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 Post subject: Help With Homebrew Transmitter
PostPosted: Jul Fri 24, 2020 6:09 pm 
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Joined: Apr Thu 09, 2020 7:46 pm
Posts: 59
Location: Fargo ND, 58102
Hello all,

I am trying to clone a short range AM transmitter, partially because I want another and partially because I think it's a good challenge and good way to go for my first build. I'd like to design a device from the ground up some day, but for now reverse engineering / cloning is as advanced as I can manage. I got the transmitter together but as far as I can tell my oscillator is not running and therefore not generating a carrier for the audio signal. When I reviewed my connections I discovered that my oscillator coil is hooked up wrong.

In the original transmitter the oscillator coil is a little metal can, like IF cans for PC boards. The can has four pins, one with a red wire, one with a white wire, and one that is soldered to the metal can itself / ground. The fourth pin has no connections. The pin with the red wire connects to variable capacitor and the bypass cap / resistor for the grid of the pentagrid converter (12BE6). The white wire connects to the cathode / grid five of the 12BE6. Between the red and white wires there is 3 ohm, between the red wire and ground there is 14 ohms, and between the white wire and ground there is 4.4 ohms. There is no continuity between any pin or ground to the fourth unused pin. (I'll attach pictures to make this easier to understand.) NOTE: In the picture it looks like pin three is soldered to the terminal where the red wire and the variable cap connect. This is just the way I bent the tab, they are not connected.

So finally the question, how do I hook up my coil? I have a simple shielded inductor with axial leads that I'd like to use, but with three connections to make and only two wires I'm not sure how to hook it up. I also have one of those "universal" oscillator coils from AES that I could use. Any guidance you guys could provide on how to hook the oscillator up would be much appreciated.

Thanks in advance!


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Last edited by Fordman on Jul Fri 24, 2020 9:11 pm, edited 1 time in total.
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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Fri 24, 2020 6:54 pm 
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Joined: Mar Sat 08, 2008 3:21 am
Posts: 619
Location: hillsborough, nc
Why do you think you can replace a three terminal coil with a two terminal coil?
Can you repost the coil datasheet with all data visible?
WL


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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Fri 24, 2020 6:57 pm 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 10186
Location: Santa Rosa, CA
A schematic diagram sure would be helpful.

Rich


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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Fri 24, 2020 8:26 pm 
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Joined: Apr Thu 09, 2020 7:46 pm
Posts: 59
Location: Fargo ND, 58102
infomet wrote:
Why do you think you can replace a three terminal coil with a two terminal coil?
Can you repost the coil datasheet with all data visible?
WL


Well, I don't really. I somehow didn't notice the third connection until I started troubleshooting. If it can be made to work I'd like to use it, but I'm not dead set on forcing the wrong part into the role. I don't have a datasheet for the original coil, if I did I would have gotten one that had the same characteristics. I haven't been able to identify it, only part in the transmitter I never figured out.

Rich, W3HWJ wrote:
A schematic diagram sure would be helpful.

Ask and ye shall receive! Sorry about the weird layout, I had to draw it from scratch and I made it as a half schematic and half wiring diagram. Hope it's still helpful. I went back and added component values to it. The coil in question is labeled L1 on the schematic. The representation of L1 is probably not correct, but the three connections are. The variable capacitor value range is ~15 pF - 384 pF.


Attachments:
Transmitter_Schematic.pdf [52.62 KiB]
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Transmitter_Schematic.jpg
Transmitter_Schematic.jpg [ 258.77 KiB | Viewed 1105 times ]
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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Fri 24, 2020 9:06 pm 
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Joined: Mar Sat 08, 2008 3:21 am
Posts: 619
Location: hillsborough, nc
Very nice.
Try
3-G
2-K
1-P

WL


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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Fri 24, 2020 10:14 pm 
Member

Joined: Apr Thu 09, 2020 7:46 pm
Posts: 59
Location: Fargo ND, 58102
False alarm, problem not solved. Please read next post

Update:

I tried hooking the universal oscillator up the way that infomet suggested and the transmitter actually started transmitting! I was able to pickup the carrier on my RCA 110K2, so I tried feeding it some music from the stereo input. I could get the music to play but it still wasn't right. There was lots of distortion and the tuning capacitor had no effect. I played around with it some more by moving test leads around to different terminals and moving the ferrite core in and out and never got much improvement.

Just for kicks I hooked up the two terminal coil and grounded out one side while both test leads were clipped on. When I grounded the side connected to the tuning cap I couldn't get music anymore, but I could now tune a squeal with the tuning cap. I flipped it around so that I grounded out the side connected to the cathode of 12BE6 and all of sudden I had nice, clear, and tuneable music!

