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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Mar Sun 04, 2018 5:45 am 
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mcoates wrote:
Well, I didnt have much luck on the ole' home brew. I'm not the greatest solderer, but I make sure nothing is touching that could cause a problem. I'm at wits end. It works like a rock. Something is wrong. Please look at the pictures and see if you guys can help. Maybe "fifties" can compare it with his. Much appreciated!!!
https://photos.google.com/share/AF1QipO ... gycllZLTRR


Hi, Mike, you may have a cold solder joint at the spot circled in the photo. What kind of soldering tool did you use, gun or soldering iron? What wattage?


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Mar Sun 04, 2018 7:19 am 
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Try connecting the output from the earphone jack of a Transistor radio to it's input & ground, volume low, tuned to a station well away from 1310. Use a radio in the same room, tuned to 1310, to see if you can hear it's output, and post back.

Okay, I did that. Murpay's law. At least I could hear something--sounded like it was a million miles away, so something is not connected right. Now to Murphy's Law. I did use the AC adapter and started with 6 volts. Could barely hear it with the aerial antenna wire in back of the receiver set. Then I reached to take the AC adapter from 6 volts to 9 volts. I mistake-fully reversed the polarity and blew the capacitor closest to the little light. Yep, Murphy's law. Well, I have a few 220 uf coming in the mail, so when they arrive, I will replace it and try again. Still, something is wrong because it's losing voltage somewhere. The voices sound so weak. Thanks for your help. I'll keep you posted when the caps come in. This isn't brain surgery. Very simple. It is just acting like brain surgery--a frontal lobotomy.

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Mar Sun 04, 2018 7:25 am 
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OK, so it is alive. I would suggest connecting a small amplifier, such as an LM386 circuit, to the input. That's what I had to do, in order to get decent, clear volume from the source. Also, be sure you are using a ground connection; very important with this transmitter.

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Sun 01, 2018 8:13 pm 
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Mike,

After reading this post I bought one of these and assembled it an hour ago.
I checked your pictures and all components look like they're in the proper place.

Then I saw the bad solder joint, went to tell you about it and saw Macrohenry beat me to it.
It seems your soldering iron isn't hot enough or you're not leaving it on the connection long enough.
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I assembled mine very carefully. When I was done I showed it to my girlfriend, Andrea. She took one glance at it and said, "That blue thing is in the wrong place." DOH!!
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Question for fifties:
The kit came with some extra inductors for the antenna.

The instructions say, "The loading must be away from PCB at least 30cm" and " C01: Never close to the loading and the modulation circuit output coil."

I take this to mean the antenna inductor should be mounted away from the PCB. Which one did you use and how?
I've been thinking of making a small antenna tuner by adding a variable cap in series with it.

Mike (a different Mike)


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Sun 01, 2018 8:35 pm 
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SkyKing wrote:

Question for fifties:
The kit came with some extra inductors for the antenna.

The instructions say, "The loading must be away from PCB at least 30cm" and " C01: Never close to the loading and the modulation circuit output coil."

I take this to mean the antenna inductor should be mounted away from the PCB. Which one did you use and how?
I've been thinking of making a small antenna tuner by adding a variable cap in series with it.

Mike (a different Mike)

I honestly have no clue about that. Mine didn't come with any extra inductors, and works just fine, so you might try it W/O using them, and see if transmission is adequate.

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Mon 02, 2018 2:17 am 
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fifties wrote:
I honestly have no clue about that. Mine didn't come with any extra inductors, and works just fine, so you might try it W/O using them, and see if transmission is adequate.


Mine came with two inductors to put in series with the antenna. 820uH for 655 kHz and 330uH for 1310 kHz.
mcoates, did you get those?

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I'm preparing a small cigar box to mount it. I have a 9 volt wallwart that puts out 600mA


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Mon 02, 2018 6:42 pm 
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I'm not done, but it is working.
The first test was to tune the radio to 1310 and turn on the xmitter with no input. There is a radio station at that frequency. When the transmitter was turned on the station went quiet.

I need a stereo jack, which won't arrive for a few days. I used the laptop for an input, but because I only have a mono jack it is only getting one channel. I needed to turn the volume up pretty high. I'm going to wait on the proper jack before I try adding a little amp to the input.

I haven't experimented with the antenna, nor do I know what the range is yet. In the picture with the laptop, I'm transmitting the Apollo 13 Flight Director's loop to a radio across the room.


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Mon 02, 2018 7:13 pm 
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Nice neatly laid out job there!

You need a resistor summing network to combine stereo channels to feed to mono. I built an LM386 audio amp circuit in order to boost the input, then dialed it down to below distortion level, and it works great. Here's a scan, using battery power (which I later chucked for a wall wart), connected to an MP3 player;


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Mon 02, 2018 7:42 pm 
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Nice Job! I like how it's all self-contained.

