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amp woes

Discussion in 'Amps and Cabs [BG]' started by Scott Lynch, Aug 16, 2005.


  1. Scott Lynch

    Scott Lynch Supporting Member

    So here I am rockin' out last Thursday with my band, when suddenly in the middle of the third set my volume drastically reduces, and putrid smelling smoke starts coming out of the vents of my old SWR basic 350. No sound is coming from the amp, save the DI, and the sound I am hearing is the wee bit of PA sound bouncing back at me (talk about a challenge, lol). Unfortunately I have gigs on Friday and Saturday with no sound reinforcement fot the bass, so I end up using my keyboardist's 160w Crate keyboard amp for the other two nights.

    Anyway, on Sunday I test the amp through one of my Aggie GS112's. I hear a constant loud hum coming from the amp, which can not be lessened or worsened by turning any knobs, playing, etc. Smoke starts pouring from the vents as well, but stops when I turn the master volume all the way down. Then suddenly, no sound at all. So I say "eff it" and go looking for a new amp.

    I return from the music store on Monday with a brand spankin' new Aguilar AG 500, ready to crank it up through my cabs, only to the dismay of finding out that one my GS112's (the one I used to test my SWR on Sunday) is not working. Here's the weird part: the tweeter works fine, but the woofer does not.

    So I have 2 questions:
    A: What could possibly be wrong with my SWR, and is it worth fixing?
    and more importantly,
    B: What could possibly be wrong with my GS112, and is it worth fixing?

    Thanks in advance for any replies. :)
    -Scott
     
  2. My guess is that the SWR probably hosed the crossover in the GS112.

    As far as the amp goes, I dunno. Judging from the symptoms you describe it sounds expensive. You could take it to a tech but you know he's gonna charge you a hundred bucks just to raise the hood and look inside. It sounds like a basket case to me.
     
  3. xb100

    xb100

    Mar 24, 2004
    NH, In
    I think what happened is you cliped your amp which sent a huge signal into your gs 112 and mad eit explode.

    In other words you had your SWR cranked, put lot of pressure on the head it self and it was probably clipping, you just didn't know it. While it is clipping you are distorting the speaker in your gs 112, and it eventually gets fried in the process.

    Maybe someone else can back me up on this, but that's what I think happened. Do you have a warrenty on either of the 2?

    Hope you get everything fixed.
     
  4. god, do you know how much that smoke costs! how could you let so much of it out!!


    on a serious note tho, **** man that sucks :(
     
  5. Well, rule #1: Amps do not repair themselves....once smoke starts coming out, why on earth did you plug it in and turn it on again??? :confused:

    Sorry, hate to give you a hard time...Anyhow, the two problems may be related. The amp may have taken out the speaker or vice versa. Or it may have been a cosmic coincidence.

    I'd certainly check the speaker out BEFORE trying it again with your new amp. I can think of a couple of failure modes (in the speaker) that could give a direct short to the amp, and thereby could have taken out the (old) amp (assuming the amp did not have short-cicuit protection on the outputs). Wiring failure, for example...loose input jack on the speaker that twisted and shorted the woofer leads.

    As far as getting the old amp repaired: used 350's show up all the time on ebay and this forum. I believe there was one a few days ago for $325 shipped. If the repair doesn't cost anywhere near this much, you could get it repaired. If you keep it, any time you gig with it you will wonder if it's going to go out on you again. Even after a few years, that thought may be in the back of your mind.

    If you sell it, and the buyer knows it's been repaired, it would be worth less (than a used, never-repaired amp) because there's an increased chance that the amp may fail again....

    Best case for repair (based on the vague description of "emitting smoke", and neglecting a couple of real simple possibilities that are kinda remote): an output transistor or two and their emitter resistors have gone. Maybe $30 in parts, plus a few hours labor...probably a total of $100 to $150. Medium-damage scenario: it took out a driver transistor or two, plus the output transistors. A few more dollars in parts but a few more hours labor, brings the repair bill over $200. Even worse case: it took out the transistors plus the power transformer. Power transformers aren't cheap...Now we're talking $350 or so....not worth repair. And of course, you may have to pay some money just to have the tech open the case and give you an estimate of the damage.

    You may want to sell it "as-is" for parts or to somebody that would be interested in fixing it themselves. For example, there's a broken Trace here in the classifieds, though IMHO perhaps there's more of a demand for vintage Traces than for SWR's. (shameless self-plug, if the price were really really good, I'd maybe be interested in it as-is, but my girlfriend may shoot me if I start ANOTHER amp repair project)

    Hope this helps.
     
  6. Scott Lynch

    Scott Lynch Supporting Member

    The master volume on the amp was at 9, maybe 10 o'clock, so I doubt I clipped the poweramp. I'm pretty sure I blew the crossover in the cab, as there are some burns on the circuit board where some wires connect to the board. As for the 350, anybody want a busted amp? :p
     
  7. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    I doubt you toasted the crossover, more likely you first blew the output transistors and subsequently sent DC to the woofer and blew it when you tried the amp at home. The tweeter wasn't affected because crossover high pass capacitors block DC to the tweeter, but woofers don't have capacitors to block DC from them.

    Initially $10 worth of transistors (and a hundred bucks labor) might have fixed the amp, but you may have done more damage to it when you tried it again. If the woofer is blown you can replace it or recone it, you won't get away without paying another $75 in either case.

    Lesson that should be learned: if something goes wrong and you aren't a trained technician don't even plug the amp into a wall socket, take it to the shop to get it checked out. If you had done so chances are good that the woofer wouldn't have blown.
     
  8. Scott Lynch

    Scott Lynch Supporting Member

    We'll let's face it. I'm young and stupid. :p And you're right. I shouldn't have plugged it back in, but I didn't know any better at the time. I'll definitely learn from this mistake.
     
  9. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    On those, you have to cover the vents with gaffer's tape to keep the smoke in. They don't work that well if you let the smoke out.
     
  10. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    You will go a long way in this world, Grasshopper.