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Is transistor faulty or something else amiss

Discussion in 'Radio and Wireless' started by Devnode, Aug 5, 2019.

  1. Devnode

    Devnode

    92
    4
    Jul 13, 2015
    Hi, all.
    I trust you are all doing great. I saw this transmitter circuit at this link,http//www.instructables.com/longrangefmtransmitter. So i wanted to give it a try. I build it on vero board and at first i could not find any ac signal any where but after some time i could trace at up to my second transistor, Q2. However i can find signal at the base and emitter but not at the collector of Q2. i have gone over this countless times looking for any sign of mistake but found none both are 2n3904 and the first one works okay. I am totally at loss as to what the cause can be. Can anyone please help.It is my first time to try radio so i'm a bit out of dept here.
    I tried to add a pic or two but it would not upload so i opted for the link. I hope it is okay.
    Thank You so much in advance.
    Devnode
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,129
    1,842
    Nov 17, 2011
    Your link leads to a 404 :(
     
  3. Martaine2005

    Martaine2005

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    May 12, 2015
    Why not eliminate the NPN by testing it?.

    Martin
     
  4. Devnode

    Devnode

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    Jul 13, 2015
    Hi, Martin. I did test it, in circuit,with the diode test feature on my dmm and it tested fine. Should i replace it and see what it does?
     
  5. Devnode

    Devnode

    92
    4
    Jul 13, 2015
    Hi. Harald. This is the one https://www.instructables.com/id/The-Ultimate-FM-Transmitter/ . I Am very sorry for wasting your time.
     
  6. Martaine2005

    Martaine2005

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    May 12, 2015
    You should really test it out of circuit. Other components can affect the readings.

    Martin
     
  7. Devnode

    Devnode

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    Jul 13, 2015
    Hi, Martin. I wiil do so as soon as i get back home. I need to go out now. Duty calls. I will let you know though.
    Regards.
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    9,129
    1,842
    Nov 17, 2011
    We do have a size limit. Recale your image to a few 100 kB, then it should work.
    How are you trying to measure at the collector? The signal there is supposed to be a high frequency FM signal. Your multimeter is very likely not suitable for measuring signals in the frequency range.
     
  9. Devnode

    Devnode

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    Jul 13, 2015
    Hi, Harald. I am using my ( very old) scope for tracing it.
     
  10. Devnode

    Devnode

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    Jul 13, 2015
    I wiil be off line for maybe an hour or two due to my work. Please dont think that im not responding to your advice but i will be away from internet for some time.
    Thankyou so much.
     
  11. Devnode

    Devnode

    92
    4
    Jul 13, 2015
    Hi,
    Martin. I did as you suggested and took out the transistor and it tested fine again 0.63 base to emmitter and 0.65 base to collector. I then replace it with a new one in the circuit with the same results. I think ill check the voltages at the various bases, collectors etc and post it tomorrow from work.
    Thanks in the meantime to all of you. You are very helpfull.
    Regards.
     
  12. Devnode

    Devnode

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    4
    Jul 13, 2015
    Hi, Martin. I have noticed that the caps used in the original project are ceramic whereas i used polyester in my project. Do you think it may have caused the problem?
     
  13. Audioguru

    Audioguru

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    Sep 24, 2016
    A multimeter is made to accurately measure the voltage at 50Hz or 60Hz. It works poorly at 1kHz and the FM radio frequency of 100MHz is 100 hundred thousand times higher and a multimeter measures nothing.

    That is the same circuit that I fixed 14 years ago:
    1) The mic preamp transistor is poorly biased so it is saturated when the battery is new and it is cutoff when the battery voltage has run down a little. I fixed it by biasing the transistor correctly and adding a 5V low-dropout voltage regulator.
    2) The radio frequency changes as the battery voltage runs down. So I connected the RF oscillator to the 5V regulator.
    3) The radio frequency changes if anything moves towards or away from the antenna. So I added an RF amplifier to isolate the oscillator from capacitance changes at the antenna.
    4) The sounds are muffled by de-emphasis in all FM radios so I added pre-emphasis like all FM radio stations have.

    I also built it compactly on Veroboard and it works perfectly and sounds perfect.
    The 0.1uF (100nF) capacitors are for audio so they should be polyester. The 0.01uF and 4.7pF capacitors are for RF so they should be ceramic. Here is my project:
     

    Attached Files:

    Last edited: Aug 6, 2019
    hevans1944 likes this.
  14. Audioguru

    Audioguru

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    Sep 24, 2016
    I forgot to say that the original C1 and C2 audio coupling capacitors have such a low value that they cut all low audio frequencies causing a man's voice to sound like a chipmunk. The values of the original R6 and C3 are also wrong and cut high audio frequencies.
    Then the sounds heard on an FM radio sound as bad as an old telephone.

    Guess what? It is so powerful that it illegally causes interference to normal FM radio stations that your neighbours are listening to.
     
    hevans1944 likes this.
  15. aurelZ

    aurelZ

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    7
    Jun 10, 2019
    Yes this one given by audiogure must work fine!
    Only one thing...on your image is 680 ohm emitter resistor and on schematic is 470 ohm
    any difference?
     
  16. Audioguru

    Audioguru

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    574
    Sep 24, 2016
    The 680 ohms resistor is the current-limiting resistor (to ground) for an LED I never installed. The 470 ohms emitter resistor R5 is 0.1" to the right of the emitter wire of Q1. C2 and R6 are on the bottom of the circuit board.
     
  17. Devnode

    Devnode

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    Jul 13, 2015
    Hi, Audioguru. I thought the caps must be changed and i already ordered some ceramics then ill give it another go. Your`s seems to be the real deal though. Can i try it? I mean this time i will find some real help if i got stuck again and i can actually learn more about radio.
     
  18. Devnode

    Devnode

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    Jul 13, 2015
    Also what diameter is your coils? I see you have ± 10 turns on both.
     
  19. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    It's for sure that, the one used for the oscillator portion is going to be an IMMENSE 10 improvement over the SWAN's . . . . . Slinky / Mickey M...O...U...S...E coil being utilized.
    Both in the respect of mechanical solidity, frequency stability and minimizing any mechanical vibrations microphonic creations.
    Now, considering that the perfboard is using .040 holes, that wire gauge would appear to be #18 enamelled and being closewound on a 1/4 in form . . . .OR . . . the companion variable trimmer capacitor specified, would easily be able to accomodate to that value of created inductance .
     
    Last edited: Aug 7, 2019
    Devnode likes this.
  20. aurelZ

    aurelZ

    55
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    Jun 10, 2019
    devnode
    if Audioguru circuit looks complex to you you can try this one.
    princip is the same simpleFMtransmitter.gif
     
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