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Eq. Parts for an AM radio project

Discussion in 'Datasheets, Manuals and Component Identification' started by eric_x, Mar 22, 2011.

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  1. eric_x

    eric_x

    5
    0
    Mar 22, 2011
    Hello everyone,
    I'm in a sort of introductory electronics course for Mech. Engineers, and our final lab project is to build an AM radio. Easy enough, but we also have to order the parts for the project, and I'm having a hard time locating some parts or finding equivalents, partly due to a lack of knowledge about what something needs to be equivalent.
    The circuit I'm trying to build is detailed here:
    http://www.circuitstoday.com/simple-opamp-radio#ixzz1G2OBYZ3D
    [​IMG]

    I'm trying to order most parts from digi-key to save on shipping, I don't know if there would be a better place to order from. The parts I am having trouble finding are:

    c1………..30-360 Pf Variable
    I cannot find one with the same specs apart from one listed for $75 through google that doesn't seem like it was meant for this application. I thought I could use two 10-180pF trimmers here instead
    SG3014-ND CAP 10MM 10-180PF-TOP/BOT ADJ OR
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=97905990&uq=634363910871594965

    T1………..Sl 100 NPN Transistor or equiv.
    I found a datasheet for this transistor here:
    http://www.sunrom.com/files/SL100.pdf
    and I can find it online, but I would like to order through digi-key
    This is what I thought would work:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ZTX453-ND

    D1………..1N34 Germanium Diode

    I found this online, it would be nice if it was available through Digi-key but I didn't see anything. I think the main reason for Germanium is that it has a smaller voltage step before it activates.
    http://www.packetradio.com/catalog/...ucts_id=2284&zenid=o2ifm3gd08ned1scd2omeld7f3

    For capacitors and resistors I opted for 1% tolerances or 10% tolerances, and 1/2 watt resistors. I thought that would be good enough for this application, and I might as well get good tolerances if I only need 4 or so.

    Thanks for any input
     
  2. davenn

    davenn Moderator

    13,620
    1,882
    Sep 5, 2009
    Hi Eric, welcome to the forums

    no you cant use 2 trimmers in parallel, you need a contineous wide variable capacitance range...
    ok, have you got an old radio receiver you can rat a variable tuning capacitor out of
    any old radio or radio/cd/tape combo unit that is otherwise dead ??
    your digikey link to that part didnt work
    anything around the 30 - 360pF is ok it ist critical, if you can get say a 50 - 500pF go for it

    T1 can be just about any NPN transistor a BC546 will probably work happily in there or say a 2N2222 it isnt critical its just a low power audio amplifier.

    D1 ... yeah you use Germanium diodes as detectors in receivers. :)
    the standard one was a OA95 maybe they are still available... I have lots of them in my parts drawers :)
    The 1N34 or 1N60 should be spot on :)

    Dave
     
    Last edited: Mar 22, 2011
  3. eric_x

    eric_x

    5
    0
    Mar 22, 2011
    Thanks a lot!
    I went to Goodwill and found an old GE alarm clock, lo and behold it had a 100-300 pf trimmer in it. Pretty cheap at 4 bucks.

    I found the BC546 amp, so I'll use that.

    Still not finding the exact diode on Digi-key, unless I want 3000 of them. Would this diode work?
    http://www.datasheetcatalog.org/datasheet/philips/1N914_3.pdf
    I noticed the part I needed was listed as high conductance and that diode was too.
     
    Last edited: Mar 23, 2011
  4. davenn

    davenn Moderator

    13,620
    1,882
    Sep 5, 2009
    No, the 1N914 and its mate, the 1N4148 are silicon switching diodes

    ok that GE alarmclock radio must also have a detector diode in it use it :)
    if your not sure which one it is, post a close in sharp/clear pic of the radio section of the cct board and I will see if I can tell you which diode to use :)

    Dave
     
  5. eric_x

    eric_x

    5
    0
    Mar 22, 2011
    Thanks. I looked in the radio and tried to find the part, I thought it was a silicon diode from what I found, but could be wrong. Unfortunately, I just got back from break and left the diode at my house. Luckily, I bought two old radios, so once I get to lab I'll tear it down and see what's inside. It's cheaper to use the $4 radio than order a new part :p

    I was looking at radioshack too because i can drive there, and it looks like they have no germanium diodes. Bummer.
    http://www.radioshack.com/family/in...ype/Diodes&filterName=Type&filterValue=Diodes
    edit: would this work?
    http://www.radioshack.com/product/index.jsp?productId=2062576&CAWELAID=107599028
     
    Last edited: Mar 29, 2011
  6. davenn

    davenn Moderator

    13,620
    1,882
    Sep 5, 2009
    the first link is to silicon rectifier diodes not really suitable

    the second link is to the 914 and 4148 diodes I had already comment on

    as I said if you are not sure I may be able to identify the correct diode in the radio you are pulling parts from :) show me a close in sharp/clear pic of the radio section

    cheers
    Dave
     
  7. eric_x

    eric_x

    5
    0
    Mar 22, 2011
    Alright, finally got the radio open. My camera isn't good at real close range, but the general layout is:
    [​IMG]
    higher res:
    http://s3.postimage.org/4hnfuaufk/P3290006.jpg
    http://s3.postimage.org/4hn0ygpzk/P3290005.jpg
    Two of the glass type diodes say "418" and the other one says "1N4" on the top.
    One of the black diodes looks like it ends in 02 and the other one ends in 9
    Although I am a little confused about which ones are diodes because they are labeled Rxx on the board then have the diode triangle symbol on them.
    Once I desolder them i should be able to read them, but I'll have to go to the lab.
    Thanks

    btw: it's a ge 7-4624a
     
    Last edited: Mar 30, 2011
  8. davenn

    davenn Moderator

    13,620
    1,882
    Sep 5, 2009
    Hi Eric

    forget about the left side of the board its the right side of the board around the general area of the tuning capacitor and that IF chip there may be a detector diode

    ... but am thinking that that function is probably done inside the IF (intermediate frequency) chip. you may need a much older radio prior to intergrated circuit days
    where the detector is discrete components

    Dave
     
  9. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Crystal sets used no external power, relying solely on the energy derived from the aerial. Thus it was essential to use a detecting diode with very low voltage drop (germanium not silicon). In your case, you have a voltage supply which can help. If you turn the diode around and add a resistor from the diode to the 9v supply to get about 0.5V at the diode, then a silicon diode will be on the verge of conduction and so a 1N914 or !N4148 could be used. The resistor will need to be about 15 times R1.
    Happy experimenting.
     
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