the amplifiers, and will be harder to diagnose and repair. Note that passing this basic test doesn't mean that the speakers are 100 percent healthy, it just means that you should hear something when they're connected to a working amplifier.

You may be able to swap parts from units with good drivers but bad amps into units with bad drivers and good amps and get something that works. This is fairly easy for the low driver which can be pulled from the front, but not so much for the high driver, which you can't reach without disassembling the whole cabinet.

I could add more on basic amp troubleshooting if anyone is still interested.

Hope this helps.

Hi maybe unrelated but I have Mackie c300z cabs they both failed .Are about 6 years old.thinking it was my amp I tried another on them and no wasn't the amp.stripped one down found the cones were solid no movement so had to rip cone out as I needed to rule out the amps.found the speaker coils jammed by the moved magnetics hence a write off so much for mackies speaker reputation as obviously is a build fault as they shouldn't move but they can not going to pay mackies exaubitent prices to fail again.