Mackie SWA1501

[Jazz P Bass]

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.

The inputs to U1 will look like the output , as they are a sample of it.
That sample will turn on U1's internal LED which changes the output pins to the FET.
That spike is weird.

[Shredso]

My 1501 blew at the gig last night. I popped it open to see what's up and did some searching and came up with this site. The f1 (alt) fuse was catastrophically blown. When I pulled it the fuse clip tab broke off. I eyed over the board and I don't see any obvious fault. I checked a few of the commonly blown resistors on this thread and they came up fine. Is it possible that vibration and a loose fuse clip caused this to blow? Are there things I should check before I power this up after changing the fuse holder (assuming I can find one that fits)? I'll keep reading this thread in the meantime hoping someone is still looking at this old thread. Thanks!

[Shredso]

After some more searching I tested some of the transistors. I know a little about board repair from pinball machines but I'm no expert. I tested all the transistors under the heatsinks and the only ones I get a usable reading from are q17 and q18. The other 6 read as shorts on both legs. I don't have the schematic so I'm not sure if some of these are FETs and I'm not really sure what readings I'm looking for. The 2SA1943 at q14 gets a short no matter what lead I put on any leg so I'm fairly certain it's no good.

[Jazz P Bass]

Schematic for the power amp is here: http://elektrotanya.com/PREVIEWS/634..._sch.pdf_1.png

[Shredso]

Schematic for the power amp is here: http://elektrotanya.com/PREVIEWS/634..._sch.pdf_1.png

Thanks. I'll take a closer look and retest. Are there certain components that I should just change while I'm in there? like a bulletproofing? I also saw some black cardboard on the floor near I was working. Chunks of speaker cone. Is reconing with a kit a viable option or should I spring for a new speaker?

[Shredso]

I just made a huge post but I don't think it went through. I'll try and replicate it. Sorry if it double posts.

I tested a bunch of components against a list of common fail stuff I found on here. Most tested bad. They mentioned the BAV20 diodes were a common fail, can you test these in circuit? I was under the impression you couldn't get an accurate reading from a diode when it was installed on a board.

I'm still unsure if I'm gonna tackle this, I guess it depends how much help is available here. I don't know a whole lot about amplifiers. Here is my shopping list so far. Please let me know if there are additional things that would be good to have on hand.

25a1943
tip122
mje15032
2k9306
(2) irf630
(12) BAV20

The 5200 tested fine but if it's cheap I may grab one.
IRF640 was on the common fail list, I couldn't find it on the schematic.

Thanks for any help.

[nevetslab]

SWA1501 with assymetrical cascode stage conduction @ turn-on & turn-off

Having SWA 1501’s, SWA 1801’s , SA1232z & SA1532z’s in the rental inventory, I get to see these in the shop often. Having just serviced a SWA 1501, after replacing overheated 330 ohm resistors on the main amp supply (R72, R73), and D45, D46 1N5245’s, and replaced the stock 22/25V caps C34 & C35 with high temp 22/63V, I found an odd behavior when I went to bring up the supplies on this one. I always remove the 8A fuse F1 (or equiv fuses on the SWA 1801’s), so you can verify all is working normal on the basic amplifier circuit, and watch the DC level at the output at J1/D33-A/D30-C while running up the variac & watching the power analyzer for any tell-tale signs of faults that I didn’t already catch when it was apart.

Normally, I’ll see the two LED’s D15 & D16 light up simultaneously (voltage ref for Q13, Q16) after you’ve got the AC Mains up above 27VAC. This one, though, only one of the two LED’s would light up. As I began looking at the voltages in the cascode stage, finding one normal, and the other ‘off’, switching the Mains off, then back on, it flipped to the other LED, with it's cascode stage normal, the compliment 'off' Raising the AC Mains higher (no fault current being drawn) to 60VAC, still have this same flip-flop behavior, now accompanied by the DC level of the output stage @ J1 at 10V or -10V, depending on which half is conducting.

I shut it down, and lifted up R16 & R63 (emitter resistors of the voltage gain stage Q5 & Q18), so there would only be the cascode circuit to deal with. Neither cascode circuit was drawing excessive current, they just weren’t running complimentary. The bases of Q8 & Q9, during this single-side conduction condition, would typically be around 170mV, and the emitters of them at around 900mV, so Q8 or Q9 were ok, depending on which leg was conducting. Shorting the bases to Gnd didn’t change the one-sided condition. The power amp stage was off, since I had Q5 & Q18 off, J1 remained at 0V. All resistors nominal, U3 nominal, semis' nominal.

Puzzled, and not finding any obvious reason for it, I replaced Q8, Q9, Q13 & Q16, though I didn’t take the time to match them. Powering up again, same behavior. At that point, I raised the AC Mains to 80VAC, and finally both sides were up. Shut down, re-attached R16 & R63 to put the rest of the amp stages into play. I had DC offset until I got past 80VAC, then it dropped to well below 100mV as I got up to nominal line voltage. Powering down, one side of the cascode circuit would stay up longer, and as a result, a DC offset, though just briefly (less than 250mS) Powering up normal at line voltage, both sides came up the same, but there would be a turn-off transient.

Maybe I just never noticed this behavior before. The amp ran fine, so after powering down and discharging the HV supply & LV supplies, I added the 8A fuse F1, and verified the switched HV was behaving correctly, conducting on signal peaks above the fixed LV supply levels.

I normally use 1/3 octave pink noise, 80Hz and lower for my signal source to watch the behavior of the switched HV supply on both the positive and negative rails, while listening to it (once past using dummy load), and all worked fine there. I don’t have a 16 ohm bass cabinet in the shop (just 4 & 8 ohm), and at moderate levels, it drives the 8 ohm cabinet fine. Went to fetch it’s mating cabinet, only to find in this last rental, the 12 ohm woofer had failed short. So, now I have two SWA 1501’s needing woofers.

I never have seen any Thiele/Small data on the original woofers Mackie used (RCF, Eminence?). I’ve seen posts regarding using Peavey Black Widow woofers, or Eminence Kappa Pro 15LF2 woofer as substitutes. I assume it’s the Peavey 1505-8DT, having the greater Xmax spec. Both woofers, though, are 8 ohm. I just don’t think there’s enough silicon in the SWA 1501’s amplifier to tolerate long term service without failures. Who offers a good match to the cabinet in a 16 ohm?

[frostyfx]

Hello everyone,

I hope someone on here will be able to help guide me in repairing a SWA1501.
I picked one up yesterday and when I got home to test it, at the first power up I got a thump in the speaker and the Power LED and Thermal LED are lit.
I switched the amp off and for a couple of Thumps that faded as the capacitors discharged I'm guessing.
I turned the speaker on again and again for a thump and the two LED's lit. I proceeded to test and see if I would get sound out of the speaker and I did.
Everything seems to be working fine except for the fact the Thermal LED is always lit.
One other thing I noticed which I am not sure if it is a problem or not, The volume on the output to full range is about twice as loud as the volume on the high passed output.

Thank you for any help in advance.

[Enzo]

Hi, welcome to the forum.

Let me suggest you start a new thread for your amp. We already have 188 posts here from a thread that started 8 years ago and has been dead for over three years. You will get better response with a new thread. No one will want to read all this thread to get to yours, if you get me.