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 Post subject: AM transmitter mostly alive: FIXED! FINISHED!
PostPosted: Feb Fri 16, 2018 1:01 pm 
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Location: Leon Guanajuato, Mexico
Hello,

I just built the transmitter on this website. Though I have some issues.

I used the improved version

http://www.techlib.com/electronics/amxmit.htm

I also used this other website for help

http://www.geojohn.org/Radios/MyRadios/ ... MXmtr.html

Both are the same except geojohn's website provides information on the coil. The first link uses a pot core to wind it's output tank coil. I opted for using a T50-SF53 toroid core with a permeability of 55, took some research to find information on this core. This is supposed to be wound for 280uh. I calculated that these transmitters cores are tapped at around 4uh. I confirmed my calculations by punching in the info for the two kinds of cores talked about on the geojohn website (T50-1 and FT37-43) to find I was getting a close amount of turns when calculating for 280uH. T50-1 gave me 172 turns tapped 20 turns, maybe not so close on the overall turns (he used 145) but the tap is correct. FT37-43 gave me 28 turns he also used exactly 28 loops. The tap also matches at 2-3 turns. For my core I got 104 turns with the tap at 12 turns. In the end due to an error I added a few extra turns leaving it at 15 turns. Figure it's not such a big difference.

The transmitter uses the tuned oscillator as shown on the website. This uses a red coil from an AM radio. It would not oscillate at the beginning until I changed the 33pf capacitor settling on 68pf. This is because these red coils can come in different configurations mostly changing the inductance. Some are 140uH and others 270uH. Mine is 140uH. Now it works well from 820Khz to 1.1Mhz. It provides bigger amplitude below 1Mhz though.

Everything is now working except it does so not so good. At the output I'm getting about 60Vp-p. Yes that's right, that's the intention of this tank circuit. Seemed strange to me but indeed it puts out this much voltage. Not sure what the right amount is though. The issue is range, not even a meter of range. A boombox I have wont even pick it up being next to the transmitter. I can tune the tank circuit and watch it peak but still doesn't get me very far. Modulation is very low. Can't go above 7% or so without it distorting. I tried driving it using up to 5Vp-p. My tube set picks it up about a meter away but hardly does so. If I put it against the wire it sounds pretty good but that's it.

I have two theories, my best one is my coil isn't correct. I don't know if this hurts modulation but it probably is messing the range up. Modulation may be the 100 ohm resistor on the modulator transistor which the website talks about reducing for more sensitivity.

Looks to be a good transmitter circuit. Just cant get it to work correctly myself. Pictures coming in a follow up post.

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Last edited by tube42 on Sep Mon 17, 2018 2:05 am, edited 3 times in total.

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 Post subject: Re: AM transmitter mostly alive
PostPosted: Feb Fri 16, 2018 5:32 pm 
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I built the Wenzel improved transmitter, and I can attest to its great performance. Listening on my GE Superradio on wideband setting it sounded much like FM. With a roof mounted ten foot antenna, I could hear it a mile away in my car. I read somewhere that the SSTrans uses the Wenzel circuit.

My only modifications were that instead of a crystal, I used my AM frequency synthesizer and I used an air coil that has a large number of taps. The freq synth let me choose a quiet channel at the top of the band to optimize performance.

I found it invaluable to have all the taps. It took a long time to find the sweet spot. There were places where the modulation sounded muffled and some places distorted, and only one place where it sounded like FM with maximum signal strength. Once I got it working, I moved on to other projects and never made a permanent version of the development version shown below. The next step would have been to record the inductances and fabricate a ferrite coil.

The silver alligator clip with the red handles went to the antenna. The yellow clip went to a 365 pF variable cap to ground. The blue clip is from the collector of the transistor that goes to the coil (see three turn tap in picture) and the green clip goes to V+, which enters the bottom of the coil in the pictures. Note that the 365 pF cap is not right at the antenna, which is the top end of the coil, but a few turns down (looks like 10 or so). IIRC, this is where I got the best fidelity and kept it from sounding muffled. Then again, that was ten years ago.

I made a PCB, which had pretty much the same form factor as the freq synth. In the photos below you can see the two stacked PCBs.


Attachments:
pcb.jpg
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800Dscn0214.jpg [ 47.18 KiB | Viewed 1788 times ]
800Dscn0215.jpg
800Dscn0215.jpg [ 62 KiB | Viewed 1788 times ]


Last edited by Macrohenry on Feb Fri 16, 2018 6:48 pm, edited 1 time in total.
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 Post subject: Re: AM transmitter mostly alive
PostPosted: Feb Fri 16, 2018 5:40 pm 
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Man I'd love to know what gauge wire and former you used and where you placed taps. Looks like every 3 turns or so?

