Another SWA1501

[kevhunter]

Hi all,

Im banging my head against a wall with a SWA1501 amp and despite reading all the forum posts I still havent found the root cause

The amp died quite a while ago during a gig and got relegated to the back of the garage for a few years ! Im having a clear out and decided to have a go at fixing it. I opened it up and there is no sign of burnt out parts .

D15 is on but D16 isnt so I suspected the +15V supply but it measured good at 15.1V, I then had a look around the first amp stage and found R16 was measuring approx 1.5K rather then 2K2. No sign of physical damage till I took it out and it had slight damage on the underside. To be on the safe side I replaced everything in the +ve and -ve side of the amp.

R16,R17,Q6,C9,D14,D12,R33,Q13,R34,Q9,Q8,R45,Q16,R47,D40, D41,C26,Q18,R63,R64.

I also tested Q7 Q14,Q17,D26,D27,D28 and D29 all measured ok.

Built it back into the chassis and turned it on but its still the same !!

I checked the base of Q8 and Q9 and there is a DC bias which explains why one one the the LEDs light !

Pin7 of U3 is giving and output of approx 3.5v with the filter board unplugged and LF IN and GND shorted together . I measured the supply to U3 and found that the -15V starts at -15V but drops away to around -13V, i suspected U3 so took it out and tested the -15v . Stayed steady at -15V . Replaced U3 with a new one and exactly the same issue

Swapped D45 and D46 around to see if the fault moved but didnt, R72 and R73 measure OK and C32 &C34 are not shorted.

Unless im missing it I can only see that the -15V supplies just U3 .

Any ideas what Im missing ???

Unfortunately my access to test equipment is limited to a DVM no variac so Ive unplugged F1 until Ive got the LV one running .

Thanks in advance for any help you can give.

Kevin

[52 Bill]

Welcome to the place.

Reading through your post, I'm not certain that I understand the actual problem. The amp light up but there is no sound?

Do you have a schematic or a link that you can post?

[Jazz P Bass]

Schematic: SWA1501 Sub.zip

[Enzo]

I am an experienced professional, and I have to say, when I get these with blown amps, it is always a pain in the ass to make them whole. I know that isn't helpful, but just to let you know not to feel bad.

[kevhunter]

Welcome to the place.

Reading through your post, I'm not certain that I understand the actual problem. The amp light up but there is no sound?

Do you have a schematic or a link that you can post?

Apologies, let me see if I can clarify

The sub stopped working during a gig, no sound and the blue led on the front of the speaker wasnt on. I had taken the amp out at the time but cant remember what I did to it apart from putting it at the back of my garage to gather dust.
I took it out of the garage last week as part of a clear out and powered up just the amp (no speaker connected) and saw that D16 wasnt illuminated (both should be - its connected to the +15v rail) . I found a high resistance on a 2K2 resistor in the first amp stage but that turns out not to be the problem.

Further investigation show that there is a high DC bias going into the amp which is not a good thing and that the -15v rail is collapsing over a few seconds to around 13v.

Connecting a signal to it does make D16 flash if the input level is high enough and connecting a speaker gives a very distorted sound

Thanks
Kevin

[kevhunter]

I am an experienced professional, and I have to say, when I get these with blown amps, it is always a pain in the ass to make them whole. I know that isn't helpful, but just to let you know not to feel bad.

Thanks Enzo, I used to be an electronics engineer years ago but have not done diagnostics in a looooooooong time. I will get there its just going to take time, and of course not having all the test gear doesnt help

Kevin

[nevetslab]

Apologies, let me see if I can clarify

The sub stopped working during a gig, no sound and the blue led on the front of the speaker wasnt on. I had taken the amp out at the time but cant remember what I did to it apart from putting it at the back of my garage to gather dust.
I took it out of the garage last week as part of a clear out and powered up just the amp (no speaker connected) and saw that D16 wasnt illuminated (both should be - its connected to the +15v rail) . I found a high resistance on a 2K2 resistor in the first amp stage but that turns out not to be the problem.

Further investigation show that there is a high DC bias going into the amp which is not a good thing and that the -15v rail is collapsing over a few seconds to around 13v.

Connecting a signal to it does make D16 flash if the input level is high enough and connecting a speaker gives a very distorted sound

Thanks
Kevin

Welcome to the Forum! You have quite a challenge on your hands with only a multimeter available for troubleshooting one of these. I've repaired a good many SWA 1501's, SWA 1801's as well as the SA1232Z & SA1532Z. All are very similar in how their Hi Voltage power supply system works to augment the lower bipolar supply voltages to run the power amp. The SWA1501 & the SA1232Z & SA1532Z only uses one of the HCPL-3100 IC for switching the HV supplies. The SWA1801 uses a pair of them. One limiting factor in restoring the SWA 1501 is the loudspeaker itself. The original woofer was a 12 ohm speaker, now discontinued and unavailable. An 8 ohm speaker will NOT work with this unique amplifier, and there's really only one speaker suitable for the system....the Eminence Kappa Pro-15B 16 ohm woofer. I've attached the data sheet for it, just in case.

