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Newbie with Capacitor problem

Discussion in 'General Electronics Discussion' started by CX15, Dec 31, 2013.

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  1. CX15

    CX15

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    Dec 31, 2013
    Hi everyone

    I am new to the "hobby" of electronic projects. (VERY new!)
    Also, my background in electronics is basically zero :(

    I just finished my first project: A FM Transmitter Kit using a BA1404 IC.
    Here is the schematic:
    [​IMG]

    And a photo of my finished project:
    [​IMG]

    Further details on the website I bought it from here:
    http://electronics-diy.com/BA1404_Stereo_FM_Transmitter.php

    -----

    And here is my problem:

    The device works half the time. However it behaves slightly erratically and I have no idea why.

    Half the time when it is powered up and transmitting, the transmitted signal is extremely noisy. You can still hear the transmitted music but the noise is unbearable.
    However, if I lightly touch the Ceramic Capacitor at position C15 with the tip of my finger, the noise disappears completely and I have a crystal clear transmission.

    The other half of the time I have the opposite: The signal is clear and when I touch the same capacitor it turns noisy immediately until I stop touching it.

    I have double checked the soldering on that capacitor and it seems fine.
    The capacitor at C15 is 10pF.

    If anyone can shed some light on the problem I will be very gratefull!!

    Thanks in advance and hopefully this is the start of a fruitfull new hobby!! (I already started playing with an Arduino!)

    Regards
    Andrew
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I don't suppose you have a schematic with C15 marked do you? It looks like the one connected between pin 9 and 10.

    Can you post a picture of the other side of your board?

    One possibility is a poor solder joint.

    The other possibility is that touching it changes the tuning enough that the signal goes into-or out of tune causing more or less noise.
     
  3. CX15

    CX15

    4
    0
    Dec 31, 2013
    C15 marked here:

    [​IMG]

    [​IMG]
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    OK, that's what I thought. Now, a picture of the other side of the board would be great...
     
  5. CX15

    CX15

    4
    0
    Dec 31, 2013
    (keep in mind this was my first attempt at soldering a ICB :eek:)

    [​IMG]

    Let me know if I should retake from different angle...
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    For your first attempt at soldering you've done a great job.

    However, soldering is a bit of an art and takes practice. There are a few joints that look a little suspicious to me.

    Heat up your iron, and touch them briefly with the soldering iron (to melt the solder) then just add a very small amount of fresh solder before removing the iron.

    The ones I am most concerned about are circled below

    [​IMG]

    The ones in green look like they're not good joints, the ones in blue are the ones for the capacitor you mentioned (so do them anyway).

    Because you used a flash, it's actually quite hard to see a lot of detail.

    What I can tell is that some joints seem to have a little too much solder, and others not quite enough. However you haven't gone either way to such an extreme that I would be worried.

    The green circled joints don't have a smooth appearance and/or the solder doesn't seem to have "wet" the wire or the pad properly.

    I'm not 100% sure this will fix your problem, but it's a simple thing to try :)
     

    Attached Files:

    Last edited: Dec 31, 2013
  7. CX15

    CX15

    4
    0
    Dec 31, 2013
    Thanks for taking time to help!!

    Will give them another once over and see if there's any improvement!
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    Oh, and also google "good solder joint" and take a look at the image results. There are plenty of examples of good and poor solder joints.

    You may be able to spot more poor joints, or even venture an opinion on whether the ones I spotted are bad.
     
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