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do polyester caps crackle?

Discussion in 'General Electronics Discussion' started by tempus, Feb 24, 2013.

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  1. tempus


    Feb 24, 2013
    Hey all;

    I've got a Sequential Circuits 6Trak keyboard that makes a crackling noise from time to time. I use this board for recording, and the recorded audio (sometimes) has this sound 10 times in a minute, other times not at all. There is also sometimes a clear DC offset (visible on the recorded waveform). Again, sometimes, not always.

    I was thinking that this may be the result of the DC coupling caps on the out becoming dried up or otherwise failing. I opened it up, and found that the coupling caps are those little yellow box caps (when I've used these before they were metalized polyester). I've never heard of these things drying up - do they? I'm a little unsure about the DC offset too, because I would think that my soundcard (which I plug the keyboard directly into) would have blocking caps as well.

    Any other ideas on where the DC offset/crackling could be coming from?

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Do you have a photo of the caps in question? Could they be tantalum (unusual, but who knows)? I wouldn't expect polyester caps to show the effect described.
  3. tempus


    Feb 24, 2013
    I use the keyboard fairly frequently, so I reassembled it, but the cap looks like this:


    There's a bunch of them all over the board. From the behaviour, I was expecting either tantalum or electrolytic myself, and was surprised to see these.

    Thanks for your reply.
  4. davenn

    davenn Moderator

    Sep 5, 2009
    No they dont dry up as they are dry for a start same with tants, but unlike electrolytics which do dry up.
    The only time I have seen those yellow ones fail is if they have been zapped with excessive voltage ( current)

    looks a nice keyboard :) , but getting a bit old now, when you opened it up, are there lots of electrolytics on the circuit board(s) ?

    Electros - first choice
    Dry joints - a close second

  5. tempus


    Feb 24, 2013
    Thanks for your reply.
    No, I was surprised to see there are almost no electros. There are a few in the power supply section (obviously) and a few scattered about near some ICs, but the vast majority are these little yellow box things.
  6. EinarA


    Feb 11, 2013
    It's rare for film caps to develop shorts but it can happen. The symptoms you describe point to coupling caps. You might have to get in and test for shorts. Being completely unfamiliar with your, or any keyboard circuits, I can't suggest any shortcuts.
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