Help in TDA 7000 Fm receiver

Discussion in 'Homework Help' started by nsw1216, Mar 28, 2010.

  1. nsw1216

    Thread Starter New Member

    Mar 28, 2010
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    I am a simple Fm receiver project. I have picked the circuit attached... i wondering whether is this circuit working if i replace the variable coil with a fixed coil with 2 turns of diameter of a pencil n 0.5mm enamelled cooper wire. i cant find the variable coil in my place.
    Is it ok for the replacement ?
     
  2. jlcstrat

    Active Member

    Jun 19, 2009
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    The variable inductor is needed for alignment. You can use anything you want, but it will need to be within spec of the schematic for it to work at all. If you have a way to measure inductance, then you'll be safe making your own. If not, it'll be hard to tell how far off you are.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    The TDA7000 IC is obsolete and has not been made for many years. Its performance was awful. The coil is called "adjustable" because when its insulated wire turns are squeezed close together then its inductance is increased and if its insulated wire turns are spead apart then its inductance is reduced. The wire insulation should be enamel paint.

    I think the coil should have about 6 turns.

    The radio is tuned with the voltage from the potentiometer varying the capacitance of the varicap diode. The frequency of the radio will change as the battery voltage runs down. The MV2105 varicap diode is also obsolete and has not been made for many years.
     
  4. nsw1216

    Thread Starter New Member

    Mar 28, 2010
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    If i replaced the MV2105 varicap diode with the BB204B double varicap diode. Is it ok using it ?
    The coil u mentioned is 6 turns, is it using the the enamelled copper wire i mentioned ?
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    I did not compare the specs of the BB204B double varicap diode with the old MV2105 because the obsolete TDA7000 "radio" has such poor performance.

    All FM transmitters use air-core 4 turns to 9 turns enamelled copper wire for their coils.
    My FM transmitter works pretty well and sounds great.
     
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  6. nsw1216

    Thread Starter New Member

    Mar 28, 2010
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    ok. Thanks for the help.
     
  7. nsw1216

    Thread Starter New Member

    Mar 28, 2010
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    A little problems occurred... I cant tune my frequency after i replaced my variable coil with 2 turns of magnet wire with 5mm diameter and the MV2105 with the BB204B double varicap. I cant figure out what's wrong in my circuit and the deadline is approaching....

    Your help will be appreciated.....
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    Two turns for the coil will make the tuned circuit resonate far above the FM broadcast band. Try 4 turns to 9 turns like I said earlier.
    Use only one varicap diode and try tuning it by changing its voltage from 2V to 20V.
     
  9. Will777

    Member

    Sep 12, 2010
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    My apologies for bumping this old thread but your design looks great!

    I have a few questions for you:

    1. L1 and L2 - what wire diameter to use
    2. On the circuit diagram you have a component marked LM2931A - I am not familiar with the symbol used - what is that?
    3. You have 2 places where you marked ground - does this mean I need to physically connect to ground or just the negative rail?

    Thanks in advance!
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    I used enamel insulated copper wire 1mm in diameter from a speaker's crossover coil. The coils are spaced above the circuit board.
    Here is a sketch and photo of my coils.

    It is a "low-dropout" 5V voltage regulator that was made by National Semi but now it is obsolete. I still have many. There are many other low-dropout voltage regulators available from nearly every semiconductor manufacturer. You can use a 78L05 regulator instead but then the circuit will not work when the battery voltage drops below 7.0V. The low-dropout regulator works perfectly until the battery drops below 5.2V.

    There is no "negative rail". All the grounds and the negative wire of the battery connect together. A schematic shows many ground symbols so the schematic is not covered with ground wires going all over the place.

    An FM transmitter operates at 100MHz so you can't build it on a breadboard (that is used for DC and low frequencies). Make it on a pcb or on stripboard (like I did) with all the tracks cut as short as possible and the parts close together. My stripboard measures 5cm x 3cm including space for two mounting bolts.
     
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  11. Will777

    Member

    Sep 12, 2010
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    Thanks Audioguru!

    Answers all my questions :)
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Please stop sending private messages to me. I answered this question about 1 minute ago.
    The 680 ohm resistor was going to power an LED but was never used.
    Sorry, I did not look at your labels for the resistors, they should be obvious.

    I have two pcb designs if you want me to post.
     
  13. retched

    AAC Fanatic!

    Dec 5, 2009
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    Audioguru, Id like to see the PCB designs, if you have them handy.
     
  14. Will777

    Member

    Sep 12, 2010
    48
    1
    Thanks for answering
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    Here are two pcb designs (I didn't make them) for my FM transmitter:
     
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