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 Post subject: Minimal Build AM Transmitter, Muntzed
PostPosted: May Thu 24, 2018 3:25 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 7984
Location: Beautiful Downtown Burbank CA
After hearing someone wanted to buy an AM transmitter for his antique radios and was going to put it NEXT to the radio, I made this "Muntzed" version of my Minimal build `386.

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A very simple AM transmitter that uses one mp3 player, a 386 amp module, an oscillator and a couple 9V batteries in parallel. NO COIL OR TUNER. It's range is about 5'. Set it on top or in back of an AM radio for clear loud hum free music. The MP3 player comes with a remote with 5 equalizer settings, vol etc.

Image

I noticed this transmitter works really well on radios with built-in antennas like AA5s but minimal with radios that need external antennas. Very directional too. My 568 Detrola with metal cabinet needs to be very close to the the antenna wire.




Only thing I did was change the `386 module's output e-cap to an inductor. Reading about the `386 and how they recommended a ferrite bead in series with the output, I tried it and it worked. Someone tell me if there is something better.

Image

I had room so it's powered by 2, 9V batteries in parallel.

BTW, setting this transmitter next to a Tecsun AN-200 loop antenna extends the range to 15'

MP3 player https://www.ebay.com/itm/Mini-5V-7-12V- ... 2749.l2649

`386 https://www.ebay.com/itm/3V-12V-LM386-S ... 2749.l2649


Last edited by Mike Toon on May Sat 26, 2018 12:24 am, edited 4 times in total.

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 Post subject: Re: Minimal Build AM Transmitter, Muntzed
PostPosted: May Thu 24, 2018 3:28 am 
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Location: SoCal, 91387
Very good, Mike! :D

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 Post subject: Re: Minimal Build AM Transmitter, Muntzed
PostPosted: May Thu 24, 2018 3:47 am 
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Location: Yukon, OK USA
Nice! Learned another new word; Muntzing. I had heard the description but not the word.

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 Post subject: Re: Minimal Build AM Transmitter, Muntzed
PostPosted: May Thu 24, 2018 3:53 pm 
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Joined: Nov Wed 27, 2013 5:59 am
Posts: 580
Location: Metzger Oregon
Here's a little more history about the term:

https://en.wikipedia.org/wiki/Madman_Muntz


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 Post subject: Re: Minimal Build AM Transmitter, Muntzed
PostPosted: May Thu 24, 2018 5:11 pm 
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Joined: Sep Tue 30, 2014 6:08 am
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Location: The Old Dominion VA 23518
Mike: Nice build.

Others - If you mention Muntz, let Bob Pease (RIP) do the 'splainin…….

http://www.electronicdesign.com/boards/ ... uff-anyhow

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Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


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 Post subject: Re: Minimal Build AM Transmitter, Muntzed
PostPosted: Jun Sun 03, 2018 1:39 am 
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Location: Vancouver Island, Canada
Thanks, Mike. 5 feet would be plenty for all but one of my radios. 8)

And I thought a Muntz was just a cheap TV. Well, it is, but there is much more to the story. 8)

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 Post subject: Re: Minimal Build AM Transmitter, Muntzed
PostPosted: Jun Fri 08, 2018 8:09 pm 
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Joined: May Wed 06, 2009 7:01 pm
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Mike,

Did you swap the input from the non-inverting pin to the inverting pin (swap pins 2 and 3 on the chip?)? I used a similar scheme with a slightly larger 386 module that included connectors for voltage input and audio output. I solder-bridged the output cap, swapped the non-inverting feed from the volume control center leg to the inverting and re-used a choke from the circuit board of a burned-out CFL bulb. I also tuned the output with a loopstick antenna from a dead AC-DC radio and a small variable cap. The result is a simple AM transmitter like the one popularized on this forum but less work and cost. ($2.37 for the chip plus first class shipping from Digikey and $1.53 for the 386 module). That circuit which I believe originated with forum member 35Z5 uses the inverting input for good reasons as you can verify with a scope or field strength meter. Just a few seconds with a Dremel tool to cut the traces and swap the pins was all it took.

Every CFL bulb I've opened has a little cubic choke in it that seems to work fine in the line between the 386 and the chip. The loopstick rod or bar antenna from a dead AC-DC or transistor radio and the compression trimmer cap are usually plentiful in the junk box. I also ordered a 1.2288 MHz chip from Digikey.

The Asian MP3 player circuit board I used is a bit larger than the one in your setup but has some other functions. Experimenting is part of the fun and Muntz himself would have appreciated the low cost!

-- Rich


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 Post subject: Re: Minimal Build AM Transmitter, Muntzed
PostPosted: Jun Fri 08, 2018 10:05 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Beautiful Downtown Burbank CA
I did not swap inputs. I understand there is two ways to connect the `386. I used an off the shelf Chinese module with only output cap changed to an inductor. MP3 player with both left and right channels tied together.

My latest version has a greater range because I left the antenna/ground wires long, separated and taped inside the box lid. It is very directional and out performs both previous versions powered by batteries!

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On an earlier version there was only an inch of solder trace acting as an antenna. I can't believe that one inch was all that was radiating a signal. It was directional also. How is this possible?
Image

This loop added 35 feet to transmit distance!
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Things not needed in this Muntzed version.
Ant coil
Var cap
Ant socket
Power socket
Oscillator socket

This is a self contained AM transmitter/MP3 player using a 32 gig TF memory card AND/OR USB thumb drive, selectable by the remote control. It is dead quiet hum free.

I encourage breadboarding to appreciate the performance.

Image


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 Post subject: Re: Minimal Build AM Transmitter, Muntzed
PostPosted: Jun Sat 09, 2018 12:20 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 29504
Location: SoCal, 91387
Great work, Mike! Fascinating. I've been meaning to make one myself, whenever I can get "roundtuit"... :wink:

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