Mackie SWA1501

[HadYourPhil]

R-25, 26, 72 and 73 are all the same value, 330 ohms

[boogafunk]

i have a 1501 on the bench. i used to own these and sold them off. now there back for repair. when i got it F1`was shot(both)alt and one near L3. ohm load on driver is 1.3 ohms(bad). replace fuses powered up, no sound, no voltage at speaker leads, signal light is dim light withought input. r54 (5watt) is open. is there something else i should look for after replacing r54. ? i have schems. the value is not clear to me, it says r33 on the schems.
any help is appreciated.

[tonykara]

R54 is a 0.33 ohm, 5 watt resistor. If that fried, I would bet Q14 fried as well. You should check ALL the transistors, diodes, etc. I had an amp that nearly required a shotgun replacement because so much was blown on it.

[Dr_Amp]

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 was recently given a pair of SWA1501s to repair and found the above advice very handy and saved me some time. There are a couple of bits of info that I would like to add to it that I found from other sources:

1) The diagrams for the SWA1501 show a 12ohm driver, this may have been true for the better American built models but it is not for the cheaper Chinese ones. I measured the working sub I was given as a comparison and it was 8ohms. In the repair that I did, I replaced the original short circuit driver for an Eminence Kappa-15 450W Bass speaker. These are about £75 + VAT in the UK. The new speaker has a much larger magnet and I would assume a larger power rating than the original which looks like a 300W unit to me. If you are replacing an original 12 ohm unit with an 8ohm replacement like the one I mention, note that by decreasing the speaker impedance you effectively increase the gain, so the input volume set previously must be reduced accordingly.

2) From the shear number of post showing that the 15V supplies in these units keep burning out, I would say that the regulation circuit is a bit under rated. In my repair I have uprated both 15V supplies using the following components:

22uF 85°C capacitors have been replaced with 105° 22uF versions (C15, C16, C32, C34)

15V 500mW Zener diodes replaced with 1.3W BZX85 (D17, D18, D45, D46)

The 330ohm 500mW resistors have been replaced with 2W 330ohm resistors

3) Before fitting your new speaker, and after complteing the repair, use a multimeter in DC voltage mode, and very carefully measure the voltage across the speaker terminals, you are looking for a DC voltage of <1V DC. If your repair is good them there will be minimal DC, if a If you haven't found all the faults yet you may have >50V, which may well cook your new speaker, and possibly give you a shock if touched.

4) Mostly, for a well rated PA system you can run with the limit lights just flickering slightly with the bass kicks and it will run without damaging the system. This is not the case with the SWA1501s. From what I can see there is no margin added for people abusing the system, so if the yellow limit light is on at all back the volume off to preserve the life of the unit.

Many people do not realise that when you have reached the end of an amplifiers capability, typically when you run out of volts to supply to the speaker at high volume / power, turning it up further will not give you more audable volume increase, for a bass speaker, it will just overheat your speakers up, leading to premature failure.

[toddm]

Blown up SWA1501

I'm attempting to repair an SWA1501. When I first saw it, it had burned up two resistors, R17 & R64. I found this site and started checking all the components and also found R63, D10, D40 & C9 all damaged by heat from The two resistors as well as the copper plane underneath. It did not appear to have damaged any traces. I also found Q5,Q7,Q15 and Q18, either blown or shorted. I replaced all of the above and powered it up and nothing blew up. I took it to our sound guys and it actually worked for about 20 minutes, then made some angry sounds and smoked a bit.

Now the damage is much worse. I'm fairly sure I missed something before, but I'm not sure what. R17 &R64 burned up again as well as R32 & R63. also Q5, Q7, Q15, Q18 and new this time around Q14 & Q17. I found D27 open this time, Didn't check it before. D26 is shorted, R57 is open, R58 is 12.8Ohms, should be 6.8. FUse f2 blew this time and D9 is shorted. The trace from Q14 collector to R16 partially vaporized and lots of it is lifted from the PCB. The trace from Q14 Base is also vaporized where it passed under the burned up R32. I don't know if R32 got it or if Q14 took it out.

I have checked every diode and every resistor is the sections that had trauma. I don't know how to check the capacitors, other than for shorts.
I have an o-scope but I don't want to power it up till I'm sure its not going to cause any damage.

After I replace and repair all the above, What else should I check before I power it up again? Do I need to check the pre-amp board?

