Amp Techs take notice: Help Needed

Discussion in 'Amps and Cabs [BG]' started by pablomigraine, Nov 20, 2005.

  1. pablomigraine

    pablomigraine Commercial User

    Feb 9, 2005
    New York
    Director of Commercial Sales - Neural DSP
    This is an extention of my previous thread about a recently fried amp, and I need some advice from some technically qualified TB'ers. Okay Long and short, I accidentally wired the bridge / mono output from my Crest LT 1000 Power Amp straight into the A / Mono input of same. (with the parallel inputs on my cabinet inbetween )There was a spark and SERIOUS speaker crackling, and shutdown. Now only side A works at all, side B, nothing

    I cracked her open and removed the board to have a better look. Now, on the B side of the board I can see the sparks came from a flat, rectangular chip or resistor or something thats roughly a centimeter square with three prongs soldered to the board, and and one face of it glued to the flat side or bottom of the heat sink, which lays on its side and runs the length of the amp back to front. The Solder connection to this peice are damaged, and presumably the peice itself is also. There is also a very small blue lined resistor just in front of it that may be damaged.

    Anybody have any idea what this thing is, and whether replacing it will cure my ails?

    The board is totally symetrical, so side A has one as well, which is intact...

  2. From your description, you need to take it to a tech...sorry...but it's not something you will be able to repair yourself, based on my hasty assessment of your knowledge of electronics......
  3. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Output transistor. Probably fried.

    This could be a tech job, because you might not be able to find an exact match of the transistor, but also, working on a damaged board without compounding the damage is a matter of technique and experience. Further, there could be collateral damage to other parts. A tech might have the guts to re-solder the connection and try firing it up, but I would not recommend doing this at home. When power transistors fail, they typically go into dead short mode, and the melted solder connection could be inadvertently protecting other parts of the circuit from damage.

    The good news is that I can't imagine it to be an expensive job in the grand scheme of things. Having an intact A side makes it an easier job, because the tech can feed in identical signals and just do a point-by-point comparison on an oscilloscope.
  4. pablomigraine

    pablomigraine Commercial User

    Feb 9, 2005
    New York
    Director of Commercial Sales - Neural DSP
    thanks fdeck. My Drummer repairs Mobile phones for a living and is very handy with delicate solder work. He's oubvious not used to dealing withing having this much voltage, but the supply stage was unharmed I believe. I'm sure I can find a compatible transistor. It's not so much the cost of repair but the time it would take. There are 3 maybe 4 reputable places to bring a probalem like this, and they all run weeks and weeks behind. I'll have lost enough money in that time from missed gigs to afford a new one altogether.

    Anything else you'd like to add or are you pretty certain this is an output transistor? I figured since it's the only thing on the amp affixed to both the heat sink and the board it could only be a certain number of things.
  5. Usually anything with 3 leads is a transistor. If its bolted to a heat sink, its relatively high power. But it's probably not the really high power final output transistor, they're much larger, look different than you described. Most likely there's more damage you haven't found. I'd bet the output stages would pop.

  6. Trevorus


    Oct 18, 2002
    Urbana, IL
    Check for fuses, too. Some amplifiers have fuses on their final output transitor stages that prevent this kind of thing from harming them too much. And also, don't do that again :p.
  7. pablomigraine

    pablomigraine Commercial User

    Feb 9, 2005
    New York
    Director of Commercial Sales - Neural DSP
    Hahahahah. Amp is going into a tech today. I will actually be scoping out an Ashdown ABM II 500 while I'm at the shop....and maybe wont be fixing it at all since I can probably trade my existing setup in for it fairly evely without too much outlay of cash. I let you know what happens. Thanks TB'ers!!!