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amplifier loss of signal strength

Discussion in 'Electronic Repair' started by ll, Jul 2, 2007.

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  1. ll

    ll Guest

    I recently installed a transformer (12VAC, 2Amp) in a musical
    instrument amplifier (rated at 10 Watts RMS), and when I test it with
    a variety of input sources (guitars, keyboards, different cables), for
    about 30 seconds, the amplifier produces a nice healthy volume level,
    and then, consistently, the volume just drops about 50%. For some
    reason, this problem sounds like it might be heat-related. Has anyone
    else experienced this?

    Thanks
    Louis
     
  2. Arfa Daily

    Arfa Daily Guest

    Several questions. Why were you having to 'install' the Tx in the first
    place ? Was it a replacement for one that had failed ? If so, in what way
    had it failed, and was there any apparent reason for the failure ? If this
    Tx is a replacement, is it an exact manufacturer's original, or a 'ringer'
    that you've sourced yourself ? If a ringer, is the output voltage exactly
    the same as the original ? Have you checked if any of the power supply rails
    are dropping as the output diminishes ? If the power supply uses monolithic
    regulator ICs ( 78xx / 79xx series ) then if these are subjected to
    excessive input voltage, compared to the amount of heatsinking that they
    have, they will rapidly overheat, which will cause them to retreat into
    their SOA, by reducing their output voltage. Also, if the mounting bolts to
    the heatsink have come loose, or the heatsink compound behind them has dried
    out and powdered, the same thing will happen, even with the correct level of
    input voltage.

    Does anything mounted on a heatsink ie regulators, output IC or transistors,
    'feel' or even smell like it's getting too hot ? Very often in small or
    cheap amps, the heatsinking is only just about adequate for the job, and the
    devices will run very hot even in normal circumstances. Never-the-less, they
    should still not be so hot after 30 seconds, that they are so uncomfortable
    to touch that you can't keep your finger tip on for more than a couple of
    seconds.

    If you suspect that you have found a device that's getting too hot, if you
    can find enough exposed metal on the heatsink, you can try clipping on an
    office bulldog clip to temporarily increase the amount of heatsinking to se
    if that makes it stay on longer.

    Arfa
     
  3. Meat Plow

    Meat Plow Guest

    What kind of amp? Why did you replace the transformer? Replace it with
    exact replacement? Schematic?
     
  4. Eeyore

    Eeyore Guest

    Are you by any chance sine wave testing it for those 30 seconds ?

    Graham
     
  5. Arfa Daily

    Arfa Daily Guest

    I took it he wasn't, as he says that "when I test it with a variety of input
    *sources* ... " and then specifically mentions guitars and keyboards.

    Arfa
     
  6. Eeyore

    Eeyore Guest

    I was a bit dozy at the time.

    I've seen that effect though.

    Graham
     
  7. Arfa Daily

    Arfa Daily Guest

    Likewise. I often deliberately test group amps with a 400Hz tone for a few
    minutes, just to get the output stages thoroughly thrashed up to
    temperature.

    Arfa
     
  8. Meat Plow

    Meat Plow Guest

    I made a looped test CD with me playing some leads, licks and riffs on my
    Les Paul through my Boss GT-6 just to test guitar amps. :)
     
  9. Arfa Daily

    Arfa Daily Guest

    You should send me the files !!

    Arfa
     
  10. ll

    ll Guest


    Yes, guitars, lots of guitars (a variety) were used to test the amp.
    The tx was an exact replacement. It seems we have narrowed this down
    to heat-related.
     
  11. Arfa Daily

    Arfa Daily Guest

    In what way had the original Tx failed ? Might have some relevance to the
    problem, which although may be heat-related, might not be a 'heat' problem -
    if you see what I mean ... For instance, it may be that one of the supply
    rails is dropping due to a monolithic regulator IC going into heat foldback,
    but that could be because something on the end of it is drawing too much
    current. I have also had this occur due to open circuit decoupling caps on
    the regulators, allowing them to hoot, and even open circuit decouplers
    around the output IC, or open circuit Zobel networks on the output, either
    of which can allow the stage to hoot ultrasonically.

    Arfa
     
  12. Eeyore

    Eeyore Guest

    Or the screw holding it to its heatsink may have come loose.

    Basically it needs proper fault finding ! Monitoring the supply rail(s) is the
    obvious starting place.

