Mackie SWA1501

[J M Fahey]

Hi crombie.
You must first get your DC stuff good, that's to say no DC on the output, voltages across transistors reasonable, Vbe "the right way", meaning around 0.5 to 0.65 forward bias, not -15Vbe on an NPN, etc., and your multimeter is good enough for that.
Only after that you can run signal tests.
Good luck.
PS: if it drives you crazy or sleepless, just abandon it for a week, I mean *forget* it, and do something else.
Maybe next time you'll slap yourself and say "OMG, why didn't I see THAT !!!!"

[clydey]

enzo any chance of posting the schematics please

[crombie]

Thanks for the help!!!!!!

I did check for any dc voltage at the outputs there is +.401 dc at pin 1 to ground on jumper 1 nothing on pin 2also I did check the preamp and it see:ms to be getting signal to the power board because when a signal is applied D44 and d 42 blinks slightly if that means much,I also get 4.7 ohms across q19 d and s i am guessing that would qualify as high ohms?

[HDSarge]

SWA1501 problems

If Q19 is measuring 4.7ohms from signal line to ground, that is low ohms. The result is to mute the signal....which is what you are experiencing, right?
I would lift either source or drain of Q19(or both), carefully power up and try some signal.

This is assuming you have verified the DC supplies.

If this works Q19 is likely bad, but there is simple circuit that drives it that should be checked.

The Thermal switch is supposed to drive the gate of Q19 to ground if the amp gets too hot. If the switch measure a short on a cold amp, it needs to be replaced. If the switch is okay, and Q19 is not....you may be done.

[WholeToneMusic]

Same ole Mackie

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.

I thought I would inquire on this thread instead of starting a new one since we're working on the same unit.
I have been working on a SWA1501 that came in with 2 burnt resistors R26 and R 73 and two blown fuses. I swapped out the resistors and fuses. I disconnected the transformers secondaries and fired it up with a current limiter to check the transformers primary. The primary was ok.
I reconnected the secondaries and the circuitry works fine until I shut it off. F2 fuse blows. The one connected from the secondary CT to ground. Also I noticed when checking the Bridge rectifier D35's voltage was +148VDC and the minus leg was 0 VDC. I had originally taken out D35 and checked it with a diode checker and it was ok. Does anyone have the Voltage readings of Q4's Source, gate and drain?
Any assistance would be appreciated.

[crombie]

Q 19 removal

Removed q19 but still no difference, I happend to check d35 and found the plus leg to be at about 116 v dc and the minus leg to be in -15v range with the neg. lead to the ground on the board.I am presuming those should be close to the same voltage, could this be the problem? or is this normal

[Small Town]

Hi crombie.

D35 should only be referenced from the fuse F1, as it is a floating supply, you should have around 130-140vdc between the positive pin of D35 and the fuse F1

[crombie]

The voltage is small

It looks as though there is only 135 v from + side of d35

[Small Town]

Ok another way to do it is measure carefully accross R1, there should be around 115vDC accross it, also there should be 15v DC accross d1, if this is all ok we need to look elsewhere.

[crombie]

voltage across r1 is 125v

voltage across d1 is 15v

[jrfrond]

We repair TONS of Mackie subs, and therefore, we stock every single part we have ever found to be an issue, and then some. This includes the speaker. ALWAYS check the speakers in these boxes, because one thing we have found is that many of them have been overdriven and the voice coils literally fall off of the bobbin, shorting the power amp outputs. If you do all of the hard work repairing the power amp and the speaker is still shorted, you will be S.O.L.

BTW- an excellent source for 4 and 16 ohm drivers for subs is Peavey. Their Black Widow series work beautifully in Mackie and other subs, and are really the only stock speakers you can find with these impedances. I also stock the recone kits for the OEM speakers.

[J M Fahey]

Maybe somebody will shoot me for this, but I find the design of this Mackie amplifier *CRAZY* and stretching the "state of the art" way beyond its limits.
It's self evident by the amount of people asking for help, plus Mr Frondelli's statement ("we repair TONS of them")
Certainly not a trust/confidence inspiring remark!!
This amp is not a +/- 70V amplifier, it's +/- 140V powered !!!!!!.
I was misled (like doubtlessly many of you) by seeing those IRF 630 as a "virtual ground", "intelligent center tap" , whatever.
I always found them *too* skimpy for that task, until I finally understood :
This amp has two 140V rails, "emulated" by a *single* 140V supply that applies the correct voltage only when needed.
When the upper output transistor conducts, they switch on the opposite lower IRF, grounding the supply negative and presto! it has +140V available; when the lower one conducts, the opposite is done.
The IRF work as switches, little or no heat generated.
Problems:
1) that serious switching takes a short but definite time ...... but .... did I hear *subwoofer* mentioned? That would make the task easier. You have much more time until the next zero-crossing appears.
2) Any objectionable click (heard as buzz) that appears will be filtered by the speaker and the low freq. bandpass nature of the cabinet.
3) When amplifying low power signals, we are taxing the switching system as much as with full power ones, because it must switch on every zero-crossing, but if Dr Frankenstein , sorry, Dr Mackie stiches there some extra body part, namely an extra conventional +/-25V supply, and we hook it to the power amp via fast diodes, á la "Class H" , said power amplifier works as a very conventional "Class AB" one, for signals up to about 50VPP, say around 20W into 12 ohms.

If you still doubt my analysis, think about this: "500W continuous into 12 ohms load" ...... you *can't* get that with +/-70V supplies, no way.
I find it a daring Idea, gone too far.
I guess a kick drum hit , amplified through this, will be certainly be chest-thumping; I also think any *continuous* signal will kill it quickly.

You can't even suggest this to a client, but if I personally owned a couple of burnt ones, I would dump that crazy board , build a conventional, time proven 300W/4r reclaiming whatever I can if worth the bother, load some excellent 15" 4r speaker as suggested above and end with a trusty sub box.
I much prefer 300 "always working" watts to 500 "iffy" ones.
*Maybe* it requires a fan mounted over the heat sink, it might even be some clip-on kludge, mounted only on stage and pulled for transport; anyway it would live on the backside, away from the public's hands.

[WholeToneMusic]

Should the LED'S on the power board all be lit?
Right now D16&15 are lit.
D44,42 & 34 are not lit.
I am getting DC voltage on J1 (around 2 volts DC)
Measurements across R1 is 50 VDC (I read on here earlier that it should be 115VDC.)
Voltage across D1 is - 15 volts.

This is with the preamp disconnected.

[Steve Conner]

Well JMF, I agree with your analysis... but surely Mackie tested their creation carefully before putting it on the market?

There must be DJs out there who can destroy any subwoofer in existence with excessive volume and bass.

[Enzo]

Well THAT is certainly true.