Mackie SRM450: 120hz hum with no signal from audio input...

[DJDoubleBass]

I'm working on a 2-way 15" powered speaker: Mackie SRM450.

At a show, it slowly lost volume until it stopped working. Opened it up: blown fuse. Threw in another fuse, turned it on and immediately blew it as well.

Tested both rails (one for high pass and one for low pass) and pretty much the entire low pass rail had shorted rectifiers and transistors.

Swapped the entire blown rail along with the 1/4 watt resistors on the low and high pass sides (probably overkill) as well as all of the larger resistors connected to the rail. Also swapped the (4) 6800uf 50v caps in the power section.

Now when it powers on it doesn't blow the fuse, but a loud bassy 120hz HUM comes through the speaker, with no control over the volume from the gain knob. I can't get any signal coming from the input section either. I double checked the chassis grounds to make sure the connections were nice and tight.

Also, when I turn it on it POPS with the green power indicator LED turning on. Soft turn off though.


Any ideas or advice on this would be VERY much appreciated.

Here's the service manual with schematics:

http://www.eserviceinfo.com/download...ie_SRM450.html

[Andrew F. Ali]

Re: Mackie SRM450: 120hz hum with no signal from audio input...

Could be AC hum not being filtered properly. Verify you replaced the caps polarity correctly. Couldn't download schematic. My system abit slow today. Look at Output Transistors or ICs. Measure the VCC to them are correct.

[stj]

Re: Mackie SRM450: 120hz hum with no signal from audio input...

if there is a pair of smoothing caps after the power rectifiers then start there.

[stj]

Re: Mackie SRM450: 120hz hum with no signal from audio input...

on page6
check the 2 rectifiers and the 4 6800uf caps

[budm]

Re: Mackie SRM450: 120hz hum with no signal from audio input...

This is BI-AMP powered speakers, so if the bass amp is not working (it blew up as indicated on post 1), I would expect that the tweeter amplifier will be working and putting out sound, unless the Tweeter (HIGH-FREQ. AMP) also failed. I would check all the output voltage on page 6 to make sure they are present with correct voltages as shown. I would also check all the works that have been done to make sure the correct parts are used and installed properly.
BTW, the woofer amp is Class-G power rail switching set up type.

[DJDoubleBass]

Re: Mackie SRM450: 120hz hum with no signal from audio input...

Awesome! Thanks for the responses everybody. I'll do all of the above and report what I find out.

[DJDoubleBass]

Re: Mackie SRM450: 120hz hum with no signal from audio input...

Referencing page 6 of the schematic I posted, I get these results:

45v- is: -46.3v actual
45v+ is: +46.3v actual

80v- is: -93.6v actual
80v+ is: +93.5v actual

15v+ is: +14.2v actual
15v- is: -14.2v actual

I've got the 2 main rectifiers, "D55" and "D56" in the mail, as well as the 4 filtering caps near the 15v + and - (C157, C158, C156, C165).

Hopefully that'll solve this problem. I'll keep you posted on my progress. Thanks again for your responses.

[stj]

Re: Mackie SRM450: 120hz hum with no signal from audio input...

why not just test the old ones?

[DJDoubleBass]

Re: Mackie SRM450: 120hz hum with no signal from audio input...

I get mixed readings with the old rectifiers, and the capacitance function on my multimeter simply doesn't work :-\

So while I work on getting a new meter and the parts are only a few bucks anyway, I'll just shoot for replacing them.

[dj_ricoh]

Re: Mackie SRM450: 120hz hum with no signal from audio input...

I get mixed readings with the old rectifiers, and the capacitance function on my multimeter simply doesn't work :-\

So while I work on getting a new meter and the parts are only a few bucks anyway, I'll just shoot for replacing them.

you can test them with Ohm settings from the meter.
ofc if that is working to.

[DJDoubleBass]

Re: Mackie SRM450: 120hz hum with no signal from audio input...

I mean, of course you can test a cap with the ohm setting to see if it's open, but can you test for it's value?

[kc8adu]

Re: Mackie SRM450: 120hz hum with no signal from audio input...

120hz is almost always bad filter caps.test with esr meter or replace.

[joshnz]

Re: Mackie SRM450: 120hz hum with no signal from audio input...

120hz is almost always bad filter caps.test with esr meter or replace.

Also heavy load caused by a bad amplifyer.

[PalGStudio]

Re: Mackie SRM450: 120hz hum with no signal from audio input...

I'm working on a 2-way 15" powered speaker: Mackie SRM450.

At a show, it slowly lost volume until it stopped working. Opened it up: blown fuse. Threw in another fuse, turned it on and immediately blew it as well.

Tested both rails (one for high pass and one for low pass) and pretty much the entire low pass rail had shorted rectifiers and transistors.

Swapped the entire blown rail along with the 1/4 watt resistors on the low and high pass sides (probably overkill) as well as all of the larger resistors connected to the rail. Also swapped the (4) 6800uf 50v caps in the power section.

Now when it powers on it doesn't blow the fuse, but a loud bassy 120hz HUM comes through the speaker, with no control over the volume from the gain knob. I can't get any signal coming from the input section either. I double checked the chassis grounds to make sure the connections were nice and tight.

Also, when I turn it on it POPS with the green power indicator LED turning on. Soft turn off though.


Any ideas or advice on this would be VERY much appreciated.

Here's the service manual with schematics:

http://www.eserviceinfo.com/download...ie_SRM450.html

_________________________
Hi, I read about your problem, and at the moment I have the same problem, SRM 450, noise at 120Hz! Could you let me know how you solved this problem?
Thank you in advance!
palgstudio 03.18