Mackie SWA1501

[Enzo]

OK, lesson number 1. Resistance is not impedance. Impedance is more or less AC resistance, while resistance per se would be DC resistance. The DC resistance of the speaker voice coil is not its impedance. Impedance is a reactive value involving the inductance and capacitance as well as the resistance of the voice coil. Typically, speakers measure resistance a little lower than the nominal impedance. 8 ohm speakers might read 6 ohms for example. Look up the data on any speaker and there will be the impedance curve, showing how impedance varies - a LOT - over frequency range of speaker.

You can take the amp module out of the unit, and connect some other amp to the speaker itself to test if the speaker is still good. Blown amps often destroy the speakers.

I think it is asking a lot of someone with no experience to try to fix one of these things, especially if they think it will save money. Learning how to fix these is not like learning to cook an egg. Ain;t going to happen inside of an hour, or a week, or...

You could remove the amp assembly and ship it to a shop and have it back soon. If you read all 165 posts here, you should know that this is a complex system and not simple to repair. Not saying you can;t do it, but it won;t be simple.

[MarkusBass]

It's a very interesting approach - ask someone to learn how to fix the amp instead of sending it to a tech. Do they do the same when they need to fix a car, or a TV set?
Learning how to fix amp may take a year or two while fixing it may cost 200-300$. Maybe you just don't have an amp tech in the nearest neighbourhood.
But there's still a hope for you. You mentioned that in one of the subs the fuse lasted 1 week. Are you sure that it was 7 Amperes fast fuse? On the schematic I see 8 Amperes. So maybe (at least in this sub) you just need to buy a correct fuse (8A instead of 7A)?

Mark

[J M Fahey]

my venue believes it would be cheaper to have me learn to fix them instead of bringing them to a shop.

Sorry but no.
Just wishful thinking.
Cool and probably justified if you had to repair your own 15W entry level guitar amp.
The minimum bench service at any shop of around $60 (depending on location) would probable be more than what the amp´s worth; and repairing it would probably mean replacing a 5 leg inexpensive power chip, so there it might be worth to give it a try.
Worst case, you risk less than $5
But this particular Mackie, although excellent sounding (you already know that) is a complex beast, designed well beyond the standard parameters.
Brilliant designer, but very close to the cliff´s edge to be comfortable.
For most of us, a tough beast.
Rather than a learning experience, it will be frustrating for you and in the long run will backfire.
Welcome to the wonderful Electronics World, start learning, it´s fascinating, but go step by step.

[ianS]

Schematic diagram

[QUOTE=MarkusBass;217888]I'm looking for a schematic of the preamp used in SWA1501. I fixed the power amp thanks to this thread (thanks to Enzo and others ) and now I'm looking at the preamp. Most probably fired opamps but I'd like to see the schematic.


I searched on line for " Mackie SWA 1501 circuit diagram " and found a pdf file named SWA1501_AMP_board.pdf full schematic.

[MarkusBass]

I searched on line for " Mackie SWA 1501 circuit diagram " and found a pdf file named SWA1501_AMP_board.pdf full schematic.

Thanks for your help but have you noticed that this file does not contain schematic of the preamp, which I'm looking for?
BTW, the sub was fixed a long time ago.

Mark

[Martin Pedder]

Just got an SWA1501 running and all appears to be OK now. It was brought in with both mains and 1A fuses blown. The customer reported a "phut" when if first failed and smoke when they replaced the mains fuse. Both +15V 330R (R25 & R72) were charred but still working, and it was definitely making cooking smells when I ramped up the mains on a variac. There were a fair few dry-looking joints which I suspect were the original fault. By the time I had removed any silicon that I couldn't test in-circuit, and put them back in, the amp had stared to work without smoking.

My question:
When running, it makes small mechanical clicks on each peak in the audio. It is a "ts, ts-ts-ts, ts" sound with each of the louder bass notes. The speaker is 3m away on another bench so I can hear the amp. I think it occurs each time the high voltage part of the amp is operating. I first noticed it in my early testing with the variac as I passed through about 110V (we are 240V mains here as UK). I heard what sounded like a relay click or a small spark as the 140V supply came to life. There is no relay and nothing is arcing. The clicking appears to be coming from around the high voltage corner of the board (L1, C11, Q4 area).

Is this normal? If not, is something dying?

Incidentally, having read ALL of the posts in this thread, I can report that this one did have D19 and D20 fitted correctly but they are mislabelled in the screen printing.

For those not sure how to tell, this is how I tested them in-circuit. They are impossible to tell apart visually, even with a jeweller's loupe. I used a bench PSU with the current limit almost at the lowest (just a few mA). Croc clip the supply directly across the diode under test. As you increase the voltage, the current limit LED lights at 0.7V for D20 in either direction. This is because it has a base-emitter junction across it. On D19, current flows with the PSU one way at .7V but doesn't light until 12V the other way round. If your variable bench supply does not have a current limit, use a ammeter in circuit and look for current starting flow with that.

One final point - It is easy to mis-read the circuit and think that D19 and D24 are 15V zeners. The zener label 12V/0.5W is on the left, where most components are labelled to the right. The 15V refers to the "_SW" power rails. Watch out for this if ever replacing one when half asleep.

[Martin Pedder]

BUMP SWA1501 Mechanical Clicks - Is this normal? Enzo, Dr_Amp, Steve Conner, ANYONE?

