problems with my SWR 750 again.

Discussion in 'Amps and Cabs [BG]' started by BassIsFun17, Aug 4, 2009.


  1. BassIsFun17

    BassIsFun17

    Jan 16, 2006
    Atlanta
    hey everyone,

    back a few months ago i had some problems with my SWR 750. the power section had completely failed, and i got my step-dad's brother to help me fix it. he was able replace the power transistors and resistors and what not for a whole lot cheaper than a shop would do it for.

    now here's my new problem. im afraid it is a sign of power failure again. it started out that i thought it was a problem with my tweeter because the sound comes right out of the tweeter. but after playing through other speakers, my head i still having the same problem. there is a crackling, sort of distorted noise that stays constant. as i turn the volume knob past about 9 oclock it makes a swirling sound as i turn it up, and when you leave it at any spot about 9 oclock the crackle/distorted noise gets louder and its a little worse sounding. i was just wanting to know if this could just be a scratchy volume pot, or if it is actually the power section beginning to fail...and if so what can i do to prevent it from doing this.

    thanks!
     
  2. Turock

    Turock

    Apr 30, 2000
    Melnibone
    I would think that a scratchy volume would only make noise when you moved it.
     
  3. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    Could be a lot of things. cracking/poping noises can be attributed to bad caps. If it's much worse when turning the knob, I would suspect bad knob.
     
  4. BassIsFun17

    BassIsFun17

    Jan 16, 2006
    Atlanta
    my brother is pretty good with electronics. is there a certain set of capacitors that we should look at? also is there a way to test if it is the volume pot?
     
  5. The problem really could be anywhere in the circuit. One thing you could try is running the preamp output into a different power amp to see if you get the same noise. If you do, the problem could be as simple as a new preamp tube. If your brother is good with circuits checking the volume pot with a multimeter should be a simple task.

    Rick B.
     
  6. BassIsFun17

    BassIsFun17

    Jan 16, 2006
    Atlanta
    thanks.
    how likely would be that its just a new preamp tube?
    also what are the most likely problems that i could find with the presented information?
     
  7. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    It COULD be, but, I doubt it. Pre amp tubes last a very long time...
    Again, it could be a ton of things. If your brother is good with electronics, finding the bad components should be a piece of cake really.
    Running the preamp out to another power section will help you find where the trouble is and will help quicken troubleshooting.
    If you're brother has a scope and can follow the signal, do so, and find where the noise is getting introduced...
     
  8. BassIsFun17

    BassIsFun17

    Jan 16, 2006
    Atlanta


    well my brother isnt like incredible with electronics, but he can figure out some problems. if its going to involve using a scope and stuff i will have to contact my step-dad's brother. only problem is, he works out of state alot, so its gonna be a while until i can actually have time to sit down with him and look at my amp.

    are there any other techniques to troubleshoot that you havent mentioned?
     
  9. RickenBoogie

    RickenBoogie

    Jul 22, 2007
    Dallas, TX
    Hate to say it, but maybe it's a good idea to take it to a qualified amp tech. At least then, you'd have it fixed correctly.
     
  10. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    Not really, no. That's why you pay somewhat sizeable cash to have a head looked at :)
     
  11. BassIsFun17

    BassIsFun17

    Jan 16, 2006
    Atlanta
    i guess ill have to get in contact with my step-dads brother. cause i really dont want to have to pay a bunch of money only to find out that i needed to replace a tube or something.
     
  12. B-string

    B-string Supporting Member

    Keep in mind the cheap incomplete repair maybe why the amp is failing again. Not so cheap if you have to repeat it over and over till something really bad fries. Just food for thought? Most of those amps go a very long time between proper services.
     
  13. chadds

    chadds

    Mar 18, 2000
    +1 My BBII just came home from service to the tune of $121.00. It's so good! Caps went.
    The tech doesn't want to gouge you. They do have parts readily available to put in. If it works they leave it if it doesn't they don't. You also want to pay for an expert to find the problem not experiment.
    Great amp by the way.
     
  14. BassIsFun17

    BassIsFun17

    Jan 16, 2006
    Atlanta
    well my step-dads brother actually has a degree as an electrician and knows what he is doing. i guess the big issue is. i bought this amp used, and the power section had previously blown when he still owned it. which means if this is a power section issue or it has blown or atleast is going to, then this will have been the third time. two of which within 6 months of each other. so i guess we just have to look for all and any problems.
     
  15. RickenBoogie

    RickenBoogie

    Jul 22, 2007
    Dallas, TX
    There's a HUGE difference between an electrician, and a certified amp tech. If you want the amp fixed properly, take it to a professional amp technician.
     
  16. BassIsFun17

    BassIsFun17

    Jan 16, 2006
    Atlanta
    well he is an electrical engineer. but he is also a musician and works on and builds amps and effects pedals. so i know he is qualified to work on amps and stuff. he fixed my power section the last time it failed. hes not just some average joe messing with my amp, he really knows what he is doing and has done this stuff for the better part of his life.
     
  17. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    Absolutly no offense meant here.
    However, if he looked at it before, and it still broke again, this quickly, he obviously did not do a complete job. He probably just replaced the broken components and went on his way, instead of doing a root cause analysis. It is crucial that step be completed every time.

    It's like this; if your amp blows a fuse, that tells you something bad happened. Some "techs" just replace the fuse and send you on your way. NOT good! A good tech will look into what caused the fuse to blow, fix that, replace the fuse and then send you on your way.
     
  18. BassIsFun17

    BassIsFun17

    Jan 16, 2006
    Atlanta
    okay. yeah we never really did figure out why the fuse blew. all i know is the 6 power transitors had failed. but we dont know why they failed. we assumed it was a bad circuit at the venue where i plugged into. which was full of other amps and stuff on one serge protector.
     
  19. BassIsFun17

    BassIsFun17

    Jan 16, 2006
    Atlanta
    alright
    update of sorts.
    yesterday i got a chance to take my amp to guitar center. i ran it through three different cabs. mesa 4x10, markbass 4x10, and GK neo 4x10. it still did the distorted crackling on the GK and the mesa....the markbass didnt seem to do it....but it could have been that i just didnt really notice it. either way 2 out of the 3 cabs showed the same results as plugged into my Eden. the manager whom im really good friends with up there, allowed me to plug another tube into my head to see if that was the issue. and that did nothing. so atleast ive now sort of figured out its most likely the power amp. ive been getting rates for techs to take it to. so i will be doing that in the next week i hope.
     
  20. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
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