Thanks in advance!

Todd

[Dr_Amp]

Hi Todd,

From your description you have had a really unlucky repair. It sounds like you have a mixture of amplifer trannys and power supply parts being taken out.

Bearing in mind I don't know your experience level I'll give a complete answer, ignore anything that you already know....

1) It sounds like you have been working on the Amplifier alone, so before plugging it in again measure the resistance of the actual speaker unit. When you do this don't just measure the speaker terminals that plug in to the amp, get another person to carefully push the cone in as you can sometimes get faults that happen when the cone moves rather than just still. you are looking for a resistance of >=8ohms to <=12ohms.

2) Unplug the input board as the main amplifier can be powered up with out it. Measure all of the silicone on the board including diodes, transistors, and IC's (for the ICs measure using the + and - rails to each of the pins to any other pin, none should be short) using diode test mode on you multimeter.

3) Measure all of the silicon on the main amplifier board, replace anything faulty. Also measure all of the resistors too. Caps do not normally go, but if they were next to a resistor that has burned up, replace them (prefereably with 105 degree rated parts).

4) Once you have the main amp board fixed and fuses replaced, very carefully, power the board up for a second and switch off. make sure the fuses have survived the power on. No speaker or pre amp board should be plugged in at this stage.

5) Power on and measure the rails both + and - 15V rails, and the main 140V bus. Be carefull when doing this not to short antything out.

6) If all is good measure the speaker connection point, there should be <1V here.

7) Switch off, and assuming you have fixed any issues with the pre amp board, fit this, power up, and re-measure all the rails and the speaker output.

8) If you have got to this stage with no blowups, and provided that the speaker is found to measure OK, rest the amplifier board on something of suitable hight, and plug it in to the speaker, switch on for a second, and if OK, leave on for longer.

Safety specs would be a good idea when powering the boards up after repairing - particularly in your case as heavy shorts can result in flying bits of transistor casing.

Hope this helps.

Dr_Amp

3)

[toddm]

Hi Todd,

From your description you have had a really unlucky repair. It sounds like you have a mixture of amplifer trannys and power supply parts being taken out.

Bearing in mind I don't know your experience level I'll give a complete answer, ignore anything that you already know....

1) It sounds like you have been working on the Amplifier alone, so before plugging it in again measure the resistance of the actual speaker unit. When you do this don't just measure the speaker terminals that plug in to the amp, get another person to carefully push the cone in as you can sometimes get faults that happen when the cone moves rather than just still. you are looking for a resistance of >=8ohms to <=12ohms.

2) Unplug the input board as the main amplifier can be powered up with out it. Measure all of the silicone on the board including diodes, transistors, and IC's (for the ICs measure using the + and - rails to each of the pins to any other pin, none should be short) using diode test mode on you multimeter.

3) Measure all of the silicon on the main amplifier board, replace anything faulty. Also measure all of the resistors too. Caps do not normally go, but if they were next to a resistor that has burned up, replace them (prefereably with 105 degree rated parts).

4) Once you have the main amp board fixed and fuses replaced, very carefully, power the board up for a second and switch off. make sure the fuses have survived the power on. No speaker or pre amp board should be plugged in at this stage.

5) Power on and measure the rails both + and - 15V rails, and the main 140V bus. Be carefull when doing this not to short antything out.

6) If all is good measure the speaker connection point, there should be <1V here.

7) Switch off, and assuming you have fixed any issues with the pre amp board, fit this, power up, and re-measure all the rails and the speaker output.

8) If you have got to this stage with no blowups, and provided that the speaker is found to measure OK, rest the amplifier board on something of suitable hight, and plug it in to the speaker, switch on for a second, and if OK, leave on for longer.

Safety specs would be a good idea when powering the boards up after repairing - particularly in your case as heavy shorts can result in flying bits of transistor casing.

Hope this helps.

Dr_Amp

3)

Thanks Dr_Amp! That does help. I have experience with digital, but this is the first analog circuit I have tried to troubleshoot.

I only measured the speaker at rest, I'll try your method of moving it a bit.

I have no schematics for the preamp board, but I can check the diodes and transistors. Nothing was obviously blown on it.

What should the tolerance be on the +/-15 rails?

On the speaker output measurement, the <1v is to demonstrate that the two "sides" of the amp are reasonably balanced?


