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Underbiased Distortion problems

Discussion in 'Amps [BG]' started by cbyrd2200, Dec 27, 2012.


  1. cbyrd2200

    cbyrd2200

    Joined:
    Aug 31, 2009
    Location:
    Boston
    Hey all!
    I just got a used Ibanez Promethean P5110 combo in the mail today and when I play through it, I get a distinguished amount of distortion that is audible at all volume levels, although the distortion itself does not get any more harsh or soft. It is not a battery in the bass problem, it is not a speaker problem, it is not a cable problem, I have tested all of these. The problem is coming directly from the head. It has the same distortion with any bass, any cable on any speaker. After a small amount of research, I'm hypothesizing that it's a faulty component in the amp causing the output stage to be underbiased. I haven't opened the amp up yet and I'm asking if anyone has had any similar problems and if they have any good solutions. Thanks and best regards!
     
  2. cbyrd2200

    cbyrd2200

    Joined:
    Aug 31, 2009
    Location:
    Boston
    So after I worked with this thing tonight I discovered it works fine through the headphone and D.I. outputs, just not speaker outs. This makes me believe it's a problem with the power amp. I saw some indentions in the bottom of the box it was shipped in where it looks like the UPS guys were throwing it around like a basketball. I opened it up and nothing is loose so now what?
     
  3. David Jayne

    David Jayne

    Joined:
    Jan 16, 2006
    Location:
    Brookfield, CT
    If you're really, really sure it's not the speaker, it's tech time. Without schematics and strong working electronics knowledge, you're done. About all you could do is the old 'poke things with a chopstick' routine. Which I can't recommend because it's dangerous, so don't do it.
    :bag:
     
  4. cbyrd2200

    cbyrd2200

    Joined:
    Aug 31, 2009
    Location:
    Boston
    Dmusic 148, when my equipment malfunctions, I don't poke it with a chopstick. I figure out how to fix it. Call me an aspiring technician, but I'm trying to develop my understanding of the problem I've encountered so that I can fix it myself. If I was going to give up and take it to a professional tech, I wouldn't have asked the talkbass community this question. So is there anyone out there who can share with me any knowledgeable info about why the power stage of this amplifier is producing crossover distortion and/or how ti fix it?
     
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  6. David Jayne

    David Jayne

    Joined:
    Jan 16, 2006
    Location:
    Brookfield, CT
    I understand. But since nobody here has a schematic, and few of us have the skills to even read one, and amps such as the one you've got are all SMT and thus are often not repairable without a major board swap anyway, this thread will probably not yield anything useful to you. Sorry to be so blunt, but that's the way it is. I wish you no ill will, Sir.
     
  7. T-Bird

    T-Bird

    Joined:
    Apr 29, 2007
    Location:
    Finland (Northern Europe)
    Hi.

    Why not?

    That's by far the fastest way to find cold solder joints or broken leads.

    DO NOT use a pencil like some Darwin Award nominees have done, the graphite is conductive ;).



    What does the scope trace tell You?
    How are the rail voltages?
    How about the DC offset in output?

    Troubleshooting amps without schematics, especially SMD SS amps, is a MAJOR PITA, and no-one who values their time is willing to touch 'em without schematics.
    Unless of course one has something irreplaceable in front of them.

    The time to spend however is Yours to spend, so nothing obviously prevents you from trying.

    The best of luck for You in your quest.

    Regards
    Sam
     
  8. B-string

    B-string Gold Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
    If you can get schematics it would yield more help. It is unknown (to me at least) what the poweramp typology is for your amp. Class a/b, Class D, class H, single "chip" solution that has a feedback loop that could be faulty? If it even uses an external bias circuit?
    Using SMPS at least reduces the possibility of failed poweramp supply rails.
     
  9. Passinwind

    Passinwind Charlie Escher Supporting Member

    Joined:
    Dec 3, 2003
    Location:
    Columbia River Gorge, WA.
    Per the Ibanez website, it's a Class D power section. Internal pics of one of the Promethean heads are here.
     
  10. B-string

    B-string Gold Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
    So we need to know then if it is a OTS module.
     

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