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AM receiver not working

Discussion in 'General Electronics Discussion' started by Gherbi, Jun 13, 2013.

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  1. Gherbi

    Gherbi

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    Jun 13, 2013
    Hi everyone!
    I made the circuit as explained here: http://www.zen22142.zen.co.uk/Circuits/rf/amrec.html
    I need to make it work.

    In the radio shop in my city I could find only a ferrite rod 140mm long; the maximum capacitance of the variable condenser is 80pF.
    For the loopstick I made 90+90 turns after calculating the inductance I needed to preserve the range 500kHz-1600: if Cmax=80pF then L needs to be about 1250uH or more, right?

    It makes a noisy motorboat sound and it is very sensitive to my presence - my movements near the device become electromagnetic signals.
    Where's the problem?

    Please help, this project is very important to me, for a school exam :)

    IMG_3121.jpg The whole device (those two yellow pins are just connectors)

    IMG_3125.jpg The LC circuit

    IMG_3127.jpg The amp-demodulator circuit (on veroboard)

    Edit:
    Maybe I'm wrong, but I suppose that the problem is in the LC circuit.
    For the inductor (the loopstick antenna) I used a 0.1mm wire (42 SWG)
     
    Last edited: Jun 13, 2013
  2. duke37

    duke37

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    Jan 9, 2011
    Motor boating is usually due to bad power supply. Make sure that the by-pass capacitors are good and connected correctly.
     
  3. davenn

    davenn Moderator

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    Sep 5, 2009
    hi Gherbi,

    welcome to the forums :)

    another big source of your problems is that you are not using the correct style of variable capacitor. You are using a small plastic one that I doubt is more than a few 10's of pF let alone the 200 - 500pF you should be using for a MW broadcast receiver

    did you see the large metal tuning capacitor used in the circuit on your reference page ?
    that is an older style of tuning capacitor ... or try this style below which is the newer style of tuning capacitor found in most small transistor radios....

    [​IMG]

    You really need to get that sorted out first before attacking any other possible problems :)

    cheers
    Dave
     

    Attached Files:

  4. shrtrnd

    shrtrnd

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    Jan 15, 2010
    Here's my input:
    1) Make sure your antenna is somewhere where you can pick-up the signal.
    I can't can't AM inside the building where I work at all.
    2) make sure your assembly is not in close proximity to anything that is going to
    drag down your AM input signal. Earth ground, Radio or Electrical interferrence.
     
  5. Gherbi

    Gherbi

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    Jun 13, 2013
    Thank you for the advices

    Duke, the bypass capacitors are the two 100nF on the left of the schematic? Anyway I carefully checked the whole circuit...

    Dave, did you mean that in your opinion the capacitor has a too low capacitance? If yes, as I said, that one is the condenser with the highest capacity I colud find in my city. Note that the AM radio station with the better signal here is on 1170kHz frequency, so a few 10's of pF should be enaugh.
    I tried testing the same circuit with a loop antenna (diameter about 40cm, 33 turns) which inductance is about 900uH, so that a 20pF capacitance would be ok to listen to that radio station but nope.
    I also tried to make my own variable capacitor, that should reach over 500pF, but nope.
    Did you mean something else?

    Shrtrnd, thank you, but my AM/FM radio works perfectly...

    More ideas? :(
     
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Today that is true but back in the day I'd see this problem manifest in the inherent weakness of point to point wiring. In the 60's I tech'd on 16mm projectors, Bell & Howell, Kodak, Revere, etc that employed vacuum tube audio amps. The gain of a 12AX7 is rather awesome and a wire that's moved a fraction of an inch could cause motor boating. These amps were so sensitive that if you rapped the chassis you'd hear it in the speaker despite the fact that the sound track pickup was photosensitive! ;)

    Chris
     
  7. duke37

    duke37

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    Jan 9, 2011
    There is a 10uF capacitor across the battery, this is the one I would go for. Also have a battery with a low internal resistance (new).

    Motor boating is instability at low audio frequency so it is the large smoothing electrolytics which need to be good.
     
  8. davenn

    davenn Moderator

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    yes, that exactly what I meant, you should be a minimum of 200pF and be able to adj up to ~400 pF
     
  9. Gherbi

    Gherbi

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    Jun 13, 2013
    Ok, it is getting a bit better.

    I tried to adjust the soldering of the 10uF electrolytic condenser as Duke suggested. I don't know if this or something else affected the result, but I could "hear" a few radio stations. But I can't understand anything of what they say, the output is very noisy and buzzing.

    I think that those I reached were a Slovenian, an Austrian and an Italian radio stations. With an earbud coming from my bought FM/AM radio I tried to recognise the frequency of those station I was reaching on the device. BUT it looks like they weren't in the range of my bought radio (530-1600kHz) and this is strange!
    I was using my homemade capacitor (with a third plate added) and the 40cm loop antenna.

    I forgot to say that for the feedback resistance I've put a potentiometer from 80kΩ to 200kΩ so that I can easily adjust it, but I think it is not a trouble, right?

    Note that the circuit isn't linked to anything external except "me": in fact, I can poorly hear something (when I do) only if I touch the ground of the circuit (the metal box or the negative pole).

    Should I do something to avoid this (and listen without touching the device)? Is it possible to improve the quality of the output?
    Thank you all for the help you're trying to provide to me.
     