Can anyone advise. The customer is pushing to get it back. I want to be confident it is OK before reassembly. Any thoughts please from those with more experience as to what it is and whether it is a problem.

Please see my post above....

My question:
When running, it makes small mechanical clicks on each peak in the audio. It is a "ts, ts-ts-ts, ts" sound with each of the louder bass notes. The speaker is 3m away on another bench so I can hear the amp. I think it occurs each time the high voltage part of the amp is operating. I first noticed it in my early testing with the variac as I passed through about 110V (we are 240V mains here as UK). I heard what sounded like a relay click or a small spark as the 140V supply came to life. There is no relay and nothing is arcing. The clicking appears to be coming from around the high voltage corner of the board (L1, C11, Q4 area).

Is this normal? If not, is something dying?

[Steve Conner]

The SWA1501 uses a Class-G amp that raises the rail voltage dynamically on loud peaks. Maybe one of the inductors in the power supply circuit is clicking when the voltage spikes up.

I can't remember if it has a switched mode power supply, but it is normal for the magnetics in those to click, fizzle and sing along to rapid changes in load.

You might want to use a scope to check for any obvious instability or RF oscillations on the output. Small glitches when the higher rail voltage kicks in are to be expected, and this design may rely on the bandpass speaker box to filter out the audible effect of them.

PS, the bandpass box may well also filter out the sound of a rubbing voice coil, which can short the amp intermittently.

[woodyc]

I just saw the voice coil pic above and wanted to add another -- in this case the speaker and mosfets saved all the fuses!



Sorry I don't have any suggestions on your repair, but good luck!

[J M Fahey]

Agree with steve c's diagnostic and add: this clever but too_close_to_the_edge amplifier uses a *single* power supply which gets switched to feed either the positive or negative rail, as needed.
Talk about transients !!
If the amp works normally but you hear some in sync clicks, I don't find that unusual.
FWIW the TO3 2N3055 I used for more than 30 years were famous for "singing" .
In fact they were close relatives to Piezo Tweeters.

@woodyc: Ouch !!!! clearly positive DC voltage.

[Martin Pedder]

Thanks guys. Tomorrow a soak test and Wednesday back to the customer......unless you hear back from me with some tale of woe.
Martin

[daviddas]

I've followed this thread to get my SWA1501 back running. (or partially running). My Power supplies are up, and it produces output. I have .06v bias between J1 & J2 but it seems to work. This seems low compared to what folks are posting on the forum.

The problem I have is that at high volumes it all goes crazy. If I put in a 60hz sin wave, I can output about 140v peak to peak as seen on the scope. However, if I turn it up any higher, the power supply starts buzzing really loudly and I get quite high frequency noise on my output signal. It will make this rattling noise for about 2 seconds and then throttle itself back to 140v peak to peak. The peak light is not lit during this. I assume that because of Q12 and Q15 it should be capable of roughly 280v peak to peak running full out?

Does this means I'm having timing problems with Q15, Q12 & Q4? How do I debug these?

[Martin Pedder]

> I have .06v bias between J1 & J2...
Do you mean J1-1 and J1-2 the speaker terminals?

I am not clear from your post whether you are driving the speaker, a dummy load or no load.

These circuits are hard to understand and others here probably know it better than me and someone has probably had this fault before.

It is tricky to debug the high voltage part of the circuit as it can be a positive or negative rail as required. One way is to use both channels of your scope in summing mode ("Add") with one channel inverted. Then using the two probes you will see the difference between them irrespective of ground. That way you can see what is going on in the U1/Q4 circuit.

[daviddas]

Martin - you've encouraged me to get to know my scope a little bit better. I'm driving the speaker as a load. It was reconed at 8 ohms - I had it reconed before I knew I should probably recone it for 16 - even though the original was 8.

Looks to me like the 140v power supply is bailing in the middle of my waves. Is that big power supply pwm'd to get supply a waveform approximately what the amp is producing? I assume that's to keep the difference of supply voltage to the output on the amp portion relatively low for low cooling. If this works how I think, the pulse train input to Q4 is going low when it needs that voltage the most.

However, I cannot figure out how to trace that input to Q4 past the U1. Inputs 1 & 2 on U1 appear on the scope to be the same as the speaker output - and I don't understand how that turns into a pulse train.


Yellow line is the speaker output. Blue is the big power supply at D31


Big power supply between D31 and D32


Blue is Q4 Gate, Yellow is speaker output

[J M Fahey]

I think that PSU simply can't supply the current demand of an 8 ohms load.
A regular amp and PSU would simply drop voltage and clip waveform tops, but that crazy amp has a *single* 140V suppy and applies it to either one rail or the other, as needed, which implies a crazy "musical chairs game", trying to guess who it will be next.
If it can't because of heavier than expected load, it misses, switches in the middle of a waveform (instead of at zero crossing where it should) and you get those *huge* spikes.
Try it with a 16 ohms load first.
The original speaker was "12 ohms" and somebody measured 12 ohm voice coil resistance, which actually makes it a 16 ohms one.
Won't reread all the thread, but I faintly remember somebody in Malaysia or Singapore had 2 Mackies, with 2 dead speakers and 1 dead amp ... and locally could only get "Mystery Brand" 8 ohms speakers, so he added a series jack to the powered one, used the other box passive and drove both speakers in series with just an amp.
Worked like a charm.