Mackie SWA1501

[crombie]

R63 And R16 still hanging in there

Both R63 and R16 not open, So I am hoping that Q5 And Q18 should solve this nightmare, will keep all post when I replace them

[J M Fahey]

I think there's more going on than meets the eye here.

Indeed.
Everytime I look at it I find more interesting details.
And I agree, Mackie is good, it always was, yet sometimes getting carried by technical novelty brings up teething pains.
I'm quite minimalist, yet find class G or H worth the extra complexity, but only above, say, 800 or 1000 W RMS.
In this case, they *should* have used an extra pair of output transistors in series, when signal peaks go over +/- 25V; they seem to have avoided that by having a switched ground connection that somehow chases the signal, meaning the +/-B tracks the signal (as in those grounded output Peaveys).
Not my cup of tea, but if it works for them ....

[Steve Conner]

In this case, they *should* have used an extra pair of output transistors in series, when signal peaks go over +/- 25V; they seem to have avoided that by having a switched ground connection

Well, if you think about it, I reckon the IRF630s are that extra pair of transistors in series with the main ones. They're "in series" because the high voltage rails are floating. So it's basically some sort of heavily disguised Class-G arrangement. It should work at reduced power with the high voltage rails disconnected, and that might help troubleshooting.

I also noticed that the woofer seems to be 12 ohms. That probably helps explain the lack of transistorage, too.

Thanks for posting the schematic Enzo, it was very interesting!

On a related note, can you buy replacement woofers for these things? I've got an amp here that would work great with a 12 ohm woofer.

[HDSarge]

This will not fix the 1501, but it may keep it out in the field once repaired.
The silkscreening for D19 and D20 is wrong on the pcb, and the parts are often stuffed following the error. So look at the schematic and ohm out D19/20. If they are reversed, the schematic is correct. So interchange them. Although both are diodes, one is a zener, the other a BAV21.

[crombie]

Thanks for the help

When you say interchange them do you mean simply reverse the direction or physically swap the two diodes??
As you probably can see, I know just enough to get myself in to deep as usual
Good thing this my own amp that I am trying to get running,
Thanks for any Help

[brianv4]

slow blow?

Hi, not a tech but tinker on the novice level. i work on my guitars, jbl eons electronic keyboards etc. i just purchased a mackie swa1501 sub from GC that doesn't work. took it home noticed 2 fuses were blown, since i had a 1501 already i decided to swap fuses to see if the new sub would work. it blew the fuse (6.3 amp one on the power board) immediately so i will have to take it to a repair center. problem is, now i need to replace the fuse in my original 1501. i know it's a 6.3 amp, 250 volt fuse but is is a slow or fast blow type? any help/advice would be greatly appreciated!

[Steve Conner]

You just bought the thing from a high street store. It should have a warranty.

If the fuse said "T" on it it's a slow blow, if it said "F" it's fast blow. It'll probably be slow.

[Enzo]

The fuses on the board are fast types, but the 6A mains fuse is a slow blow.

But really, get the dead new one into warranty repair, don;t mess with it inside.

[HDSarge]

SWA 1501 Diodes misplaced on pcb

Sorry to not be clear. The orientations (cathode and anode) of D19/D20 are correct. The problem is the the silkscreen is wrong, the parts match the wrong labeling. The result is the parts are oriented correctly, but placed wrong in the circuit.
With an ohmmeter (power off) verify that the BAV20(it is labeled on the diode) is between the emitter and base of Q11(2N3906). If it is not, and instead is connected between R38 and audio ground then the parts are wrong and need to be carefully removed and interchanged. Follow the orientations shown on the pcb just place D19 in D20's position and D20 in the D19 position.
I have never seen the diodes in question damaged by this error, but correcting it has removed several 1501 from the repair "frequent flyer" list.
A recent replacement amp pcb from LOUD still had this error. So even though they are aware of the problem...check the locations. In this business the repairman is his or her own QC, and simple checks speed work and significantly reduce the dread call backs.

[crombie]

Thanks to HDSarge

Thanks These Diodes were indeed out of place, I have another question as to R17 and R64 if those were replaced with 47 ohm resistors would that cause a big deal?? are they real fussy in that spot? I am really not sure what would suffer. I ask because I happen to have plenty of 47's but not a 56 in my many resistors.

[HDSarge]

SWA 1501 resistor values

I am sitting at my university desk preparing for the start of spring classes, so I apologize that I do not have a schematic close, but I remember R17 and 64 as rail to base resistors in the preamp section of the 1501.

If R17 and 64 are overheating, they are the effect, not the cause.

There are a number of transistors between the rails that are terminated with R17 and 64. These all need to be checked. Something is allowing(excessive) current to flow.

The values chosen for these resistors help establish the current through the front end of the amp. Changing the values throws another variable into your troubleshooting. Why not series connect a 10 ohm and a 47 ohm?

[brianv4]

thank for the responses, the new one is going in for warranty repair i just needed to replace the fuse in my old sub with the correct type.

[crombie]

resistor value

Thanks
I will give it a try there is not a lot of real eatate in that section on the board.
I will order a c pack of 56r's and just do it right. I did notice that after changing Q5 and Q18 the resistors didnt smoke but that was with the 47ohms in there. I did not give it an audio test, I am presuming that is where the change in the resitor value will be noticable.
Thanks
I will keep you posted when the 56's get installed

And Thanks so much for all the help so far

[crombie]

Resistors changed and still no sound

Hello to all,
Well it looks like its back to the search and hunt, I am still baffeled!!
D44 and D42 light green looks like to that point allis well When I put a signal in I get ever so slight sound coming out of the test driver. It all most seems like there is a road block for the signal.
by changingQ5 ang Q18 that stoped the resistors from burning but that is as far as I can go, I am very limited as to equipment no scope or signal genertator, Is it wise just to start changing all the transistors in the preamp section??
Thanks for any advice

[HDSarge]

SWA 1501 still not working

Have you tried tapping J2 (pin1/2) with your finger? That is the power amp input and you should hear a good thump from the driver.
If that does not produce sound. Check Pin 4 and 8 on IC3, should be -15v/15v.
If okay, then what about pin 7 (output)? should be very close to zero.
Next, there is a mute circuit Q19(FET) check ohms D to S, with power off, should be high. If not sure, you could remove Q19, or lift either S or D and power back up. If you still have no sound, I would set the VOM to "diode check" and test all the diodes and transistors. Make sure the 25V supplies are working.
If you get sound from tapping J2, then head back to the preamp. Is there continuity from J8 to J2. Are T14/6 mechanically intact (no legs broken at the pcb). If you input a signal can you run get an HF output? There are numerous mute FET's on the preamp, check them (T13 is on the output). The 25V supplies are reduced to 15V for the opamps. Check pin 4 and 8 on several opamps. Check pin 1/7 on signal path opamps, should be low.

Go slow, work safe with power on, make written notes so you do not lose track. Good luck.