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Testing capacitors in amp circuit

Discussion in 'Troubleshooting and Repair' started by msintros, May 28, 2016.

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  1. msintros

    msintros

    9
    1
    May 26, 2016
    I have some powered studio monitors (Event 20/20) that have been misbehaving for some time. The electronics in the left part of the circuit have been producing a white noise type distortion at medium to higher volumes, especially in response to bassy sounds. Then one day they started producing intermittent bursts of static regardless of the input. It is definitely the electronics and nothing to do with the speakers themselves or the cables.

    Capacitors seem to be the most likely culprit (I could be way off base, though) and so I set about trying to test some of them today. In particular I noticed (discovered this somewhat accidentally) that if I brush certain contacts on the back of one part of the circuit, the sound would be normal for about a second at which point the static would start to gradually come back. This tells me probably a capacitor is charging up and then malfunctioning. This part of the circuit contains several 22microfarad electrolytics. I also tested the two large 10Kmicrofarads and those came back with normal readings. However, when I went to test the 22's, some of them came back normal while some of them read over 9K microfarads. I have never really tested caps before and I thought that if it was a bad cap, it would read much lower than what one would expect. I have tried searching for info about this but can't come up with much. What does it mean if my meter says some astronomically high number when testing the cap? Are these caps also bad or is it the circuit itself that is somehow coming into play here and I would have to remove the capacitor from the board to properly test it? In particular I find it suspicious that the readings for these smaller caps are about the same as the larger caps on the board.

    The closest authorized Event repairer is asking $80 minimum to even look at these things so I think if I can figure out which are the bad components I can replace them myself for cheaper. It's certainly within my skillset to replace components but I have no real experience with diagnosing this kind of thing. Thanks in advance for any suggestions!
     
  2. HellasTechn

    HellasTechn

    1,559
    215
    Apr 14, 2013
    That is most likely the reason you get that.

    Actually i would test all caps away from the board and discharged.
     
  3. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    Large capacitors are measured by inputting a small current and then measuring the rise in voltage. If the capacitor is leaky, some current is lost so the voltage rises more slowly as would a higher value capacitor.

    Measure the leakage current at rated voltage.
     
  4. 73's de Edd

    73's de Edd

    3,365
    1,406
    Aug 21, 2015
    .

    Sir msintros . . . . . . .


    How frequent is your problem occurring and has it proven to be more related to the equipment being cold or after a warm up ?

    I fully think that I could give you a methodical testing procedure IF I had the units schematic.

    "I ain't finding nuttin" . . .except a 3 page "propaganda flyer " and that Event is now Rode and they don't have anything on them . . . . or else, certainly want you to think THAT.

    73's de Edd

    .
     
  5. msintros

    msintros

    9
    1
    May 26, 2016
    I don't have a schematic unfortunately. I had contacted them at one point and the best they could do was give me a link to a schematic for the 20/20bas, which is a slightly newer model, but unfortunately that link is dead now (I wish I had saved it at the time).

    To answer an earlier question, the behavior is fairly consistent. I used to simply leave the speakers on all the time. At that point, the problem manifested any time the lower frequencies reached a certain amplitude. The left tweeter would distort (sound from the other speaker on the left channel and both speakers in the right channel were normal). One day I decided to turn them off and back on and when I did so, there was a significant burst of static coming out the left, after which it began producing regular intervals of static bursts coming out the tweeter. The interval seems to slow after they have been on for a while but will still produce bursts if the volume is turned up beyond very quiet.
     
  6. elebish

    elebish

    177
    12
    Aug 16, 2013
    There should be a small non polarized (NP) electrolytic capacitor in the crossover network that blocks the low frequency from getting into the tweeter. I would just replace it since that would be simple and inexpensive. The 22ufd may be the culprit!
     
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