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 Post subject: A Great Homebrew Transmitter
PostPosted: Feb Thu 15, 2018 1:07 pm 
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I'm looking for a transmitter that has FULL legal range. I cannot afford to buy one as they are very expensive. I did buy one on ebay (new) recently, but I couldn't take it out of the room without losing signal. So, it's cheaper to build one myself, but I'm limited in reading of schematics. For instance, I know the symbols for coils, resistors, capacitors, transformers because I have repaired about a dozen radios. So, I want the easiest schematic to read for a transmitter with full legal transmitter range. Should I purchase a wooden breadboard? I will have questions. Thanks, guys!!!

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Thu 15, 2018 2:40 pm 
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Location: Roanoke, VA
A major problem may be that range of an amplitude modulated transmitter depends largely upon modulation characteristics, and the vast majority of homebrew transmitters fare poorly in that regard.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Thu 15, 2018 4:28 pm 
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Location: Metzger Oregon
Have you looked at the 6GY6 based transmitter over on the homebrew section of this site? I'm with you in that I know a cap from a resistor but don't know RF design - but there are several people there willing to help, I've even received parts from other forum members to get me going.

To Dale's point, I believe this design is fairly good in terms of modulation characteristic, but maybe others could better talk to that better than I could.


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Thu 15, 2018 4:33 pm 
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I did take a look at that home-brew, but unfortunately, the pictures were removed, which were a large part of the learning. http://www.antiqueradios.com/forums/vie ... 2&t=221704

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Thu 15, 2018 5:10 pm 
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If you can't access the pictures I'll move them and revise the links...

viewtopic.php?t=68236

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Thu 15, 2018 8:45 pm 
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I have one of these, and it works well;
https://www.ebay.com/itm/DIY-655kHz-131 ... Swk6pZfyS3

Cheap and easy...

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Thu 15, 2018 9:40 pm 
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35Z5 <---I can see those pictures. Thank you.

Fifties or whoever can help:

Concerning https://www.ebay.com/itm/DIY-655kHz-131 ... Swk6pZfyS3

I'm having a lot of trouble with terminology. Can you or anyone help on these points?

(1) Frequency : 655kHz(Base), 1310kHz(Harmonic), FIXED <--I assume this runs at 655 khz only. I don't understand "harmonic."

(2) Input Impedance : More than 10k ohm <---Will this work throughout the house, or just in the room with the output radio?

(3) Ground/Power earth is indispensable <----Must ground the set?

(4) Power Source : DC 6-13V, 9mA/12V <---How can you run at 12 volts if the source initially calls for 6 volts. Won't that burn the set up?

(5) Circuit : Tr(C1815) x 2 <---Don't understand.

Thanks to all !!!

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Thu 15, 2018 10:06 pm 
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mcoates wrote:
35Z5 <---I can see those pictures. Thank you.

Fifties or whoever can help:

Concerning https://www.ebay.com/itm/DIY-655kHz-131 ... Swk6pZfyS3

I'm having a lot of trouble with terminology. Can you or anyone help on these points?

(1) Frequency : 655kHz(Base), 1310kHz(Harmonic), FIXED <--I assume this runs at 655 khz only. I don't understand "harmonic."

(2) Input Impedance : More than 10k ohm <---Will this work throughout the house, or just in the room with the output radio?

(3) Ground/Power earth is indispensable <----Must ground the set?

(4) Power Source : DC 6-13V, 9mA/12V <---How can you run at 12 volts if the source initially calls for 6 volts. Won't that burn the set up?

(5) Circuit : Tr(C1815) x 2 <---Don't understand.

Thanks to all !!!

Sure old buddy.
(1) The receivable signal is at the second harmonic (655 x 2 = 1310)
(2) Don't worry about the minutiae; I've had it working with an MP3 player as well as a radio receiver, for the input source.
(3) Yes, but in my case, I've simply connected a 15 foot long lead to the keeper screw of an AC outlet cover plate, and the range in certain directions is over 100 feet. This won't radiate signal equally in all directions, but I can receive it everywhere inside the house.
(4) I've found that a wall wart putting out about 8.5 volts was the "sweet spot" for best modulation.
(5) That would refer to the two Transistor part numbers.

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Thu 15, 2018 10:11 pm 
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Thanks, fifties. For that price, I can't help but give it a try :D

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Thu 15, 2018 10:27 pm 
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It's very easy to assemble, and I'm sure you'll be pleased with it. Experiment with different wall warts for the P/S, but figure between 8 - 9 volts is the best range. Please post your results.

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Thu 15, 2018 11:25 pm 
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I certainly will. One more question--It looks like it has a plastic or metal container, but either they do not show the top, or it has none. Which is it. Thanks again!!!

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Fri 16, 2018 12:31 am 
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He used to sell a black metal chassis to mount it in, which I also bought, but it doesn't appear that he offers it now. You would just be getting the PCB and components. Here's a thread I did a few years ago on mine;
viewtopic.php?f=12&t=280783&p=2345984#p2345984

Edit; here's the PCB and the metal chassis;
https://www.ebay.com/itm/DIY-655kHz-131 ... 0005.m1851

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Fri 16, 2018 12:53 am 
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classicelectronicsguy wrote:
I believe this design is fairly good in terms of modulation characteristic ...

