Mackie SRM450 Problem

[rf7]

The owner ran a power amp into the input of this powered speaker. It still functioned afterwards, but the compressor was kicking in at the wrong times. Ended up replacing all the op amps (U17, 18, 22 & 25) in the control circuits, and it seemed to sound fine. Owner took it home and brought it back saying it had no low end. Compared it to his other (working) speaker and the 40 to 120Hz region is way down. I compared the signals and voltages internally between the 2 speakers and it's definitely off, but being a closed loop system, I can't figure out where the problem actually is. Everything seems to affect everything else.

Any ideas?

[Jazz P Bass]

Class D 350 watts. Servo loop driven woofer.
You need a schematic!
Call LoudTechnologies & ask for one.

[rf7]

I've got one, thanks.

[Steve Conner]

I'm going to guess the DC blocking capacitors on the input jack are toasted.

Failing that, the procedure for troubleshooting a feedback loop is the same as it's always been: Work your way round it, checking voltages and waveforms as you go, until you find the place where things don't make sense. For instance, an amplifier stage with lots of input but no output.

PS: See here, http://www.mackie.com/products/srm450/history.html the SRM450 has a sliding high-pass filter hooked up to the limiter, that could be stuck at 120Hz.

[rf7]

Finally got back to this problem.

Originally Posted by Steve Conner

PS: See here, Mackie - SRM450 - A Young Person's Guide to SRM450 Design History the SRM450 has a sliding high-pass filter hooked up to the limiter, that could be stuck at 120Hz.

That was exactly the problem, turned out to be a bad op-amp in that circuit.

Had to cut a trace and feed the signal in to the power amp stage first, to eliminate some feedback loops and simplify the troubleshooting.

Thanks guys, for your help on this one.