FM to AM for Valve (Tube) radio

Discussion in 'The Projects Forum' started by cork_ie, Jun 5, 2012.

  1. cork_ie

    Thread Starter Member

    Oct 8, 2011
    348
    58
    I'm here scratching my head how best to approach this.

    I have an old MW/LW valve radio which I repaired many years ago but rarely use it now. I have to switch it on every so often to stop the capacitors from deteriorating. So I may a well use it rather than let it just sit there.
    MW is now switched off in Ireland and LW seems to attract every kind of possible interference. I have been toying for some time with the idea of making some sort of FM receiver and feeding the output into the aux input of the radio. How difficult would it be to go one better and make a combined FM receiver AM transmitter and place it near the set but use a separate power supply. I am quite willing to use any modern devices (IC's etc) which can make this task simpler but want to preserve the sound quality as far as possible.
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
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    I suspect your best bet is a repeater. FM receiver / AM transmitter. You are pretty stuck with the limited AM sound though. FM is rated for 20Khz, AM 10Khz high frequency response.
     
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  3. absf

    Senior Member

    Dec 29, 2010
    1,492
    371
    I am not sure if this is a good alternative. But you can read the information provided and decide for yourself. :D

    The chip TDA7000 is a one-chip FM radio. Datasheet is here:
    http://www.escol.com.my/Datasheets_specs/TDA7000.pdf

    There is also a kits designed for this chip but the PCB was not included here:
    #PE-01
    http://www.escol.com.my/Projects.html#Project__06

    You could build the circuit and test it using a normal Amp and listen to see how good is the sound quality.... Then put the whole circuit inside your valve radio and connect the output to the valve's audio input. Of course you have to tap a little DC supply for the added circuit. But the biggest problem is how to combine the tunning of the FM with your AM....:confused:

    Allen
     
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  4. cork_ie

    Thread Starter Member

    Oct 8, 2011
    348
    58
    Thanks a million, the TDA7000 is just what I was looking for. I remembered reading somewhere that there was an FM receiver on a chip, but couldn't find it when I went looking.
    I think I will be able to sort the power supply and AM Transmitter easy enough.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    Fair warning, the chip may not be very sensitive. Commercial radios have really high gain and selectivity that you may not be able to duplicate with a simple circuit.

    Then again, I've been wrong before.
     
  6. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    No.
    An FM radio cuts the 19khz stereo pilot tone so its audio response is to 15kHz, not 20kHz.
    An AM radio cuts the 10kHz interference whistle between adjacent station frequencies so most begin cutting audio frequencies above only 2kHz.
    Most AM radio statrions boost frequencies around 3kHz so that an ordinary AM radio sounds as good (?) as a local telephone call but many telephone lines have an attenuation of 10dB (the limit is -15dB) at 3kHz.
     
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