Connect with us

President Jackson Uniden TX noise...

Discussion in 'Electronic Repair' started by Giulia, Apr 5, 2007.

Scroll to continue with content
  1. Giulia

    Giulia Guest

    Hi ,
    i have a problem with a president jackson cb ,
    in tx there is about 100hz noise (i tx with car battery no PSU),
    the noise there is even if carrier is unmodulted (fm and am mode)....
    Can someone halp me ?
    Schemtics are aviable and i can use an oscilloscope.........

  2. Guest

    Guest Guest

    are you related to marqueer?
  3. Sounds like audio oscillation. What mic are you using? What happens if
    you key the transmitter with no mic? Problem still there?
  4. Giulia

    Giulia Guest

    Telstar Electronics ha scritto:
    i put an612 pin to ground mic is cutted out .....or carrier
    oscillator or power control loop from tx power transistors.......

  5. Dloyd cyber terroist

  6. Ignore Brian (Telspam) -- he'll tell you that your ground plane needs
    to be nine square feet.

    Refer to your schematic and use your scope to trace the oscillation
    backwards through the audio chain starting at the audio power amp.
    You'll find the problem stage eventually. Don't forget to carefully
    inspect the board for bad solder joints and loose pads.
  7. OK... you have pin2 grounded for no input signal. Now use your scope
    and trace backward from the RF output collector... moving back through
    the audio chain... noting the origin of the 100Hz oscillation.
    Normally those radios have only a single transistor buffer for the
    mic... followed by a high power op amp (usually mounted to chassis
    wrapper) for the audio. Your signal tracing should take long at all.
    Hope that helps.
  8. james

    james Guest



    That is a subaudible frequency.
    How could anyone detect it without a scope?

  9. jakdedert

    jakdedert Guest

  10. Radiosrfun

    Radiosrfun Guest

    The range of "normal" human hearing is 20-20,000 Hz. How is 100 Hz
  11. 100Hz?????
    If you can't hear 100Hz... you need to get your ears checked Jim.
  12. james

    james Guest

    Obvisously you lack the knowledge of what is the subtle difference
    between audible and audio frequency. Also I did forget that CB radios
    are not necessarily well designed radios. In a well designed radio,
    any audio frequency below 300Hz is considered subaudible.

    Why? Because the power density in the human voice below 300Hz is
    negligible. Most well designed receivers for communications purposes
    do not have audio response below 300 Hz. Also human hearing threshold
    increases as the frequency decreases below 300Hz. That is the source
    of the audio freq uency needs to have a larger PSL in order for it to
    be heard. Sensitivity to audio frequency in Humans is not flat like
    that of speakers and some sound systems. Instead SPL, Sound Pressure
    Level, needs to increase as frequency decreases below 300Hz to gain
    the recognition. For most Humans the power of a 20Hz signal needs to
    be about 30dB higher than that of 100Hz.

  13. james

    james Guest


    That maybe true but the Human ear does not perceive sounds with a flat
    response for that frequency range. Instead as frequency goes very low
    and also very high the power density for a given frequency may have to
    increase or decrease. The human ear does not have a flat range of

    Also in communications the Human voice has very little power below
    300Hz. Therefore most communications equiptment considered below 300
    Hz as subaudible. A well designed receiver for voice communications
    should reject frequencies below 300Hz. In fact they should also reject
    frequencies above about 3KHz also.

    The range of 300 to 3000 Hz is where the ear is most sensitive to
    frequencies. Also the Human voice contains most of its power between
    this range.

  14. Giulia

    Giulia Guest

    Changing in VR5 (some related to modulation) remove the noise ,
    i put vr5 more near ground.

    Noise seems generated from somewhere feedback at hight swr....
    regulation of vr5 remove noise but what kind of effect can it have on
    modulation (in am all semms right RF evelope in the scope and audio
    received from another cb)?

  15. Radiosrfun

    Radiosrfun Guest

    RF feedback into the microphone? If you feel it is due to high SWR - why not
    check and reduce it? Try your tests again. What kind of mic are you using?
    If you mentioned it, I didn't see it. I've seen D104s do that. On 2 meters -
    I've seen RF get into the power source and do all kinds of weird things to
    the signal.

    Check your SWR then - go from there. IF it IS high, even if the mic/audio
    issue isn't cured, at least another problem will be - and save you from yet
    a future one.

    Are you using an "Amplifier" with this set up?
  16. Radiosrfun

    Radiosrfun Guest

    I "believe" you said you're running the radio off a car battery. Is it
    "Actually" mobile or in the house? If in the house, are you running a
    trickle charger in the process? IF the modulation was cranked up all the
    way, were you running anything in the background maybe the mic picked up?
    Many items can cause a hum - you won't hear - that the mic will pick up. If
    it is mobile, any chance something in the vehicle is causing it? With the
    modulation cranked up - maybe it was enough to allow "that" to be heard.
    I've also seen a transmitter make some weird sounds on key - up - with a
    weaker battery. The antenna SWR as you alluded to - the antenna not
    grounded - could also create issues along those limes. I've seen lots of
    weird things happen to cause noises such as you describe - not always
    internal. Recheck your Antenna coax connections too.
  17. jakdedert

    jakdedert Guest

    You're backpedaling.

    What you clearly said was that it could not be detected without a scope.
    Then you come up with a bunch of gobbledeegook about 'well designed
    receivers'. That's not the point, and that's not what you said.

    Suffice to say that you were simply wrong. 100 Hz is clearly audible.
    It doesn't take a scope to detect. A pair of common ears is all that's
    necessary. Whether it should be heard in a CB receiver is debatable,
    but if it's there, there's no reason why one shouldn't be able to hear it.

  18. To be fair, "subaudible tone" is a fairly common euphemism for CTCSS or
    "PL" tones. As you point out, though, they're only "subaudible" because
    they get filtered out before the final audio stages.

  19. Hogwash. The .3-3kHz limits were standardized (by the military)
    because researchers discovered, over half a century ago, that the
    bandwidth carried the majority of -intelligibility- in the human
    voice. IOW, by limiting the frequency range of the vocal spectrum,
    transmitted speech can be understood using much less power than when
    transmitting using the full voice bandwidth.

    Hmmm..... my Onkyo is a well-designed receiver for communications
    purposes and it is flat down to 2 Hz.....

    Huh? When was the last time you looked at a weighted equalization

    That depends on how much you bump up the boom on those 24" JBLs inside
    your sub-compact riceburner. The sad thing is that when those kids go
    deaf they will collect welfare for their self-inflicted disability.
  20. james

    james Guest


    I stated how could one detect it without a scope? please reread my
    post and comprehend better.

    100 Hz is hearable or percieved. As I stated before there is a subtle
    difference bwetween audible and hearable. Yes you can "hear" 100Hz if
    increase the signal power compared to a 1000Hz tone. But at equal
    power desity, a 100 Hz tone by most people is not "hearable" or
    audible. The same goes for the higher end of the audio spectrum.
    Granted some have better sensitivity to a wider spectrum of audiio

    Besides this tangent has gone far beyond the scope of the original
    post and realy was not my intention. My intention was to discuss the
    detection of the offending signal with and without a scope. Not to
    discuss what is audible to your ear or the frequency response of your

    thank you

Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day