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Eq. Parts for an AM radio project

eric_x

Mar 22, 2011
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Mar 22, 2011
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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
op-amp-rad.JPG


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
 

davenn

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Sep 5, 2009
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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
 
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eric_x

Mar 22, 2011
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Mar 22, 2011
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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.
 
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davenn

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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
 

eric_x

Mar 22, 2011
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Mar 22, 2011
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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

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
 
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davenn

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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
 

eric_x

Mar 22, 2011
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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

Alright, finally got the radio open. My camera isn't good at real close range, but the general layout is:
p3290001b.jpg

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
 
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davenn

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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
 

duke37

Jan 9, 2011
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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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