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.

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.

[RAMSYS]

Hello,
Just new here but I have the same problem. Looks like the pass filter stays active. No really low signal.
What did you do to repair. If got the schematics downloaded but which amp do i need to replace ?

[Jazz P Bass]

While not being privy to exactly which opamp the OP changed out, to repair the circuit you will need a signal generator & a scope.

The schematic has excellent information on which frequencies are being changed at each stage.

Follow the schematic, inject the proper signal & trace it through.

[km6xz]

Why change anything, you do not have actionable information yet. How low does it still work? I suspect the whole low frequency is malfunctioning or blown driver if you do not get strong 300 hertz notes.
What test gear do you have? If you do not have a good signal generator, oscilloscope and voltmeter and know what they tell you, you will be far ahead by finding a repair shop that will save money, time and damage to your unit. It is not an easy amp to repair for beginner. If you start swapping out parts and are not expert in soldering double sided boards, you are going to make most reputable tech not accept it. If they do, and you have messed with the boards, they will charge much more for the added hassle or just replace all the user damaged PC boards.