  10. davenn

    davenn Moderator

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    Sep 5, 2009
    well its quite possible you are hearing broadcast shortwave services in the say 2 - 4MHz range, maybe higher. You would have to hear a station ID to try and confirm that

    Again this would be cuz of the small value capacitor and probably lower inductance than what's needed for the MW band


    please dont be too offended BUT .... I really shake my head some times when people are presented with a working circuit
    BUT they dont follow the instruction, rather they do their own modifications and then wonder why it doesnt work

    1) bring the 2 coils closer together on the ferrite rod, so that the connecting wire between them isnt that long length that you have wrapped around the rod
    2) make sure just one of the coiles is wound lightly on a paper roll so that it will slide along the rod a little bit
    3) drastically reduce the length of the wires between the ends of the coils and the tuning capacitor
    4) drastically reduce the length of the wires fro mthe tuning capacitor and the circuit board
    5) get a larger value tuning capacitor .... you or a friend/family member electronics repair shop somewhere close by must have an old dead radio that you can scrounge the capacitor and even the coil out of


    Dave
     
    Last edited: Jun 14, 2013
  11. Gherbi

    Gherbi

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    Jun 13, 2013
    I thought this too, Dave, but I didn't know there are AM radio transmissions in short wave. I suppose this would explain also the bad quality of the output, if the circuit is designed for medium and long wave.
    I'm going to follow your instructions as soon as I can.

    I see! In my case, are you referring to the potentiometer or still to the capacitor? (Or both?)
    Anyway, today I'm gonna do everything possible to find an enaugh capacitor, I swear :)
     
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Civilian and Military aircraft occupy a VHF - UHF frequency spectrum. They're also AM.

    Also, most AM receivers can demodulate and FM signal by tuning it slightly off frequency. Google "Slope Detection".

    Chris
     
  13. Gherbi

    Gherbi

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    Jun 13, 2013
    Chris, I looked for slope detection... that's very intresting! I'll try to explore this topic.

    Dave, before reducing the length between the two coils I had a doubt. I don't know why, I thought to check if the soldering of the coils on the loopstick antenna was well done, using a tester. Zero volt.
    I just shortened and re-soldered that wire (it wasn't soldered at one end!).
    I looked for an old radio to steal the capacitor, but I couldn't find my nutty scrap-collector friend, I'll retry tomorrow.

    Anyway it seems that now I found a good reason why the device didn't work.
    I will try to re-assemble my radio and will let you know!

    P.S.: Obviously, Dave, if it won't work anyway, I'll do what you suggested...
     
  14. davenn

    davenn Moderator

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    I was referring primarily to the coils, the capacitor and the long lengths of wire between them and the circuit board

    Hey its all about learning ... but the learning is easier when instructions are followed LOL
    and the faults/problems are easier to find as constructing something to the plan eliminates a bunch of possible problems straight away

    Ohhh you will find shortwave broadcast AM transmissions up to at least 20 MHz

    The 1.8 - 30 MHz part of the spectrum is also shared with amateur radio operators... me being one of them. We have little slices of frequencies righht across that range .... these are called the HF ( High Frequency) bands
    from 30 - 300 MHz is the VHF bands, amateurs have 50 - 54, 144-148 MHz ( with some country variations) in that range.

    cheers
    Dave
     
  15. Gherbi

    Gherbi

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    Jun 13, 2013
    Ok, it works now.
    Thank you Dave, I'll discover all the AM local transmissions then :)
    In the coming days I'll shorten the wires and mount a new capacitor (that friend of mine told me he probably has got one).
    Is this (the length of connections) the reason for which I still must touch the metal case (ground) to cancel the noise?
    Maybe I should also cover wires with an aluminium foil connected to the ground?
     
  16. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    What noise? Is it mains (50 - 60Hz) hum, static, hiss, what? The very nature of that design is going to make it sensitive to many sources other than an RF signal.

    EDIT: Long wire connections are and instability definitely a no, no and can contribute to malto noise. ;)

    Chris
     
    Last edited: Jun 16, 2013
  17. Gherbi

    Gherbi

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    Jun 13, 2013
    Hey Dave, look at this: 2150 pF!

    IMG_3130.jpg
     
  18. davenn

    davenn Moderator

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    Sep 5, 2009
    wooohooo

    I would assume thats the total pF of both sections?

    seems a bit high ( but not impossible) have never seen one over ~ 700pF
    I cant remember if you said that you have a capacitance meter....
    if so measure each section individually
    you would just need one section anyway :)

    give it a good cleanup with a small paintbrush, to get rid of anything that may short out
    the blades etc

    Dave
     
    Last edited: Jun 16, 2013
  19. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    It's a thing of beauty but as Dave said that capacitance seems rather high for what I see there. I would guess a total of 900 to 1000pf but hey, I'm guessing.

    Chris
     
  20. Gherbi

    Gherbi

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    0
    Jun 13, 2013
    Yes, it's the total capacitance! (and I confess I calculated it by measuring plates... let's forget it)

    Anyway.
    Now it works very well with headphones, BUT:
    - does not work with speaker (makes different noises depending on which speaker)
    - does not work if I don't touch the box (the usual motorboating)
    - reaches one radio channel only, at 540-550 kHz
    I tried both with one and with two capacitors, same result. It's very strange, because there are other channels above and below that frequency, very close to it.

    IMG_3132.jpg Here's a picture of the situation...
     
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