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Perhaps the least expensive AM transmitter yet!

Our good friends at Sayal Hobbies still have a few of theses pretty nice little AM transmitter kits that might be the ultimate low cost entry level transmitter kit.  The AM Microphone Broadcast Station Kit - C6752 is a small but efficient, experimental AM broadcast station using a high-gain electret microphone, two transistors and a RCA jack so that you can also connect to the earphone output of your MP3 or CD player.
     The reviewer over at the Christian Radio Project  says..." it is very impressive for its cost and simplicity of construction and may potentially be a good candidate for use in a low-cost neighborhood AM station.  It truly has all the makings of an effective and yet very simple transmitter..."
     This review goes on to note that the tuning capacitor that comes with the kit can make adjusting the frequency frustrating but after all, this is a fully functioning transmitter for under $10.00
     An easy way to cope with the supplied tuning capacitor is to use a long, non-conductive tuning stick (rod with a chisel tip at one end that will fit in the tuning slot of the capacitor) made of plastic, wood or similar material.  The tuning stick should be long to keep your hand from getting too close and detuning the circuit.
     This is a very basic kit and includes everything you need, clear instructions, a minimum of parts and simple assembly requirements.
     MODS - If you would like to get a better variable capacitor, you will need to replace the 3.5 - 13 pf trimmer that comes in the kit with a better unit from a supplier like Dan's Small Parts and Kits or your local source. Connecting to a real RF ground, probably best to connect to the negative battery terminal, will always boost range. A 12-18 volt power supply could be used in place of the battery.  Adjusting (by sliding the iron powder rod in and out of the wire-wrapped cardboard sleeve) the loading coil connected to the antenna would also kick out the range a bit.
     Soldering, kit building, an AM transmitter and lots of hacking fun await!

12 comments:

jaywfitzgerald said...

Will it work with the C6444 20w amplifier?

Blog Author said...

The $17.50 C6444 20W amplifier could be used as an audio source but the full power of that amplifier would not be needed. You might need to adapt or attenuate the amplifier output for best results

Jay said...

I found out earlier that I need a RF amplifier and not a audio amplifier. Anyway, just opened up the kit and, well,the diagram of where the parts go and how they look on the schematic is a little confusing. Yes, I'm a novice. What I want to know is...I'm thinking of not using the mic and replacing the RCA connector with whatever size the jack is for Walkman headphones. That way I can use RCA out of my mixer into 1.8mm or whatever that is. I also wanto to have 13V as my power supply. So, in addition to replacing the pF trimmer what else do I need to do? Also, got any good ideas for an antenna?

Blog Author said...

"in addition to replacing the pF trimmer what else do I need to do?"

You might consider using a couple of 680 ohm resistors to combine your stereo chanels into a mono feed

"got any good ideas for an antenna?"

Yes, check this post on the blog

Where to get, or how to build, an antenna

Jay said...

Is there anyway to change this into a shortwave transmitter?

Blog Author said...

Yes, shortwave (at the low end of the SW dial) is no problem for this kit - the only change needed is to wind a new coil using a standard formula described below.

I'm not sure the circuit would oscillate at the higher end of the SW spectrum but the two spots on the SW dial most users are interested in would be the so-called "free radio" band in the 6 MHz to 7 MHz region and the Part 15 frequency of 13.556 MHz.

Calculating the new coil is a three-part process, calculating the new inductance, determining the coil construction and connecting the new coil to the kit.

Part 1: The formula for determining the new inductance is "inductance times capacitance equals a constant divided by the desired frequency squared." In math teacher language that's L*C=K/(f)2 where K=25330.3 and f = frequency in megaHertz (MHz) L = inductance in microHenries (uH) and C = capacitance in picoFarads (pF)

In this transmitter the capacitor we care about is C8, the 3.5pF to 13 pF variable cap. We will use the center position for calculation so the unit will be adjustable.

The free radio frequency is around 6.9 MHz so the math for the new coil would be

L*8pF=25330.3/(6.9*6.9) or

L*8=532.04 and using our high school algebra we solve for L and divide both sides by 8 and get L=532.04/8 or 66.5uH

For the Part 15 frequency of 13.556 the formula is L*8=25330.3/(13.556*13.556) or 17.2 uH.

Part 2: To determine the size, shape and number of turns on the new coil, use the Coil Calculators here:
http://eweb.chemeng.ed.ac.uk/jack/radio/software/newcoil3.html

and here:
http://www.vwlowen.co.uk/java/coil.htm

Part 3: After constructing the coil, add a tap at 25 percent of total turns from the bottom end of the coil. The tap on the coil connects to the "black" / "red" terminals (these pads share a common track on the board). The bottom end of the coil connects to the terminal labled green and the top end where 75 percent of the turns are connects to the terminal labeled pink on the instruction sheet.

T. Zombie said...

Do I need to substitute a resistor in place of the microphone when I remove it (it measures about 1.5K resistance)?

Blog Author said...

A quick look at the schematic here: http://tinyurl.com/4chjqs6

indicates Q1 is already biased by R1, R2 and R3 so I'd try it without the resistor first.

In fact, from the schematic, there doesn't appear to be a need for a jumper wire either.

I'd start by just leaving that part of the circuit open by simply not soldering anything at all to the mic pads.

Dave said...

Just wondering if this kit could be adapted for use as an inexpensive rf signal generator.
You say it can oscilate at up to
13mhz which would be more than adequate.
What about frequencies in the LF range of about 50khz to 100khz and up to the MW range?
Do you think it would oscillate that low given the appropriate L/C combinations.

Blog Author said...

Sure, just use the calculations shown above to choose a frequency and modulate the transmitter with any tone you like.
Another blog has a simple signal generator plan here:
http://tinyurl.com/simple-sig-gen

another here:
http://tinyurl.com/simple-sig-gen2

or buy one for under $10.00 here:
http://tinyurl.com/Buy-Signal-Generator

Jay said...

I'm having problems getting my C6752 to work. The copper wire from the ferrite bar wasn't marked properly. Is that where I screwed up or did I just assemble it wrong. Need help. PLEASE!

Blog Author said...

Wrong is a strong term :-)

Perhaps just need to do a bit of checking and see what's where. The board is pretty easy to work with so check for solder bridges and dry pads, then do some basic trouble shooting:

1. Make sure all the right components are in the right places. I use a Volt Ohm meter to check resistors against the color code because the colors are not as distinct as they used to be.

2. Make sure D1, Q1 & Q2 and C1, C2, C4 & C5 have the right pins in the right holes.

3. Use a Volt Ohm meter in continuity mode to check the leads coming off the coil. The coil in the kit is actually two coils wound on the same tube. The leads identified as red and green are the leads for the small coil, black and pink leads are the larger coil. THat should help you confirm the pairs.

4. This circuit is a bit fussy to tune. After you verify everything, try again and remember that a fairly tiny movement in C8 can take you all the way through the AM broadcast band.

All part of the fun, really!

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