Mackie SWA1501

[oc disorder]

Got a Mackie SWA1501 here where some of the board mounting nuts have wobbled loose and caused some short circuits.
I would have thought Loctite or locknuts should be used in these subwoofers
as its the 2nd one I've seen with this problem.
I recommend to anyone with a powered sub to dismantle it and check that all the nuts 'n screws are nice and tight and suggest putting Loctite or similar on the threads to avoid a major breakdown.
I'm trying to get a schematic (unsuccessful so far) and would like to know the value of R63,R64 and R72. Can probably figure it out without a diagram except for those burnt resistors.
Thanks

[Enzo]

R63 2.2 ohm .5w
R64 56 ohm
R72 330 ohm

R72 drops to a zener with filter - D45, C34 paralleled Check them for shorts. +15 supply

R73 serves the -15v rail with D46, C32. Just for refernce.

[oc disorder]

Thank you Enzo for taking the time to look that up.
Your help is globally appreciated !

[HadYourPhil]

I have two of these beasts that are kicking my butt. I have the schematic and have replaced a handful of semiconductors to no avail. Anyone worked on these things and able to give me a tip or two? Thank you.

[Enzo]

Here's a tip, ask a more specific question. What are you trying to fix on them, and what problems are you seeing?

I just get out my meter and test every transistor and diode on the board. I find that faster than real troubleshooting most times. Once those are sorted out, you will probably find opened resistors.

[HadYourPhil]

Biggest issue is the + - 15 volt supplies and the 1 amp fuse therein. Also, it's not clear what the higher voltage supply is, as that's not given on the schematic.
These amps are hard to test because of their power. I had a complaint on one of an intermittent input. While troubleshooting the preamp board by tapping on it, I inadvertantly created a transient that literally made sparks fly! That fried several diodes and transistors in short order, but not the outputs! They seem pretty tough...
And I agree. Testing the semiconductors first is a good thing.

[Enzo]

1A fuse is CT of low voltage winding. That feeds bridge D43. That makes +/-25vDC, C29,C28 filtering. That feeds two separate zener regulated +/-15vDC pairs. One per 5532 IC, plus a couple other things.

Make sure none of the 330 ohm resistors are open, and no shorted zeners. By supply rail:
+15v: D45,C34,R72 - 1N5245,22uf/25v,330 ohm
-15v: D46,C32,R73
+15v_SW: D17,C15,R25
-15v_SW: D18,C16,R26

I often find one of those rails burnt up.

I highly suspect your transient was not a signal pulse that damaged anything, rather it sounds more like a momentary short. The preamp board uses the +/-25v rails, not the 15s. It has its own tracking +/-15v regulators. SO it can independently blow that 1A fuse. Leave the preamp disconnected until the power amp is whole again.


The schematic specs 100VAC for the high voltage winding, so that means it rectifies and filters to about 140vDC. You can think of that as +/-70v if you like, though it is not referenced to ground. That is across #1-#2. Chances are if you have high voltage, it is correct unless you lost a filter cap.

But there is that little aux power supply circuit around Q4. Check that whole thing out. Bad Q4s are common, and check the diodes and resistors. I have not had to replace the IC yet, but you never know.

[crombie]

swa 1501

i also have an issue with a mackie 1501, i replaced the burnt resistors and turned the unit on and resistors r64 and r17 started to smoke, does anybody have an idea as to what should be replaced to stop them from cooking ??
Any help would be greatly apreciated I also replaced a few other resistors that seemed to hold up when powered up.. The resistors are on the 15 volt rail

Thanks
Dave

[HadYourPhil]

I have powered one amp, minus the preamp, to 80v on a Variac. It seems to be behaving. I do not have a speaker connected (these were brought minus the speakers), and notice about .02v DC offset. R1 is getting quite warm, though not overly so. I have an ammeter in place of the 1a fuse, it measures no current draw. And, the + and - 15v LED's are lit.
Any further tips, maybe on the preamp?
(off to check it next).
Enzo, you are a big help!!

[HadYourPhil]

Success! (at least with one)

The strangest thing had happened to this amp's preamp. T-14, the pass transistor in the +25v rail of the preamp's power supply, had broken off at the printed circuit board!! That explains the fireworks I received when I tapped on the board. I was able to resolder it as there was enough lead still on the transistor.

It has been playing just fine for about 15-20 minutes. Current measured through the 1A fuse is about 15ma.

On to amp #2, which also has a broken T-14!! This one only had the emitter lead broken.
In the future, I'm checking that transistor first.

[HadYourPhil]

Well... No luck with the second amp. Once again, there's a preamp issue. As mentioned above, T-14 had a broken emitter lead, which was repaired. Then, I found T-5 was shorted. That was replaced. Next, R84 had overheated. It was replaced. Still, no luck. R84 gets quite hot and I have no -15 LED on the amp chassis. On the preamp chassis, D3 lights up, and as more line voltage is supplied, D1 lights up. Also, I measure about 7-8v on the speaker terminals. If the preamp is disconnected, the amp works normally.
Now, I'm really stumped!

[Enzo]

Resistors are never at fault when they get hot, something else is drawing excess current through them.

If connecting hte preamp boards messes with your power supplies, then look on hte preamp board. As I mentioned earlier, the preamp has its own voltage regulators, it takes +/-25v from the power amp and regulates those down to whatever it wants, so you either have a shorted power supply on that board, or a shorted IC or bypass cap.

[HadYourPhil]

As mentioned above, T-14, the partially disconnected transistor is the pass transistor in the positive supply. The failure of T-5 in the negative rail is a mystery, as is the burnt resistor in the LF output. There's some draw there, because the "thermo" LED (D-3) lights up.
I'll check the bypass caps next (with an ESR meter, BTW).
Thanks for your suggestions. It's too bad we can't discuss this on the phone.

[Enzo]

Oh, we can do this without the phone. I only have so many hours in a day I can contribute to the world at large for free. I finally had to turn off my email button.

[HadYourPhil]

I hear you! I'm trying to make a living taking care of everything from TV transmitters to stuff like this Mackie, and it is all more time consuming than financially rewarding.
I do greatly appreciate your limited time.