One weird observation though, I can tune the music in on the 110K2 in three different places. What I mean by that is, if I bring the dial of the radio to say, 800 kHz, I can tune the signal in three times with the entire range of the tuning cap in the transmitter. Am I just experiencing frequency multiples / images with the radio due to the large frequency range of the transmitter and the antennas being so close?

Anyway my plan for now is to connect the two terminal coil between the tuning cap and the cathode of 12BE6, and then connect the cathode to ground. I'm assuming I'll need a resistor here to bias the cathode correct? What resistance value(s) should I use for that?


Last edited by Fordman on Jul Sat 25, 2020 1:27 am, edited 1 time in total.

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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Sat 25, 2020 1:26 am 
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Joined: Apr Thu 09, 2020 7:46 pm
Posts: 59
Location: Fargo ND, 58102
Update 2:

Okay now this is getting weird. I installed the coil as I said I planned to and lost the performance I had before. Volume control became ineffective, tuning cap didn't do anything, and there was just a blanket of extremely distorted audio across the entire band. I pulled it apart again and tried to replicate what I got before and have not been able to. No idea what kind of magic I had a few hours ago but I could sure use some now.

infomet, I just want to clarify, when you said:
infomet wrote:
Try
3-G
2-K
1-P


You want me to hook terminal 1 of the universal coil to the plate of 12BE6, terminal 2 to the cathode, and terminal 3 to the grid right?

Anyone else have ideas?


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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Sat 25, 2020 1:32 am 
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Joined: Mar Sat 08, 2008 3:21 am
Posts: 619
Location: hillsborough, nc
Yes


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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Sat 25, 2020 1:48 am 
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Joined: Apr Thu 09, 2020 7:46 pm
Posts: 59
Location: Fargo ND, 58102
Glad I asked, I had been reading G as ground, not grid. I did not get a signal hooking it up as you described. No carrier or garbled audio. Definitely seems like the inductor has to get to ground somehow to have a signal.


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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Sat 25, 2020 3:40 am 
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Joined: Mar Sat 08, 2008 3:21 am
Posts: 619
Location: hillsborough, nc
OOPS, sorry, moving too fast, wife waiting to start movie...
Yes, G=gnd, so 3 to ground, 2 to cathode, and 1 to G1, which I called the plate.
It would be the plate in a triode oscillator.
Sorry for wasted time.
Just look at the configuration: Tuned circuit from plate to ground, with a feedback tap for the cathode, part way up from ground, like a tickler.
Looking at your circuit, I don't like the gridleak across the blocking capacitor. It's trying to put RF on the grid and preventing development of DC bias.
I suggest moving the low end of it to ground.
Here's a place to read about mixers: https://www.rfcafe.com/references/qst/p ... 41-qst.htm
WL


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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Sat 25, 2020 7:01 am 
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Joined: Mar Thu 21, 2019 3:53 pm
Posts: 171
I have redrawn your schematic so it is easier to understand it.
I have build an AM transmitter but in a different configuration. It consists of a VFO, RF power stage and an audio amplifier which modulates the plate of the RF power stage. The VFO coil in my transmitter is a homebrew unit.


Attachments:
Transmitter_Schematic.jpg
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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Sat 25, 2020 3:15 pm 
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Joined: Mar Sat 08, 2008 3:21 am
Posts: 619
Location: hillsborough, nc
Nice redraw!!

I'd love to see the mixer output on a scope!

WL


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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Sat 25, 2020 3:17 pm 
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Joined: Mar Sat 08, 2008 3:21 am
Posts: 619
Location: hillsborough, nc
Biasing of the utput stage will be critical, since it has to be linear.
WL


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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Sat 25, 2020 6:48 pm 
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Joined: Apr Thu 09, 2020 7:46 pm
Posts: 59
Location: Fargo ND, 58102
Thanks for the redraw, I know that schematics are usually laid out to follow the signal path so it's good to have a version that reflects that.

I tried hooking up the coil as suggested, and now we've got life! I'll get a video up later today, but it's running like a champ! I did change up the grid leak resistor as suggested and sent it to ground instead. Thanks for the link too, the whole point of this for me is to learn, so additional resources are always welcome! I'd show you the mixer output if I had a scope. Been thinking about getting one, so if I do I'll come back and update the thread.

For now here's a picture of the finished product. I'll have to add the pilot light to the schematic too, but it's just a 12 volt LED running off the filament winding of the transformer.

Thanks again for all the help!


