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Need help in 88-108MHz preamp circuit!!

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by rahulan999, Aug 25, 2008.

  1. radiomate

    radiomate New Member

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    Even excellent high-end FM receivers have AFC, so obviously the lesser ones will have it too, and this should not be material for critisism ;-)
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The TEA5711 radio IC and the Sony one were designed a long time ago and their radios are cheap.
    Good modern radios like my car radio use a crystal controlled frequency synthesizer and don't nedd AFC to "fix the wandering frequency" of a station.

    My cheap clock radios costed $1.25 and my Sony Walkman radio costed $19.00 about 15 years ago and they have the TEA5711 or the Sony radio IC with AFC.
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I don't see any relation between 'cheap' and 'AFC'? - modern PLL tuning systems usually have AFC anyway, to accurately tune, and determine the tuning during searching.

    AFC is more a sign of a high quality receiver than a low quality one, and no manually tuned VHF FM receiver should be without it.
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    My car radio doesn't have AFC. It snaps from one station to the next very accurately.
    Stations here are exactly 200kHz apart and my car radio changes its frequency in 200kHz steps.

    My home stereo also doesn't have AFC. It doesn't need AFC because the stations in my area have their frequencies crystal controlled "and don't wander all over the place" like a cheap radio station or a cheap radio.

    My 44 years old FM stereo receiver has an analog tuner and also doesn't have AFC. It has a tuning meter to fine tune the center of a station's frequency. Then nothing changes the frequency.

    Imagine the mains frequency in a 3rd world country changing all over the place so much that a simple clock won't work accurately. Are their FM station frequencies the same??
     
  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I would say your car radio almost certainly has AFC, it's an integral part of a PLL tuning system.

    All FM stations are a standard step apart - it's 150KHz in the UK (I think?).

    All broadcasters are accurately crystal controlled, the AFC is to keep the radio accurately in tune (for a manually tuned one), or to accurately search tune a PLL one. It'a VERY likely your home stereo has AFC.

    Again, it's most likely that your old receiver has AFC, analog tuners like that certainly benefit from it.

    No, but their radios (and your's) will drift, or require AFC for their auto-search function.
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Hi Nigel,
    My upper-quality radios don't drift so they don't have AFC.
    I just saw (heard) a cheap radio station's frequency fly past me. Maybe my cheap clock radios and my cheap Sony Walkman radio can pick it up beause they have AFC.
     
  8. radiomate

    radiomate New Member

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    Mains frequency changing all over the place? Why, are they running their generators with a guy on a bicycle or somtin? In any case, the mains frequency does not affect the tuner, the power is steped down, rectified and filtered as we all know.
    All analogue FM radios have AFC, and it can be switched off on the good ones to allow tuning weak stations when they are near strong ones.
    I remember H.H.Scott saying that AFC affects the sound quality, so it can be switched off on their receivers. H.H.Scott were famous for their FM tube tuners/receivers in the 50's and 60's.
     
  9. rahulan999

    rahulan999 New Member

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    Previously I have two stereo boards of the same IC. One thing I noticed is that there was a pot(10k or 100k) near to the IC on both of them. It was set on the middle by default. But I couldn't see any changes when I tried to turn the pot.
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Guess what? My 44 years old FM receiver model 341 was made by HH Scott and it does not have AFC. It was one of their first ones that used an IC in the IF. It still works perfectly.
     
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The datasheet for the TEA5711 radio IC shows a 47k pot as a rheostat in series with a 68k resistor at pin 27 as the 19kHz/38kHz stereo VCO adjustment. They say to adjust it when receiving a stereo signal. Look at the stereo indicator and rotate the pot both directions to see where the light turns off and set the pot in between.
     
  12. rahulan999

    rahulan999 New Member

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    Yeah, now I remember that 'VCO' is written near to that pot. You mean it should be adjusted for the indicator should just light up when receiving a stereo signal?Its for that only?Anyhow my receiver indicator is functioning properly with the factory settings :)
    Also from hereonwards I should try to collect that IC n number as possible if its discontinued as you told. It's a collectible right?:) I love it so much. thats y.
     
    Last edited: Sep 19, 2008
  13. radiomate

    radiomate New Member

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    rahulan999 it is best to adjust the VCO pot with a not so strong signal, as with strong signal there will be hysteresis. Like audioguru said, the best position is in between where the stereo LED goes off. With a strong signal, you might turn the pot all the way to one side and the LED will not go off. If you look at my TEA5711 receivers, the pot is almost in the middle, where stereo separation is best.
     
  14. radiomate

    radiomate New Member

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    rahulan let me know if u get ur hands on the TEA5711 IC.. I was lucky to procure only 2, and with the help of a friend in Germany. The IC's are stocked all over the world, but they can only be purchased in large quantities, which is not practical to the homebrewer.
     
    Last edited: Sep 19, 2008
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Digikey doesn't have any TEA5711 radio ICs.
     
  16. radiomate

    radiomate New Member

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    No they don't, neither does Mouser Electronics, and no NTE equivalent.
     
  17. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Do you have a circuit for it?, google doesn't seem to find one - but here's a far older Quad FM16 circuit that does have AFC (as all decent radios do).
     

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  18. rahulan999

    rahulan999 New Member

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    radiomate,
    On your receiver, the power supply used is only 3v! How long the alkaline battery lasts in hour?What is the safest supply voltage. I'm using a 9v battery with a 100E series resistor to power the IC. Is that any problem.
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Look at the datasheet for the TEA5711 radio IC. It gives the supply range of voltages and gives the current. The spec's are with a 3V supply.
    Look at a battery maufacturer's website to see the datasheet for the size of alkaline battery you use. AAA, AA?
     
  20. radiomate

    radiomate New Member

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    rahulan999 the TEA5711 supply can be from 1.8 to 12V, and the TDA7050 from 1.6 to 6V. I am powering my receivers with 3 and 4.5V. You are safe with a supply lower or equal to the max, the ICs can run with supplies slightly higher than the max, manufacturers set the max lower than the REAL allowed max to be on the safe side. The batteries last for a long time, in terms of weeks if not months, my receivers have been running with the same batteries since mid-July (2008) with no loss of performance. Use your voltmeter to see how much your TEA5711 is running from, but it is safe since it is obviously less than 12V.

    We need to see pictures of your work and TEA5711 receivers! :)
     
    Last edited: Sep 20, 2008
  21. radiomate

    radiomate New Member

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    Last edited: Sep 20, 2008

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