Mackie swa 1501

[vinceherman]

I have been working on a SWA1501 I bought that came in with 2 burnt resistors R26 and R 73 and a blown fuse. I swapped out the resistors and fuses. As soon as I tried to put a fuse in, even while unplugged it blew the f1 fuse. I later was able to put it in and thought it didn't blow, so I decided to test it. I was able to get sound to go the subwoofer, it just sounded very weak and when I would turn it up the thermal light would flash and the power would cut out for a second. upon inspection the f1 fuse was blow the whole time again. I also read the subwoofer should be 12ohm and when I tested it, it was about 1.5 ohm. The subwoofer seemed semi blown. It would make sound but very low and when I pushed it down it got louder like it was getting a better connection. I am only interested in repairing the amp and then purchasing the correct subwoofer to replace it. I have read multiple forums and seemed to get close on the answer, but have not got there. Please, if anyone can help I would really appreciate it. Thanks a million.



Any assistance would be appreciated.

[Enzo]

If it blew the fuse while not even turned on, that means the power supplies were still charged up from the last time, and that was enough to pop the new fuse.

Resistors never burn up on their own, something always causes it. Most likely shorted transistors in the circuit. Am I right, this power amp is the board with the square holes in the center, and transistors all around the holes bolted to the heat sink? I generally just go around the whole collection testing each part for shorts. But quite often small transistors near the edge burn up too. There are small resistors that burn up.. seem to recall some small vvalue resistors there. Little zeners blow.

Sounds like the voice coil on your speaker has melted.

[vinceherman]

Thank you very much for the reply. you are correct on the amp set up. I am very new to trying to repair this stuff and have not taken any classes on repairs yet. I was only able to find two things I could see were damaged. I know how to test the resistors but not how to test the transistors or what a Zener is. thanks for all the help. any further clues would be appreciated.

[Enzo]

I am an experienced professional, and I do not look forward to rebuilding these models myself. You can read up on testing transistors with your meter (or "DMM" instead of meter for searching)

Burnt parts are visually apparent, but most bad parts look no different. What happens is some part will fail which then causes cascade failures of other parts. I would not suggest this amp as the place to start learning.

[vinceherman]

Thank you very much. everything I have read confirms they are hard to work on. Thanks for your time.

[nevetslab]

Thank you very much. everything I have read confirms they are hard to work on. Thanks for your time.

Take a look at the threads on this forum, using Advanced Search. Type in Mackie SWA1501, and you'll find 21 threads concerning the repairs on this amp. The original 12 ohm woofer is no longer available. An 8 ohm woofer will load the amp down too much, so you're left with the Kappa Pro 15B.

I've attached the data sheet for that here:

Kappa_Pro_15B.pdf

These are a very tedious amp to service.....I too have worked on many, along with the SWA 1801 and the SA1232Z & SA1532Z. There are fuses to isolate the switched hi voltage rail(s) from the lower voltage rails that the power amp runs off. I always spend the time getting the basic power amp and front end amp/LP Filter system working first, before moving on to the high voltage system. You do have to get all the parts that failed replaced first, since the high voltage system is on the same board as the power amp circuit is on. I've also changed to MJL21193/MJL21194 power xstrs, as they're far more rugged than the parts Mackie used.

To work on the these amps, you do need a suitable bipolar lab supply, capable of powering the power amp circuit...I forget if it's +/- 18V or +/- 25V without pulling up the schematics. It would help if you understand how the switched HV power amp circuit works.

You can run this open circuit while getting the system back up and running. However, you MUST have a working woofer in the cabinet, as if that woofer you have is marginal or bad, it can kill the amp. There is NO circuit protection on the power amp/HV supply system, which has been a big problem with these amps. We stopped using them in our rental inventory, along with the SWA 1801's, as the repairs/maintenance on them was just too costly over time. Clever circuit. DJ operators are known killers of the system.

[nevetslab]

Schematics for the SWA 1501

I'm not sure if you have the schematics for this amp, so here they are:

AC Input PCB Assembly Schematics and Layout (23020680).pdf
Amplifier PCB Assembly Schematics and Layout (23020690).pdf
Input PCB Assembly Schematics and Layout (23020700).pdf
Parts List (090-387-00, April 2004).pdf
SERVICE BULLETIN -- Fuse Modification.pdf
SWA Diode Swap service bulletin.pdf
HCPL-3100.pdf

I had written up service notes on the SWA 1801, and posted them here on the forum in the past. I'll check my service records to see what I had written up during servicing the SWA 1501. I hadn't formalized a service document on that, but would have notes in my database on it. I see this is the one whose power amp runs off of +/- 25VDC until the signal requires more output, at which the upper and lower rails switch on and off to supply the higher voltages for those moments. You can find these SWA 1801 service notes here:

http://music-electronics-forum.com/t35220-post328871/ posted 1/2/14

I just compiled what service notes I had in my database on the SWA 1501, one from 2013, two from 2011, which were the first two of these I had serviced. It's a running dialog thru the procedure, so there is comedy of errors thru the notes.

Mackie SWA 1501 Service Notes.doc

[charlesaudio]

I know, it's an old thread...

Thank you for your notes nevetslab. I just serviced one of those amp. It seems to work ok but it does a quite loud ''thump'' when being turned ON and OFF. Since there is no relay protection circuit I think it's normal but would have appreciate a comment on that.

Thanks for your help.

[nevetslab]

I know, it's an old thread...

Thank you for your notes nevetslab. I just serviced one of those amp. It seems to work ok but it does a quite loud ''thump'' when being turned ON and OFF. Since there is no relay protection circuit I think it's normal but would have appreciate a comment on that.

Thanks for your help.

I don't recall loud Thumping when turning the amp on and off. There is a Mute circuit that's part of the +/- 15V supply regulator resident on the Preamp/Crossover PCB assembly. It talks to the Muting FET T13 (J107 N-Ch Switching FET) on the output of the LF Op Amp U3A that feeds the power amp circuit. That Mute control line also connects to the power amp via ribbon cable J8, which brings in the unregulated +/- 25V from the power amp / power supply PCB, as well as carrying the LF drive signal to the power amp. Q19 FET is an additional Muting circuit that also has the Thermal Switch connected to mute the Power Amp if overheated. I'd look at that Mute circuit in the power supply regulator first to make sure it's doing its' job. If the mute circuit and the controlling FET's are all ok, then its possible you're dealing with an aged filter cap(s). If you have a dual trace scope & X10 probes, I'd start by monitoring the turn-on supply voltages on the main power supply filter caps, then check that of the +/- 25VDC supply caps, then the output of the regulated +/- 15V to see that all are coming up symmetrically. By now, you've got a lot of time/hours on the filter caps in the amp. I've had to replace the main buss caps in the power amp before (on SWA 1501's, SW1801's, SA1232's & SA1532's).

I think the MUTE control line has a delay to it, starting off at 0V, and charges up to a negative voltage, since cap C5 is being charged by R14. The negative voltage on that mute line would unmute the FET's after the voltage charges up.

If that is so, the Thermal Switch is a COR format. If it's COR, you could manually short it and check if the turn on/off thump still occurs. Of course, all this assumes the Muting FET Q19 works. Likewise with the Muting FET following U3A on the crossover amp.