    Graham
     
  13. Meat Plow

    Meat Plow Guest

    Probably could in MP3 form. Would have to dig it up tho. These days when
    someone brings one I just get out a guitar :). I won't work on any SS
    stuff these days, just vintage Marshall, Fender, Ampeg, Traynor and a few
    stray oddball Silvertone, Alamo, Gibson, etc...Back in the day my
    services were pretty heavily sought. Tried to train a couple young
    apprentices because I had so much work but it was too hard to train and do
    top quality repair at the same time. Nobody wanted to do tube repair and I
    didn't want to do SS guitar, still don't unless you come groveling on your
    knees with a wad of green in your hand. I used to do my fair share SS of
    high power pro audio amps like Soundcraftsman, BGW, Crown, SCS, Peavey.
    Those I did have some help from a friend who did Yamaha warranty work but
    he didn't know shit about a tube :)
     
  14. Meat Plow

    Meat Plow Guest

    Ok one last time, why was the transformer replaced? What led up to that
    diagnosis?
     
  15. ll

    ll Guest


    Thanks - the tx was replaced because there was 0V at the secondaries.
     
  16. Meat Plow

    Meat Plow Guest

    So the secondary was 12vac? Just trying to picture what kind of amp this
    is and what the PO is. Why don't you tell me more, you've been fairly
    vague so far.
     
  17. Arfa Daily

    Arfa Daily Guest

    I did detail the mounting bolt problem in my initial reply, Graham

    Arfa
     
  18. Arfa Daily

    Arfa Daily Guest

    Know what you mean. None of the kids seem to have any aptitude for high
    voltage stuff any more. I far prefer working on tubed stuff to SS. At least
    it's well behaved, and fixes - and better yet *stays* fixed - with a minimum
    of fuss. The circuitry is straightforward, and understandable. At a pinch,
    you can pretty well fix any tube amp without schematics, just by having an
    understanding of the principles of operation. Unfortunately, the music shop
    in my village only has one person doing their service work - me ! So I have
    to take on all-comers, although it is a proper music shop, always having
    several long-haired 'musos' hanging about talking, or jamming in the play
    test room, so a large proportion of the gear is tubed anyway.

    Back in the old days, I knew Jim Marshall. Their works is just down the way
    a few miles from where I live. Jim himself custom built an amp and console
    for a very good friend of mine who ran a mobile disco, when such things were
    new. I used to run the light show for him. We were out on the road 6 nights
    a week, and I reckon that it's got to be one of the best periods in my life
    ....

    Arfa

    Arfa
     
  19. Meat Plow

    Meat Plow Guest

    Sounds like one of the shops I did work for. I wasn't the resident but was
    the only Fender and Marshall specialist around. I really tried to stick
    with a couple brand names but always got tossed others. Back then I had
    long hair tied in a pony and a face full of beard so I had the rocker
    image plus I was that rocker too :)

    I used to run lights and sound in between gigs for friends or customers
    caught in a jamb. Lots of times I would get up and fill in on a set too.
    Those were some great days of my life.
     
  20. ll

    ll Guest



    That sounds great. Did Jim do much custom work, outside of
    Marshall?
    I would love to learn about tube repair - my father did quite a bit of
    that, as well as in organs. Speaking of that, Emerson's organ
    technicians must get quite a workout!

    Back to the amp in question - it's rated at 10watts RMS.
    I did a few more 'listening' tests with it yesterday, and made more
    comparisons. I was able to have the amp on for about 15 minutes
    before the volume 'cut out' occurred (no variation in the input source
    - same guitar). I then turned it off and then back on - the signal
    was strong again. This isn't consistent, though, as I could turn off,
    wait about 10 seconds and then turn on again, and it might or might
    not have the full volume.
    I tested the line out and headphone jacks, and they each produced a
    healthy audio output. One item of note: when plugging into the line
    out, the amp's internal speaker went back to that 'cut out'/ half
    volume sound. This would occur, even if just plugging in an
    unconnected guitar cable. When the internal speaker was producing
    that weak output, the amp into which the line out was plugged was
    producing a healthy signal. The headphone jack had the same effect.
    After I had turned it off and on several times, I would hear a
    'scratchy' sound (like turning dirty pots) just after turning back on.
    Now I'm wondering if speaker wires/connections might be suspect?

    Thanks
     
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