Thanks,

Todd

[Enzo]

Be aware that when you try to measure resistance of a speaker voice coil, a moving voice coil generates a voltage as it moves through the magnetic field. This will seriously confuse your meter.

[toddm]

Be aware that when you try to measure resistance of a speaker voice coil, a moving voice coil generates a voltage as it moves through the magnetic field. This will seriously confuse your meter.

True but just while it is moving, if i'm able to hold it still, it shouldn't be a problem. Enzo, do you recommend not testing the driver this way?

Thanks,

Todd

[Enzo]

NO, go ahead. If there is a broken tinsel wire that only evidences itswelf at the end of excursion or something, it is good to find out.

I just wanted you to know that you will very likely get readings moving all over the place, as soon as you try moving the cone. try it. For that matter, flip your meter to volts and push on the cone, see how many volts you can create.

[Dr_Amp]

Be aware that when you try to measure resistance of a speaker voice coil, a moving voice coil generates a voltage as it moves through the magnetic field. This will seriously confuse your meter.

Hi Enzo,

you are indeed correct, since the speaker will act as a generator when the cone is moving, however, if it is pushed in to a fixed position it will be fine. I have had issues in the past where the either the kapton coil former has not been quite cylindrical, or has been warped due to overheating, and where the glue holding the pole piece has given out, and this can not always be seen by direct DC resistance measure, hence pushing the cone in to check it at the extremes of movement. I should have explained this in my original post, so thanks.

Dr_Amp

[Dr_Amp]

Thanks Dr_Amp! That does help. I have experience with digital, but this is the first analog circuit I have tried to troubleshoot.

I only measured the speaker at rest, I'll try your method of moving it a bit.

I have no schematics for the preamp board, but I can check the diodes and transistors. Nothing was obviously blown on it.

What should the tolerance be on the +/-15 rails?

On the speaker output measurement, the <1v is to demonstrate that the two "sides" of the amp are reasonably balanced?


Thanks,

Todd

Hi Todd,

I didn't notice any tolerances on the diagrams that I have for the 15V rail, but I would say if it is within +/-0.5V then you are good to go. From memory the one I just repaired measured about 14.7V on each rail. I'll attempt to post the diagram for you to use - it's hard trying to fix electronic equipment when you are not familiar with the architecture of the circuit.

Yes, checking for <1V shows that there is balance, but more importantly that you have probably fixed the major faults that will burn up your speaker cone i.e. the ones that could apply full DC bus (140VDC) to speaker terminals.

The diagrams are attached - Happy hunting!

Cheers

Dr_Amp

[Enzo]

I have FIVE of these goddam things lined up in the shop here. Five failed amps, five dead speakers.

No one breaks anything small anymore.

[toddm]

MORE INFO

OK I had some time today to fool with the amp.

I found the fuse on the AC input blown also, didn't notice it till i tried to power up.

Replaced fuse and It powered up with no drama- Pre-amp disconnected.
+15_SW = 16.45V
-15_SW = -15.47V
+15V= 14.99v
-15V= -15.37v
D43 across + and - = 52v
J1/1-2 = 0.78V

The 140VDC measured at 146v right at D35.

R1 is getting hot-can't hold my finger on it.

Voltage input to U1 is 16.36vdc across pins 7 and 8. Voltage across pins 1 & 2 is 0.948V.

I hear a little buzzing sound every once in a while, sounds like a wasp making a low level pass, but not that loud. its once very 20 seconds or so.

I measured between the +140 and both sides of D9. Anode is at 150V and cathode is at 25V

no fuses blowing, no smoke.

Does this all seem reasonable?

Also does anyone have a schematic for the pre-amp board? Ive got the one from enzo for the amp board.

[Appaloosa]

Hi, I see this is an old thread but I seem to be having some of the same problems many of you have faced. My 1501 recently blew the speaker, Q4 and several of the 330 ohm resistors. I notice when I power up the main board without the preamp board plugged in, it seems to hold up- nothing starts smoking anyway. I do notice that R1 gets very hot to touch, i assume this resistor shouldn't get this hot? or would this be due to having no speaker hooked up or something? I also notice I get around 23V DC at the speaker terminals. Should I be looking for another shorted resistor or faulty diode? or is this scenario acceptable. any advice or help is greatly appreciated. thanks