Anyone familiar with the mackie swa 1801 that could help diagnosis a problem. Thanks

[serg7676]

Trying to repair this amp plate to no avail by myself. Need some advice I have the schematic and parts handy. Thank you.

[The Dude]

Best if you give us a little more info. What's wrong with the amp- distorted, dead, noisy, etc. What have you done so far? Have you narrowed the problem down to any particular circuit(s)? Also, if it's possible for you to post the schematic here, everyone can follow along.

[serg7676]

I figured I start by introducing myself. I'm Sergio and basically consider myself an amateur but I do have a slight background in electronics by completing an electronics course and I actually repair power tools and am around electronics and components all the time. MY ISSUE/HEADACHE LOL is this damn Mackie swa1801. I play in a band as well and have two of these mackies and an extra amp board as a spare. Well the spare is dead and the second Mackie is down. I tore into this amp and replaced R2 22ohm 20W which I thought was the main issue considering that the amp didn't power on. Well there was more a lot more. Bulging HV Caps the usual Q3,Q18,Q6,Q14,Zener Diodes, Resistors and more. Basically I replaced almost all the transistors, diodes, Caps, even U2 NE5532AN. I powered up the amp using a lamp limiter and great its up and running no blown fuses nothing smoking. Great I think. Well I have major Distortion past 1/4 way of gain. Because of the distortion is one of the reasons I tried to replace U2. Made sure the input board was ok by running another cable from the parallel input to another amp and speaker and signal was good. Also measured the resistance of the Speaker itself and was good even pushed in on the speaker cone and check measurements to make sure nothing was glitching. This amp is driving me crazy and I'm at the point where I spent so much time troubleshooting that my brain is acting up and I must be overlooking something simple. Few main components that I have not changed out are both l3 5uh, C1, C33, and HCPL-3100's. Now I just noticed that my rails aren't coming up to the 18 volts now and have a hum without any signal going in. Thank you in advance. Sergio.

[serg7676]

Mackie SWA 1801 schematic

Amplifier PCB Assembly (23020800).pdf

[serg7676]

Amplifier PCB Assembly (23020800).pdf

[MarkusBass]

I assume you already know this thread: http://music-electronics-forum.com/t35220/
I think that not much more can be added. When this amp fails, a lot of components fail and there may be several tracks vaporized. I had this amp with burnt speaker. So my advise is to check everything again carefully - something is overlooked.

Mark

[serg7676]

Thanks for taking the time to reply Markus I did have two burnt traces which I repaired. I will take a closer look to make sure everything is still ok then I post back my findings. Last night I noticed if I lift a leg on both steering diodes my 18 volts does come back up. So going to take a good look again at the second stage of the amp. Thanks.

[serg7676]

Well I double checked everything had one more bad transistor that I over looked. Got my rails back up +18 and -18 are fine as well as +91 and -91 are up as well. Still have distortion. I think it has to do with the pwm of the amp and I am not familiar with that aspect of the amp I don't know how to check to see if its on the input side or on the low pass filter side. Any Ideas as to what it might be would be a great help. Thanks.

[serg7676]

Well I just found that my n-channel switch keeps blowing. I had installed a new one at first and just went through the whole amp plate again and found out that a
j107 n-channel is blown. Checked all the resistors in that circuit all seem fine replaced with another j106 still had distortion. Pulled the amp out to test and find j107 blown again. Any Ideas?

[serg7676]

well just got a hold of a somewhat oscilloscope. Now correct me if I'm wrong I put the negative probe on the negative to the speaker and put the positive probe on the positive of the speaker and ended up with a sine wave that was clipped on the positive side. Now I'm guessing it means my bias if out of specs. Now next question is what part of the amp controls the bias I know there is two resistors but does the transistors or does the hcpl-3100 optocoupler control the bias.

[MarkusBass]

...ended up with a sine wave that was clipped on the positive side. Now I'm guessing it means my bias if out of specs.

I don't think that this is the case. It is still possible that the problem is related to bias circuit but I would rather search for failed driver, or failed short circuit protection. Why do you think that this is bias?

Mark

[serg7676]

Well I thought it was due to the fact that the sine wave was clipped. It is very well possible like you said that its within the bias circuit. I'm just not to familiar with the bias circuit on this amp. Last night I was doing some tests at both hcpl-3100 and was comparing them to a good amp plate that I have and noticed that on hcpl-3100 at location U3 Pin 7 to ground is not even close to being the same reading as on the good amp. I double checked q14 Irf540, q6 Irf540, all the diodes and resistors in the nearby location. Everything there is looking good. I don't know if q12 2n3906 would effect it but that is where I'm at with this amp right now. When I get home tonight I will try to move on to the next location and troubleshoot some more. Any Direction I should take at this point would be a blessing. Ty.