Attachments:
IMG_20200725_110107.jpg
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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Sat 25, 2020 8:30 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3900
Location: Lexington, KY USA
Nice looking project!

Don't forget to use with an isolation transformer. Else keep everything, including any signal source, inside an insulated enclosure.

They never did make any tubes with double insulation on the heaters.

The 12BE6 is not the best modulator tube, due to the remote cutoff grid. This has been all discussed at length in other threads.

Ted


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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Sat 25, 2020 9:10 pm 
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Joined: Mar Thu 21, 2019 3:53 pm
Posts: 171
Nice job Fordman, you may even put it in the living room without having the wife shouting what is this piece of junk doing in my living room :lol:
By the way, where did you get the six side box?


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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Sat 25, 2020 10:02 pm 
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Joined: Mar Sat 08, 2008 3:21 am
Posts: 619
Location: hillsborough, nc
Great!


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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Sun 26, 2020 2:53 am 
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Joined: Apr Thu 09, 2020 7:46 pm
Posts: 59
Location: Fargo ND, 58102
Usually Lurking wrote:
Nice looking project!

Don't forget to use with an isolation transformer. Else keep everything, including any signal source, inside an insulated enclosure.

They never did make any tubes with double insulation on the heaters.

The 12BE6 is not the best modulator tube, due to the remote cutoff grid. This has been all discussed at length in other threads.

Ted


Good points Ted. Like I said in the OP all I can manage for now is cloning the existing design, but I have a few design changes in mind to implement eventually. I'd like to go all tubes and swap the 1N4007 diode for a rectifier. I want to go with all 12 volt or 6 volt heaters so I can run them in parallel off a transformer to eliminate the series filaments on line power. Getting the heaters on a transformer is the first priority. I'll do some reading on a better modulator tube, but do you have a suggestion? Thanks for the input!

JGTubeamps wrote:
Nice job Fordman, you may even put it in the living room without having the wife shouting what is this piece of junk doing in my living room :lol:
By the way, where did you get the six side box?


Thank you! I may be a complete noob with tube type electronics, but I can at least make my experiment look clean :lol:. I got the octagonal box from Antique Electronic Supply. Here's the link: https://www.tubesandmore.com/products/c ... 38-octagon
If you're considering using it be aware that's it's pretty small. Even the circuit for this little transmitter is PACKED into it. When I get around to doing a redesigned version I probably won't use this box.


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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Sun 26, 2020 3:44 am 
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Joined: Mar Thu 21, 2019 3:53 pm
Posts: 171
Thank you for the link and perhaps you could provide the results of its operation regarding the transmitting range, quality of the signal and transmitted harmonics?


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 Post subject: Re: Help With Homebrew Transmitter
PostPosted: Jul Sun 26, 2020 5:07 am 
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Joined: Apr Thu 09, 2020 7:46 pm
Posts: 59
Location: Fargo ND, 58102
JGTubeamps wrote:
Thank you for the link and perhaps you could provide the results of its operation regarding the transmitting range, quality of the signal and transmitted harmonics?


No problem. Right now the transmitter transmits over about 650 kHz to above the broadcast band. You can shift this by playing with the values of the variable cap and the oscillator coil. The coil I installed today is adjustable, so I'll probably adjust that a bit to see if I can shift the range down a bit to cover the entire broadcast band.

As for the range I'd say it tops out at about 15-20 feet, depending on the antenna orientation. If you have a radio room it should be just fine for the sets in there. If you're trying to cover your whole house, this won't cut it unless you move it around or build a few of them. You can get the signal way out past 20 feet, but it's pretty weak.

I have no issues with the quality, as long as the volume control on the transmitter is kept fairly low. Adjust the volume too high and it starts to distort the highs and lows. Let the amplifier in the radio do the work. It's not Hi-Fi by any means and I'm sure the audiophools would say it's garbage, but for casual listening I have no issues at all. Here's a short video, you can get an idea of how it sounds here: https://youtu.be/EduFlJOPmsI

I don't really know how to measure transmitted harmonics. I know that I pick the signal up at several places on the dial, but I think that has more to do with the really strong signal overpowering weaker ones because all the places I pick it up are normally where a weak station would be.

If you are interested in building one for yourself send me a PM and I'll share my parts list with you, as well as the updated schematic which includes the pilot light and the rerouted resistor discussed earlier in the thread. The parts list has everything I used, the quantities needed, and links to where you can buy it, so it really has everything you need minus tools and hardware. There are better transmitter designs out there, but if you want to give it a shot or are just curious I'm happy to share. That goes for anyone else reading too.


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