The transmitter has two inputs, I assumed they are for left and right channels.
Do they make up the resistor summing network?
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How about something like this? Don't the two resistors on the transmitter input do the same thing?


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Mon 02, 2018 8:53 pm 
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Yes, exactly.
If you notice in my scan, there is a resistor coming from each channel's output (I know they look like they're in series, but they're separate) tied together and going to the input of the amplifier.

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Mon 02, 2018 9:11 pm 
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Ah -ha! You made a separate network for the amplifier. What did you connect the output of the amplifier to? In your picture, the wires are cut to the input to the amp.


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Tue 03, 2018 1:56 am 
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SkyKing, my advice is to use a pair of Edcor WSM15K/15K transformers with the secondaries wired in series.

As I posted earlier that isolates the circuit ground from the audio input ground which is important to keep hum under control.

One of Edcor's stereo to mono matchers might work as well and may be smaller.


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Tue 03, 2018 3:14 am 
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SkyKing wrote:
Ah -ha! You made a separate network for the amplifier. What did you connect the output of the amplifier to? In your picture, the wires are cut to the input to the amp.

Correctamundo. The green wire coming out of the amp is it's output, connected to the left lug (high side) of the volume control. The center VC lug (yellow wire) connects to the point on the PCB where the two input resistors tie together. The right (low side) of the VC connects to ground.

Tube Radio wrote:
SkyKing, my advice is to use a pair of Edcor WSM15K/15K transformers with the secondaries wired in series.

As I posted earlier that isolates the circuit ground from the audio input ground which is important to keep hum under control.

One of Edcor's stereo to mono matchers might work as well and may be smaller.

He doesn't need them. Hum is almost non-existent.

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Tue 03, 2018 4:26 am 
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Maybe so, but if the transmitter is grounded and the source also is grounded it could cause a ground loop which may introduce hum.

Plus if the signal level of his device is low enough to where it won't generate 100% modulation the two transformer secondaries in series will give more voltage without resorting to a preamp circuit.


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Tue 03, 2018 4:29 am 
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fifties wrote:
He doesn't need them. Hum is almost non-existent.


Actually, there was no hum when using a superhet to pick up the signal, but there was some hum when using a regen receiver.

Tube Radio, I think the Edcor transformers are too expensive for this simple project.
I'm going to add the LM386 amplifier to the input. Fifties, do you think there would be a problem if I went from the amplifier output to the 1000 ohm resistors on the xmitter, or should I bypass them like you did?
Since you have the amp there, have you ever tried using a microphone?


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Tue 03, 2018 7:34 am 
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SkyKing wrote:
fifties wrote:
He doesn't need them. Hum is almost non-existent.


Actually, there was no hum when using a superhet to pick up the signal, but there was some hum when using a regen receiver.

Tube Radio, I think the Edcor transformers are too expensive for this simple project.
I'm going to add the LM386 amplifier to the input. Fifties, do you think there would be a problem if I went from the amplifier output to the 1000 ohm resistors on the xmitter, or should I bypass them like you did?
Since you have the amp there, have you ever tried using a microphone?

No harm in experimenting both ways. Try my way first, and if you don't like the results, try the other.

Since I can't sing, I've never tried using a mic... :wink:

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Tue 03, 2018 6:01 pm 
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fifties wrote:
Since I can't sing, I've never tried using a mic... :wink:
Do not feel left out, my friend. Rap was invented for such as you.


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Tue 03, 2018 6:25 pm 
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SkyKing wrote:
fifties wrote:
He doesn't need them. Hum is almost non-existent.


Actually, there was no hum when using a superhet to pick up the signal, but there was some hum when using a regen receiver.

Tube Radio, I think the Edcor transformers are too expensive for this simple project.
I'm going to add the LM386 amplifier to the input. Fifties, do you think there would be a problem if I went from the amplifier output to the 1000 ohm resistors on the xmitter, or should I bypass them like you did?
Since you have the amp there, have you ever tried using a microphone?


If by some chance you do have hum from the transmitter or you find that the signal strength in your radios varies depending on what source is connected you may have to use the transformers.


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Tue 03, 2018 9:08 pm 
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The only time I ever encountered hum, it had nothing to do with a ground loop. It was RF being fed into the house mains through the power supply, and then being subject to cross modulation in the power supply rectifier in just one particular receiver. I added a couple of line bypass capacitors to the transmitter power supply, and that solved the problem.

On the other hand, for those who like to make their circuits as complicated as possible, there's no limit to the permutations that you can incorporate.


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Apr Tue 03, 2018 9:54 pm 
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At least with my transmitter I noticed a reduction in hum when I isolated the audio ground from the circuit. Of course I connect my transmitter to my stereo system versus using a MP3 player or other similar source.

I might try line bypass caps on mine as well to see if that reduces the hum further.


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