Help with audio diagnosis methodology

Discussion in 'The Projects Forum' started by wanders, Mar 27, 2012.

  1. wanders

    Thread Starter New Member

    Jan 19, 2012
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    Been using a self built, Heathkit audio preamp for many years. Never had reason to try the moving coil amplifier until recently; then found that the right channel is not functioning.

    I've been experimenting with hobbyist level electronics repair, with some success and failure. Going to try to fix this myself. I have, and am ok with a digital multimeter. Someone gave me a function generator and oscilloscope which I've yet to use.

    I have the original Heathkit schematics. The m/c amplifier board is easily accessible and removable from the pre-amp. The pcb is fairly simply by today's standards. My questions refer to test equipment capabilities and methods.

    It would seem to make sense, with the preamp power on and the m/c board in place, to attach the function generator to the m/c imput jacks of the preamp (I already checked for continuity on both channels from the jacks to the selector switch board; no problems.) Assuming that the function generator would send a signal through the m/c amplifier and I could check voltage at each point through the two circuits, comparing right channel to left channel. When I find a difference between the two channels, I could pull the component and check it. I think that that assumes that the function generator will send a signal though the board without any additional current or signal; and I do not need to connect to a load, like a speaker, or headphone output?

    Is my thinking correct; what am I missing?

    Thanks for the help.
     
  2. panic mode

    Senior Member

    Oct 10, 2011
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    i'd say your reasoning is right... go for it.
    make sure that output from signal generator is sufficiently attenuated (you can check with scope).
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You need an oscilloscope.
     
  4. wanders

    Thread Starter New Member

    Jan 19, 2012
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    The specified output from the m/c cartridge is 0.25 mv. Is it reasonable to assume that a normal function (signal) generator, could generate such a low voltage output? If not, it seems possible that I could damage the amplifier circuit.

    "You need an oscilloscope."

    Is that because a multimeter would not be able to read voltages at various points within the circuit, or because the scope would provide superior analysis, or something else?

    Thanks for the critique.
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Scope gives better info. Shows both shape and amplitude of signal. many multimeters don't have enough bandwidth to measure the full audio range accurately.
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    If you are trying to locate an Audio fault, I have a rather simple approach and a tester to nail any analogue Audio signal.
     
  7. wanders

    Thread Starter New Member

    Jan 19, 2012
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    Interested to hear Your ideas.
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I have been using this method to trouble shoot audio for ages.

    Mind u that this method applies to Analogue signals only, that is the Audio bandwidth.

    I simply made a Audio amp using any low wattage IC. Preferable is anything that can be operated with a battery around like "AA" or a li-on cell. So it would be isolated and can be carried around.
    A volume should be used. The amp drives any low power descent speaker that sounds perfect for listening music.

    It can be into a PC speaker as I had. Stereo Operation is not needed. The Input is connected to a Probe as in an oscilloscope probe.

    With this you can probe audio circuits as the way you would probe using a scope.

    The amp (the one u need to repair) should be powered and an Audio signal applied to the relevant inputs. And use the little audio tester to trace audio.

    This way is easier since the ear is better to isolate any distortion and varying audio signals
    I prefer this cause I can play my favorite songs and trace signal since I know how the song sounds.

    If you prefer pictures, lemme know.
     
  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    But Keep in mind not to probe High Voltage areas. You shud always have ur DMM to measure any voltage at the test point if u think it is higher than the IC in the tester can handle.

    I can probe around 50VDC without a problem with the one I am using. I can't remember the IC no. I have to take my one apart. But I think it is LM386.

    The input should be coupled with a NP capacitor around 1 or 2uf at 63VDC to AC couple the tester.
     
  10. wanders

    Thread Starter New Member

    Jan 19, 2012
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    R!f, thanks for the ideas.
     
  11. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    By the way.
    The test probe should be like a scope probe. Infact any old 20Mhz probe will be OK. The input wire is long so it should be shielded to prevent picking up noise
     
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