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80's TV UHF tuner

Discussion in 'Electronic Repair' started by [email protected], Aug 20, 2005.

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  1. Guest

    Hi All,

    I have an older TV set with the UHF tuner going up to channel 83. The
    tuner is a mechnical type.

    The output of the tuner goes back into the VHF tuner and you select the
    U on the VHF tuner to operate the UHF channels.

    In down conversion of the UHF band, what is the IF frequency of the UHF
    tuner? The set is an old RCA.

    pdrunen
     
  2. Mark

    Mark Guest

    44 MHz, same as the VHF tuner, the VHF tuner is configured as an
    amplifer when in UHF mode.

    Mark
     
  3. WB2MEP

    WB2MEP Guest

    Same IF as VHF: 45.75 MHz.

    When the VHF tuner is set to "U", its local oscillator is disabled,
    and its RF amp & mixer amplify the IF out of the UHF tuner
    without doing any frequency conversion.

    Those mechanical UHF tuners have no RF amp stage, and
    use a diode mixer, therefore no gain. The VHF tuner
    makes up for this by acting as an IF preamp before passing
    the IF signal to the IF stage.

    This configuration is likely due to the lack of cheap, low-noise
    UHF transistors in that time period, along with there being
    much fewer UHF stations on the air, so the UHF tuners
    went unused in many sets. By the time digital-controlled
    tuners became commonplace in the late 80's, the UHF
    tuners got real RF amp stages, and didn't pass their
    IF outputs thru the VHF section for amplification.


    Mike
    WB2MEP
     
  4. Barney

    Barney Guest

    What country are you in?
     
  5. Guest

    What is the problem you are trying to solve, or were you just curious
    about how it worked?

    Bob Hofmann
     
  6. Guest

    Thanks All,

    No problem with the set, just trying to play around with the UHF tuner.
    It appears to have an input for 18VDC and line for the AFT. The TV
    had some other problems and I removed the tuner to see if I could do
    anything with it.

    I don't see anything on the O'scope out of it but will try a scanner at
    44 MHz to see if I can hear any stations. Guess I will need a wideband
    FM scanner.
     
  7. If you're talking about putting the scope at the output of the UHF
    tuner, that's not likely to show anything.

    As someone pointed out, those tuners used a diode mixer, so you not
    only see no gain, but there is a loss between the input and the output.
    The signal is going to be very weak with that sort of tuner. I'm not
    sure when active tuners kicked in, but even those won't have too much
    of an output.

    Better to make sure the oscillator is oscillating. Use the scanner
    to tune in the tuner's oscillator signal. After that, either the
    diode is good, or it's not. Beyond that, if there's a problem it's
    not an active component, because you've just accounted for them
    all.

    Michael
     
  8. Mark

    Mark Guest

    yep, the uhf tuner outpout will be too weak to se eon a scope but you
    may be able to use a scanner tuned to 41 to 47 MHz. The UHF tuner
    ocsillator will be very unstable compared to the narrowband signals in
    a scanner. The drift is not a problem for 6 MHz wide TV signal but
    might be a problem for narrow 30 kHz signals etc.

    If you work at it, you mat be able to generate an AFT signal from the
    scanner to stablize the tuner. Have fun

    Also, most cell phone signals are digital these days so you won't be
    able to use this method to hear them.




    Mark
     
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