Mackie SRM450

[Corben]

I got a srm450 thats not powering up no lights or output. I got to admit im not that great at switchmode power supplies. But from what i see it is rectifying the ac to dc and the caps are good. I checked the output transistors and they seamed to be good. well all except i couldnt really test that power ic on the heatsink. I sent a schematic request to mackie but i think i got the wrong one back cause it doesnt seam to match the board layout. If any one could point me in the right direction id be glad. I noticed a couple people have this same issue on some other places online when i was googling to.

[Enzo]

If you have a switching power supply, then I bet you have a SRM450v2 instead of a SRM450. I that the case? DId they send the SRM450 instead of the SRM450v2 maybe? If so, call them back and tell them you need the other one.

[Grant_More]

Hey Corben,

I've got a set of these and had the exact same problem. I may have a solution for you! I got a set of schematics from Loud and I've attached them here.

If you look at the power supply section of the schematic, there's 10R series resistors in the +/-15V rails (R282/283). I found these had gone open circuit. They're only 1/4W chaps - I replaced them with 1/2W devices and I've been gigging with them for over two years with no further problems.

Earlier revisions of the SRM450s didn't have these resistors - they were added for some reason so I wouldn't remove them - just beef them up and maintain the 10R resistance.

Check that and see how you get on. I remember having to remove the main PCB (and all the transistors from the heatsink) to get in to them.



Cheers,
Grant

[pecorporation]

Just had one of those units in with those same resistors opened up. Good cue on the beefed up wattage, but also look at (or just replace) D46 & D53 zeners to make sure they're not leaky or shorted.

[WholeToneMusic]

I have been working on a few SRM450 v1's and 1 in particular had the above mentioned issue. I also found that the R282 & R283 were open. Replaced them with higher wattage Metal Oxides and powered up. Now it's really behaving strangely. The Power LED came on. I applied a 1kHz signal to the input to test all the transistor bases for a sinewave to make sure they were all passing signal. When the volume is swept past half way point to 3/4 up the signal light goes out, it comes back on past 3/4's up to full output. I turned it back to 1/3 volume and after a while the thermal circuit trips. Not sure what to make of these symptoms.

[plec.22]

Got one of these older gray cabs.. just power's up with clip led and thermal led on.. o/p devices to both amplifiers are ok.. part of this model is smd parts and standard parts for the rest. did try the link up the page but will not open?

need schematic or as anyone else had this odd fault?

[Jazz P Bass]

Check the + & - 15 volt supplies.

The led's are opamp driven and loss of a rail will certainly make the opamp slam on.

The SRM 450 is here: Mackie SRM450 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics (linear power supply)

The SRM450 V2 is here: Mackie SRM450 v2 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics (switch mode power supply)

[plec.22]

Solved the faulty srm450 Gray older powered speaker with clip and thermal leds on...open circuit resistor within it's low 15+/- reg circuit a 10 ohm...

time for a tea break! Thanks to Jass p bass for schematic and pointer to the low rail voltages...

[dmsr63]

Wanted to start a new thread, but will see if anyone catches this...
Mackie 450 V1. (I have two) used one at a small party, after two hours, the sound crackled badly and got worse. It does it on both the horn and speaker. I checked the 80 volts, 45 volts and 15 volts. All are fine. Did notice the input board has a three wire white cable to the main circuit board and was pinched under one of the xlr connectors. Removed the pinch, still no fix. Tried the other amp to this speaker cabinet and it is fine. The problem is in the amp. I notice by ohming out the xlr to where it leaves the input circuit board has different readings between the good unit and the bad. I will attempt to "swap" those to see if that could be the issue. (The sound is really broken up and weak) It's hard to unsolder without a solder sucker. I do know that now, after removing the input board, the three wire white cable that was pinched now has an open. Will return soon with an update if I can get the other input board soldered in...

[Jazz P Bass]

Oh, it is fun stripping back that wire & reinserting it.

[dmsr63]

Gave up... Not enough electronics knowledge to go deeper. Soldered in a new three wire cable to the main board from the input board. Removed main board and looked with a magifying glass for crack and bad solderes. Re-assembled. Plugged in a mic and it seemes to work, the tried music and nothing... Back to the mic and still nothing... Cant do micro surgery...

[billoni]

Hey Corben,

I've got a set of these and had the exact same problem. I may have a solution for you! I got a set of schematics from Loud and I've attached them here.

If you look at the power supply section of the schematic, there's 10R series resistors in the +/-15V rails (R282/283). I found these had gone open circuit. They're only 1/4W chaps - I replaced them with 1/2W devices and I've been gigging with them for over two years with no further problems.

Earlier revisions of the SRM450s didn't have these resistors - they were added for some reason so I wouldn't remove them - just beef them up and maintain the 10R resistance.

Check that and see how you get on. I remember having to remove the main PCB (and all the transistors from the heatsink) to get in to them.



Cheers,
Grant

I also have the Mackie SRM 450 and am trying to replace the R282 and R283 10 Ohm resistors. I'm trying to do a DIY repair on the unit, but I'm having a problem locating exactly where the resistors are. I've scoured the circuit board at least 3 times and can't find R282 and or R283. Could you point me in the right direction?
Many thanks in advance!

[g1]

He mentioned that all units do not have them, just the newer ones.
Do resistance checks from the +/-15V regulators to the IC supply pins. If you have continuity then you don't have to worry about those resistors.

[weegiebombermef]

to Grant More
Can't seem to download the Mackie SRM450 schematics. Am I doing something wrong--clicked on link in your post to Corben but nothing happens

weegiebombermef

[oc disorder]

[QUOTE=Can't seem to download the Mackie SRM450 schematics. Am I doing something wrong--[/QUOTE]

Hi weegiebomber no you're not doing anything wrong... the forum developed a fault some time ago and some of the attachments
no longer work.
See if this works
Main PCB Assembly Schematics (055-227-xx, Rev E).zip