If you can get the LF power amp/front end/filter circuit working, you can remove the 8A fuse F1, which will at least allow checking the system under drive without having the switched HV supply applied. That's where I normally begin, after having removed the PCB assembly, finding/repairing any/all component failures and burnt/open circuit traces that DO occur in failure. These are tedious to repair, but, once restored, they do kick ass. BUT.....a weak point to them....there is NO Circuit Protection other than the fuses.

I have a lot of service notes from the repairs I've made on these amps. I've attached those notes, along with the service notes I've written up on the SWA 1801. I'll have to give some thought on how to best proceed with only a multimeter available.

Kappa_Pro_15B.pdf
Mackie SWA 1501 Service Notes.doc
Mackie SWA 1801 Service Notes.doc

[nevetslab]

Hi all,

Im banging my head against a wall with a SWA1501 amp and despite reading all the forum posts I still havent found the root cause

The amp died quite a while ago during a gig and got relegated to the back of the garage for a few years ! Im having a clear out and decided to have a go at fixing it. I opened it up and there is no sign of burnt out parts .

D15 is on but D16 isnt so I suspected the +15V supply but it measured good at 15.1V, I then had a look around the first amp stage and found R16 was measuring approx 1.5K rather then 2K2. No sign of physical damage till I took it out and it had slight damage on the underside. To be on the safe side I replaced everything in the +ve and -ve side of the amp.

R16,R17,Q6,C9,D14,D12,R33,Q13,R34,Q9,Q8,R45,Q16,R47,D40, D41,C26,Q18,R63,R64.

I also tested Q7 Q14,Q17,D26,D27,D28 and D29 all measured ok.

Built it back into the chassis and turned it on but its still the same !!

I checked the base of Q8 and Q9 and there is a DC bias which explains why one one the the LEDs light !

Pin7 of U3 is giving and output of approx 3.5v with the filter board unplugged and LF IN and GND shorted together . I measured the supply to U3 and found that the -15V starts at -15V but drops away to around -13V, i suspected U3 so took it out and tested the -15v . Stayed steady at -15V . Replaced U3 with a new one and exactly the same issue

Swapped D45 and D46 around to see if the fault moved but didnt, R72 and R73 measure OK and C32 &C34 are not shorted.

Unless im missing it I can only see that the -15V supplies just U3 .

Any ideas what Im missing ???

Unfortunately my access to test equipment is limited to a DVM no variac so Ive unplugged F1 until Ive got the LV one running .

Thanks in advance for any help you can give.

Kevin

As to what else the –15V supply is feeding, its’ the same as what the +15V supply is feeding: the collectors of Q8 & Q9 in the voltage gain stages. I’m wondering if D16 is bad? Each of these LED’s serve as a good voltage reference to set the base voltages for Q16 & Q13. Assuming the LED voltage is 2.2VDC, that would put the emitter of Q16 at 1.6V, and with the base of Q8 at 0VDC, it’s emitter would be +0.6V, placing 1V across R45 47 ohms, for a current of 21.3mA. With 21.3mA thru the current string of the voltage gain stage, you’d have 1.19V across R64 (56 ohms), with 0.59V across emitter resistor R63 2.2 ohms of Q18, which would be 268mA. That does seem too high for the current thru the second half of the voltage gain stage, but whatever that LED voltage is, that’s how you would derive the potentials. Both of these LED’s are necessary for the power amp circuit’s symmetry and balance.

Do you have a Light Bulb Limiter circuit built, thru which you can power the amp from the AC Mains? If not, you’ll want to build one up. I thought I had one of the several articles on building one handy, but don’t seem to find it. Here's one of many you can find on line.

https://www.doktorsewage.com/tools-o...-bulb-limiter/

You’ll need to get the front end of the power amp working and properly balanced before we tackle the high voltage switching circuit.

Removing the 8A supply fuse F1, as you’ve already done lets the amp run on the bipolar 25V supplies.

Under normal operation, the HV rail switching connects to the let current flow thru the MosFET Switching , as can be seen in the output scope photos, using X10 probe monitoring the +/- supply rails.



The middle trace in these scope photos is from the Monitor Output of my Amber 3501a Audio Analyzer, while the X10 scope probes are attached to the +/- supply terminals at D32 Anode & D31 Cathode. Signal source is 50Hz 1/3 Oct Pink Noise, which yields random amplitude sine wave.