Some guidance on high end audio amplifier repair?

Discussion in 'Technical Repair' started by hspalm, Mar 13, 2011.

  1. hspalm

    Thread Starter Active Member

    Feb 17, 2010
    201
    8
    Hello all forum users.
    After repairing a couple of my colleges tv's by simply replacing bulging caps on their power supplies, I got thrown this massive amplifier at me to try and fix. The message was "either you fix it, or I'll throw it", and I like a challenge. I'm mostly fond of digital electronics, but I do know some basic analog and thought this might give me some valuable experience, if I got it to work.

    The amplifier is made in Norway and weighs at least 20 kg, no information to be found online.

    One of my main concerns is that it is normally driven by a preamplifier, which I don't have. The input signals connects by a XLR connector. To test the amplifier (of course not final test) to trace and detact signals on the PCB, is it okay to use my signal generator positive on the positive polarity terminal of the XLR, and the signal generator ground on the returning terminal?

    I guess you need to know what the problem here is, and it's really just one dead channel. Completely dead. The owner said it started failing slowly (channel alive/channel dead, repeat) but completely died after a while.

    I have read through this page
    http://www.bcae1.com/repairbasicsforbcae1/repairbasics.htm
    and from what I can see it's very informative as a guide to start of identifying modules and components. So far I've identified the power supply, and each channels amplifier PCBs with the driver transistors and output transistors.

    http://sandakerpalm.no/andftp/IMAG0110.jpeg

    Click the link for an image.

    Any hints or tips before I go further on this! Thanks in advance!

    Henrik
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    No problem...show me lot's of pics and give a brief explanation on the problem.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    It is a stereo amp so it has two identical circuits.
    It is easy to measure voltages and compare the good channel's voltages to the bad channel's voltages then replace parts near the wrong voltage.
     
  4. hspalm

    Thread Starter Active Member

    Feb 17, 2010
    201
    8
    I'm sorry for the late reply guys!


    Are you serious? I'll give you tons of pics if you'd be so kind to help me. Just wait till I get back to the workshop. Any parts in specific you're interested in seeing?

    Yes, this was also my initial plan, and still is. But regarding the question about how to apply an input signal to measure, do you have any comments?
     
  5. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
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    One method of troubleshooting is to use an input like a sine wave or a triangle wave (the triangle wave is nice because it's easier to see distortion when it occurs), then start tracing through the circuits starting from the front end. Since you apparently have a good channel to compare to, this could be a worthwhile approach.

    In contrast, another method is to use a function generator to supply the required signal for each stage and start from the back end. Then you get to hear the output. As you work forward to the input side of things, you'll eventually find the stage that's bad by poor or no sound. This tends to be a method where you want the schematic and troubleshooting information; otherwise, you might inject an inappropriate signal. Sometimes you also need a DC offset in addition to the AC signal so that the stage is biased properly.

    I'd recommend starting off with quite small signals and slowly increasing the amplitude. You don't want to overload the input and since the input is a preamp, you know the signals will likely be small. I'd suggest starting off with a signal on the order of a millivolt or so, although I suggest you wait for comments from some experienced audio troubleshooters, as I'm not one of them...
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Show me the PCB from both sides, Take high res and up em to photo bucket, this way I can get them if I need to see close ups.

    From the pics I can guide you and tell you which points to check regarding voltage measurement and the likes..

    You might need take the amp apart. The first pic does not help at all. I like to see the whole thing. Just to get a general idea of the whole thing
     
  7. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    You are correct to wonder about the signal from the preamp. Low signal levels are the most important part of the way to safely test. Try to also use high value resistors (start about 33kΩ and drop by magnitudes to 3.3kΩ and 330Ω) on your input signals from your signal generator. High impedance, so that power is attenuated is a normal characteristic of preamps.

    Ground attenuation can cause problems so try to see if either of the inputs is connected to the ground of the amp and that side should not need a resistor.

    Also keep in mind that if the amp was used with a preamp then the preamp could have been the problem. Unless they have used the preamp with another amp then you might find that both sides of this broken amp are working. Don't be confused by this until you test the preamp.

    And since you mentioned Norway as the source of the amp, I suspect you have a Bang & Olufsen amp. (They are a Danish based company.) If so I would definitely try to fix it and if you can't then don't throw it away. Even broken you might manage to sell it.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    Feed a sine-wave signal with the amplitude of the amplifier's specs to the defective amplifier and to a good copy of the amplifier. Compare the outputs on an oscilloscope.
     
  9. hspalm

    Thread Starter Active Member

    Feb 17, 2010
    201
    8
    This sounds like a plan, and if I am lucky enough to get R!f@@ to point me at some key test points this is the approach I will persue.

    I understand how it is important to you to get the whole PCB with both sides and this is not achievable with the amplifier assembled. I will take it completely apart and provide you with photos.

    I will check with my college if he have tested the pre-amp with another amplifier after this one was discarded. Thanks for all your info and fulfilling answers. someonesdad suggested a test signal with the amplitude of about 1 mV, do you have any comments?

    Unfortunately I don't have a function generator at hand, but my oscilloscope is standing here ready to work for me. Or with me. I don't have any specs on the amplifier as it's sort of like a one-off from a local company situated in Trøndelag, Norway. It's brand is Adyton Nor (sorry, no Bang&Olufsen here, Potato Pudding
     
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