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OT: hi-fi amp distorting on one channel

Discussion in 'Amps and Cabs [BG]' started by alexclaber, Aug 9, 2004.


  1. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    Sorry for the non-bass related post but it is amp related! I'm just hoping one of you amp gurus can help me out...

    This evening I noticed my hi-fi distorting on one channel, a really nasty dirty unmusical distortion, like fuzz gone wrong. I've switched the leads, speakers and sources and it's definitely coming from one side of the power amp. Anyone any idea what this is likely to be - blown output transistor, deceased output capacitor etc? What's the best course of action - any self-diagnosis I can do?

    FWIW the amp is a Rotel RA-971 MkII running biwired into Mordaunt Short MS-20i Pearls.

    Help!!!

    Alex
     
  2. It could be any number of things from a simple cold solder joint or strand of wire shorting something to a device failure. It's probably not a blown up output device or a cap but it's really hard to tell sight unseen. I'm not familiar with that particular amp so I can't really advise you much. Take it to a qualified tech. Sorry I can't be more helpful...
     
  3. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    Thanks Mark!

    One more bit of information - the distortion increases as I turn the amp up until it gets almost completely fuzzed out. I've had problems with bad contacts with previous amp and then turning up solved the problem (I presume due the higher voltage defeating the 'insulation' in the bad contact). However if something live is shorting then I guess the higher the voltage, the more likely the short is to occur.

    Alex
     
  4. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    If I had to guess, I'd say it's likely to be a transistor problem, only because I've had similar symptoms from amps before and it's always been a transistor or 2. But that's hardly expert advice.......

    Sounds like you like the amp. It's been my experience that getting a favourite amp fixed often works out better than buying a new one and hoping for the best. I'm with Mark, take it to a reputable Hi-Fi repairer.
     
  5. notanaggie

    notanaggie Guest

    Sep 30, 2003
    Sounds most like an output device may be open, a bad connection or other fault might cause it.

    That increasing distortion is a clue. The driver transistor is presumably carrying the load, but it usually cannot give much power, so as you turn it up, it clips way early on that side.

    Sounds like a busted speaker, usually.

    Oh, and don't turn it up, or on, until fixed. if you do, you may cause the problem to spread to more parts, as you will probably blow the driver as well.

    It may be a simple repair, but I would not count on the bill reflecting that.

    Rotel stereo equipment is generally decent, but I prefer their tomatos with diced green chilis.......

    sorry couldn't resist.
     
  6. i don't know that he'll get that joke. he's in london. i didn't ever see rotel (tomatoes) in a store until i moved south. my southern friends like to make fun of my rotel RA-02 integrated.

    robb.
     
  7. notanaggie

    notanaggie Guest

    Sep 30, 2003
    That's OK, one groan is enough for any self-respecting wise-acre.
     
  8. Frank Martin

    Frank Martin Bitten by the luthiery bug...

    Oct 8, 2001
    Budapest, Hungary, EU
    If its only one side, than something in that channel has most probably gone wrong.
    if its got headphone outs, then try it with it - if its all clear and good then, then somethign in the ppower-amplifier part is bad.
    (BTW did you try cleaning it out to get dirt and dust out of it?)

    i can only also say that a qualified repairman can most likely repair it
     
  9. jondog

    jondog

    Mar 14, 2002
    NYC metro area
    This is good advice, if you like it pay to get it fixed now. I just killed my stereo last month trying to self diagnose. I tried switching everything around and turning it back on and up about 7 different times (swap cable, swap speaker, etc). On the 8th time it just stopped working altogether and I had to throw it out. :bawl:
     
  10. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    Took it to a very knowledgeable hi-fi-fixing-guru-type-chap last week and it's now fixed! Hurrah! Blown output transistor on one channel. Back in action, total damage £35 - cheaper than buying another £250 amp for sure!

    Excellent.

    Alex

    P.S. robb, I'm only in London sporadically now - since the change of jobs I'm based from home in sunny Brighton. A much more pleasant experience all round!

    P.P.S. So Rotel have a division in the tinned goods business - who'd a thunk it...
     
  11. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    ........


    Where do I send the invoice? :)
     
  12. Bah! Lucky guess.... :spit:



    :D
     
  13. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    One lucky guess after another.... yep, I'm brilliant. Ask my mum!

    :p
     
  14. notanaggie

    notanaggie Guest

    Sep 30, 2003

    Hey there, we gotta share, buddy..............

     
  15. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    *Marge Groan*
    hmmmmmmmmOK but I'm donating my half to charity..........

    See how effortlessly I made you look bad :)
     
  16. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    Ah, I am dim. I misunderstood the guy on the phone - turns out it was a driver transistor that was blown not an output transistor.

    Alex
     
  17. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Pffftt... fancy getting those two mixed up :)

    Damn I'm in a silly mood lately.