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Probably simple AM radio repair

Discussion in 'Electronic Repair' started by Dave, Jun 22, 2005.

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  1. Dave

    Dave Guest

    Hi,

    I acquired a Tandy Chronomatic 246 clock radio that is in great condition.
    The only problem I have is AM reception below 900 kHz is almost non-
    existant. I can turn up the volume to max and touch the internal antenna
    to hear stations at low volume. The FM band works great.

    I did replace a leaking 4.7u electrolytic cap that was nearest to the
    adjustable yellow choke and tuning cap, but it made no difference. I also
    adjusted the chokes a little to see if I could get better reception near
    900 kHz, but no luck.

    I'm suspecting there might be a broken wire in the internal AM antenna?
    The wires in position on the antenna are:

    Red-Black-Blue White

    If I touch the white wire, much better reception. But it is soldered to
    the PCB securly.

    Is there a way to test the antenna, like ohm it out? Any other ideas?

    Thanks,

    Dave
     
  2. Asimov

    Asimov Guest

    "Dave" bravely wrote to "All" (22 Jun 05 05:18:25)
    --- on the heady topic of "Probably simple AM radio repair"

    Da> Subject: Probably simple AM radio repair
    Da> Xref: aeinews sci.electronics.repair:51188

    Da> Hi,

    Da> I acquired a Tandy Chronomatic 246 clock radio that is in great
    Da> condition. The only problem I have is AM reception below 900 kHz is
    Da> almost non- existant. I can turn up the volume to max and touch the
    Da> internal antenna to hear stations at low volume. The FM band works
    Da> great.
    Da> I did replace a leaking 4.7u electrolytic cap that was nearest to the
    Da> adjustable yellow choke and tuning cap, but it made no difference. I
    Da> also adjusted the chokes a little to see if I could get better
    Da> reception near 900 kHz, but no luck.

    Da> I'm suspecting there might be a broken wire in the internal AM
    Da> antenna? The wires in position on the antenna are:

    Da> Red-Black-Blue White

    Da> If I touch the white wire, much better reception. But it is soldered
    Da> to the PCB securly.

    Da> Is there a way to test the antenna, like ohm it out? Any other ideas?

    Da> Thanks,


    Dave,

    The loopstick antenna can often be optimized by moving the pickup coil
    along the length of the ferrite rod. However, I don't think this is
    the real problem. There might be a break in the tuning capacitor or
    its trimmer. Perhaps the rf xformer/mixer cans has an open coil.
    Perhaps the local oscillator/mixer (transistor, IC) is stalling below
    900KHz. This might be the supply bypassing electro for that circuit
    being open, try tag soldering a good one across it. It should be
    obvious where the problem lies by signal tracing with a scope or
    demodulator probe. Was this an existing problem or a sudden event?

    A*s*i*m*o*v

    .... That was a fascinating period of time for electronics
     
  3. NSM

    NSM Guest

    Unlikely. Probably someone unskilled "tuned it up". You need a signal
    generator and basic skills.

    N
     
  4. Ken Weitzel

    Ken Weitzel Guest

    Part of routine maintenance. Tighten all the screws :)

    (OP - this an old joke; don't even think about it!)

    Ken
     
  5. NSM

    NSM Guest

    I saw a few tube radios brought in for repair after that was done. Then
    there was the guy who disassembled his record changer and brought it all in
    to me in a bag for me to figure out.

    N
     
  6. Arfa Daily

    Arfa Daily Guest

    Don't forget that electrolyte is very corrosive, and has a tendency to creep
    where you don't want it to - it loves things like RF cans . . .

    If the decoupling cap that you mentioned really leaked, and it was near an
    RF can, suspect that it's gotten inside, and eaten the windings .

    Geoff
     
  7. Dave

    Dave Guest

    Thanks for the advice everyone! I should mention after tweaking the chokes
    around the variable tuning cap, reception is acceptable to 750 kHz, but
    there are a few stations below that I'd like to listen to. I bought the
    radio with its current problem at a garage sale, it doesn't look like it's
    ever been opened. I'll inspect the variable cap and chokes.

    I do have a 5MHz function generator and a decent scope, is there some
    advice on the net for testing receivers by injecting signals? I am mainly
    a digital troubleshooting type and don't really know that much about tank
    circuits and such.
     
  8. NSM

    NSM Guest

    Peak the IF at 455 KHz.

    Adjust the coils to peak at 600 KHz, and the trimmers to peak at 1500 KHZ.
    Repeat as required.

    Not a big deal really. BTW, if the ferrite rod is cracked it won't work
    well.

    N
     
  9. Guest

    Dave:

    Where are you located?

    There are many antique/tube type clubs out there that could probably
    figure out what's wrong in a few minutes. Those of us brought up on
    tube type am-fm radios can do what's needed in our sleep, but writing
    out the process would take ten times that long, even for just an am
    radio.

    Regards,

    H. R.(Bob) Hofmann
     
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