It should be significantly better than most homebrew designs because it is screen modulated.

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http://plymouthcolony.net/starcity/radios/


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Fri 16, 2018 3:08 am 
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Location: Saskatoon
Dale H. Cook wrote:
classicelectronicsguy wrote:
I believe this design is fairly good in terms of modulation characteristic ...

It should be significantly better than most homebrew designs because it is screen modulated.

If you're referring to the 6GY6 transmitter, it's actually suppressor modulated. The 6GY6 is a dual control pentode.
Most of the better performing transmitters designed and built on this forum are based on dual control pentodes, such as the 6888, 6GY6 etc. They give excellent modulation, whereas the old pentagrid converter designs are generally pretty awful.


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Sun 18, 2018 8:32 pm 
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Location: pensacola fl
Hi aal.The coverage area depends on several variables. If you take the part15 rules in the part 219b section then the efficiency of the final is important if you want maximum coverage area. That means the complexity of the transmitter can go up. Class C final high level modulated is a good compromise on that front. Other schemes like Doherty and class E or F rf funals can get tricky to less experienced builders/operators. The suppressor grid modulated transmitters here work well but can not fully modulate linearly and would gain from adding negative feedback to allow more modulation depth. Continental did this with screen grid modulation in their 314-315-316 models. The 315 is the transmitter harris corp. used as a benchmark to compare their digital dx series boxes with. The efficiency of those continental 315 boxes was about 33 percent. compared to 65 to 70 percent to a high level modulated rig of the same era like a harris mw5 that used a high efficiency pdm modulator to save power and offer a higher audio bandwidth without a huge and expensive modulation transformer and modulation reactor. Hi level modulation can be achieved inexpensively at our power levels so a pdm is not needed and would complicate the design. Sstran in their amt5000 used a series high level modulator and they used a class E final. It is efficient but tuning is critical to get it that efficient and it is very sensitive to antenna effects on this and so can need retuning often. This is why I aid to use class C as it is more tolerant with load changes as far as modulation characteristics go. To sum up if the dual control pentode versions do not give good enough results and you wish to stay inside of part15.219 then go up to the next step. Hope this helps.


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Sun 18, 2018 10:12 pm 
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Location: Phoenix, Arizona
Unfortunately AM radio has gone the way of the 6 volt car. My 6 volt 54 Packard can only pick up one AM worth listening to daytime only.

The good part about this is it leaves a lot of spectrum to broadcast to your radio collection.

I use a 6888 tube modulator with a 6C4 oscillator and a 6V6 PA and an audio amplifier stage and it sounds quite decent on a Mcmurdo Silver Masterpiece V console radio.

I drive it with an FM tuner that way I get their processing.

Stay below 1 MHz (megacycles for us old guys) as the early radios such as AK breadboards don't tune much higher.

I have about 10' of transmit antenna laying on the floor and ground is connected to the house ground wiring.

If your house doesn't have modern grounded wiring prepare to fight getting loud hum and buzz out of the transmitter signal

The FCC part 15 rules require you to construct the transmitter and the lawful power is so low you have to be about a foot away.

There was a guy who sold complete units and he got busted and fined by the FCC.

If you can find an old talking house transmitter.

The one tube pentagrid converter transmitter has way too much FM'ing to stand to listen to it on a good radio.

Jim


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Feb Mon 26, 2018 3:01 pm 
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I have built the 6GY6 transmitter and have been well pleased with it over the last few years.

If you're interested in trying it or just want to look at the circuit I can email you a schematic.

Grounding a transmitter is a double edged sword as some radios hum less with the transmitter grounded and some hum less with the transmitter ungrounded.

Also use a 1:1 audio transformer to isplate the transmitter's circuit ground from the audio input which will eliminate any effect the audio cable and connected device has on the transmitter's operation. The transformer should be mounted in or on the case the transmitter is in.

Concerning the transmitetr add a VU meter circuit which can be in an external box that goes between the audio source and transmitter. If the transmitter has an audio level control just remove the control from the transmitetr and install a 10K audio taper control in the VU meter box on the input.

You set it up by playing music and using a scope to view the modulated signal. A trimpot is used to set the VU meter such that it just does read in the red when the audio peaks which is identified by brighter sections of the trace at the center of the scope screen.

That way you have a visual indicator of the modulation level ignoring the numbers on the scale only paying attention to keep the needle out of the red area.

The 6GY6 schematic shows a VU meter circuit.

Keep in mind it has to be an actual VU meter such as what some reel to reel tape machines have as the "VU meter" a lot of lower end devices have is basically a standard meter movement.


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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Mar Fri 02, 2018 3:05 pm 
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Sounds like a winner. Definitely worth trying. Thanks for posting!!

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Mar Sun 04, 2018 3:37 am 
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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

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 Post subject: Re: A Great Homebrew Transmitter
PostPosted: Mar Sun 04, 2018 5:17 am 
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Mike,
it looks like you wired it OK. What voltage are you using?

1. I would recommend starting with a nine volt wall wart.

2. This circuit DEFINITELY needs a ground, as well as an antenna lead. I use the retaining screw on an AC outlet plate.

3. 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.

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