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Thread: Crackling solid state amp

  1. #1
    Join Date
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    Default Crackling solid state amp

    Working on an Elka AC1280R guitar amp. Speaker coil has gone open cct due to intermittent spikes, almost -ve rail voltage, at the O/P.

    Since starting work on the amp the spikes have subsided but O/P rests at -2V DC. On a cold startup, when filter caps are flat, O/P will briefly peak to around +15V for a few secs then sits steady again -2V.

    Hooking up an external speaker a really harsh crackling sound is heard right through the program signal or when sitting idle and the -2V persists.

    Disconnecting the preamp from the power amp the O/P will drop to around -65mV and the crackling gets quieter.

    Switching the amp off the O/P will slowly rise from -2V to around +15V unless the speaker load is connected which dissipates this and the O/P comes to rest at 0V.

    The O/P measures 0V AC at all times while idle.

    All the transistors are measuring 500-650 mV across their junctions as are the diodes. Supply rails are a constant +/-48V.

    I've been right accross the power amp board twice and resoldered every nipple, this is what has brought some stability to the amp.

    I'm considering replacing the 2 output and 2 driver transistors. Can power transistors like that get stressed and even though they measure ok still not function correctly?

    Any experienced advice with this would be greatly appreciated. Cheers.

    Patrick.

  2. #2
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    Patrick;

    Off-set voltage on the output can come from a problem in the pre amp power supply.

    I Don't have a schem for the ELka, but if it uses op amps, check them for a clean +15V /-15V.
    If that low volt power supply is off, this can set up all kinds of mischief.
    ampdoc


    myspace.com/akgservice

  3. #3
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    Thanks for the tip ampdoc. Measuring directly at the volt regs on the preamp board I get +15V and -14.9V. They dont budge loading the amp up either.

    Worth trying new power transistors on the power amp board you think? Don't like to shoot in the dark like that but I'm at a loss as to what else to try.

    Cheers,

    Patrick.

  4. #4
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    Patrick;

    Check the output pins of the op amps. If you have more than a few tenths of a volt on any of those, you could have a bad op amp. Also, those ic's can get unusually warm when they fail.
    ampdoc


    myspace.com/akgservice

  5. #5
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    Hi Ampdoc

    i checked over all those ICs, nothing get hot. All the O/Ps measured a few mV DC and 270-300 mV AC, all 14 of them, 7 dual packages.

    i'm inclined to think the problem is in the power amp board. When the preamp is disconnected the same crackle is still very evident just quieter. If the input of the power amp is open cct due to being disconnected will that lower the overall gain of the power amp making the crackle quieter?

    Ahhhh......the adventure of amp repairs.......

  6. #6
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    Quote Originally Posted by Padster View Post
    If the input of the power amp is open cct due to being disconnected will that lower the overall gain of the power amp making the crackle quieter?
    No, I wouldn't think so.

    This is telling me that the noise is present in both preamp and power amp, and gets amplified even more when the preamp noise is routed thru the power amp....

    The one circuit that is common to both sections is the power supply.

    Do you have a scope ?
    I'd like to see if all of your B+ lines are stable and clean.

    Maybe you have a crackly power supply voltage somewhere.

    The DC offset problem could be part of this problem(although it is possible it's 2 separate problems)

    Are you luck enough to have a schematic ?
    ampdoc


    myspace.com/akgservice

  7. #7

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    I am working on a Mike Mathews Freedom amp which is a transistor amp that is crackling as well.

    Due to the age of the amp I replaced all the filter caps for precaution.

    This amp is all transistors and no op-amps so it's a little different from yours but my gut tells me the crackling is a leaky transistor and most likely a transistor that is going directly to the V+. That being said I am thinking the power transistors are the place to start since they go directly from V+ to ground and probably have had the most stress on them through the life of the amp.

    From what I have seen and I am not saying this is always the case but when an op-amp goes bad it will blow an open or put out a tremendous amount of white noise.
    Crackling reminds me of that bias knob on a Fat Boost...you know the way it crackles when you turn the knob which is passing voltage accross the pot?
    So a leaking transistor with voltage on it kind of makes sense would do this in a random fashion and be heard as a crackling noise as well.

    The Mike Mathews Freedom amp has all sorts of "magical" transistors that I understand made the original Big Muffs so special so I am really not into replacing things I do not have to replace as it will change the tone for sure. That being said I am waiting on the power transistors and once they get here I will replace the ones in the amp and come back and let you know how that worked out.

    Did you say that the voltages were steady on the power transistors in your amp and do you have a schematic?

  8. #8
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    hey guys

    Thanks for all the input with this project. The crackling has ended up taking care of itself. Left the amp on overnight and in the morning, no more crackle.

    Still doing the DC dump for a few seconds at power up so i installed a time delay relay on the power amp output. Quite the hacks approach i know but its good enough for the second hand shop that i'm doing the job for. Been running the amp solidly now for a couple of days, it works, plenty of volume, plenty of tone, plenty of that tranny crunch on the overdrive channel. The amp will make some kid happy for at least a couple of years i'd say.

    Managed to rig up a little scope using free downloaded software and a few resistors as a voltage divider on the audio input of the laptop. Pretty happy to get any of this going since I am travelling at the moment and working with limited tools and whatever work comes along. By this stage the crackling had stopped and still hasn't come back so don't know if it was power supply or something else. A fairly reasonable shape from the all the O/Ps of the transformer, a slight flat spot just before the peak of each half cycle.

    Thanks again for all your input on this ampdoc and geodude, may your soldering irons be forever hot and your joints never dry!!!

    Cheers,

    Patrick.

  9. #9
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    oh yeah, and managed all that without a schematic, not that i managed all that much at all, but anyhow, the amp works.

  10. #10
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    Way to go Patrick!

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