I thought about adding various taps to my toroid but seeming how people had built the circuit successfully I thought it would be fairly easy to get it going as is.

I actually made two toroids, both using the same inductance values. Only one works. Go figure.

Attached are the pics.

Image

Image

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 Post subject: Re: AM transmitter mostly alive
PostPosted: Feb Fri 16, 2018 6:32 pm 
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It's 18 gauge. Taps are sometimes every five turns, sometimes 3 or so. That was ten years ago. I wish I'd made better notes of the final settings and what frequency I was on. Here are today's photos.

I made the following edit to my previous posting:

The silver alligator clip with the red handles went to the antenna. The yellow clip went to a 365 pF variable cap to ground. The blue clip is from the collector of the transistor that goes to the coil (see three turn tap in picture) and the green clip goes to V+, which enters the bottom of the coil in the pictures. Note that the 365 pF cap is not right at the antenna, which is the top end of the coil, but a few turns down (looks like 10 or so). IIRC, this is where I got the best fidelity and kept it from sounding muffled. Then again, that was ten years ago.


Attachments:
8002018-02-16 11.15.02.jpg
8002018-02-16 11.15.02.jpg [ 56.9 KiB | Viewed 1781 times ]
8002018-02-16 11.15.31.jpg
8002018-02-16 11.15.31.jpg [ 49.17 KiB | Viewed 1781 times ]


Last edited by Macrohenry on Feb Fri 16, 2018 6:49 pm, edited 1 time in total.
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 Post subject: Re: AM transmitter mostly alive
PostPosted: Feb Fri 16, 2018 6:42 pm 
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Do you have any way of measuring the inductance you ended up at?

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 Post subject: Re: AM transmitter mostly alive
PostPosted: Feb Fri 16, 2018 6:57 pm 
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tube42 wrote:
Do you have any way of measuring the inductance you ended up at?
The entire coil is 104 uH. Makes sense because it was almost certainly at 1680 or 1690 kHz. I wound it spaced one wire diameter for better Q and to make it easier to add taps.

I added this edit to the previous postings:

Quote:
The silver alligator clip with the red handles went to the antenna. The yellow clip went to a 365 pF variable cap to ground. The blue clip is from the collector of the transistor that goes to the coil (see three turn tap in picture) and the green clip goes to V+, which enters the bottom of the coil in the pictures. Note that the 365 pF cap is not right at the antenna, which is the top end of the coil, but a few turns down (looks like 10 or so). IIRC, this is where I got the best fidelity and kept it from sounding muffled. Then again, that was ten years ago.


My meter doesn't go below 1 uH so I don't know how accurately it measures that three turn tap. My meter shows it to be 1 uH. The coil is a solenoid, so the inductance of each tap should be easily deduced.


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 Post subject: Re: AM transmitter mostly alive
PostPosted: Feb Sat 17, 2018 4:13 am 
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Location: Milwaukee,WI
I also built the Wenzel improved transmitter and was very pleased with the results. I used the original crystal oscillator. I wound my coil on a toroid because I didn't have the supplies to wind it as per the article. My testing on a 3 foot piece of wire yielded a good signal at 5-20 feet away which is where my radios are located. Tuning was very touchy. Just a slight bit off of ideal and modulation went down and then distorted. But at that sweet spot it worked very well.

Last year I built my prototype with physically larger parts and after testing I tore it apart to rebuild it properly with smaller components and smarter layout. I haven't yet rebuilt it because I got sidetracked working on other stuff and have been using a Talking House Transmitter I picked up cheap. But eventually I will build my own again and sell the TH.


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 Post subject: Re: AM transmitter mostly alive
PostPosted: Feb Sat 17, 2018 4:43 am 
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forumuser wrote:
I also built the Wenzel improved transmitter and was very pleased with the results. I used the original crystal oscillator. I wound my coil on a toroid because I didn't have the supplies to wind it as per the article. My testing on a 3 foot piece of wire yielded a good signal at 5-20 feet away which is where my radios are located. Tuning was very touchy. Just a slight bit off of ideal and modulation went down and then distorted. But at that sweet spot it worked very well.

Last year I built my prototype with physically larger parts and after testing I tore it apart to rebuild it properly with smaller components and smarter layout. I haven't yet rebuilt it because I got sidetracked working on other stuff and have been using a Talking House Transmitter I picked up cheap. But eventually I will build my own again and sell the TH.


Do you recall the amount of turns used on your toroid and it's type?

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 Post subject: Re: AM transmitter mostly alive
PostPosted: Feb Sun 18, 2018 4:09 am 
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Sorry, no I don't remember. It was lots of trial and error for the tank circuit and I didn't write down the values. I remember it was on a larger red toroid core and it wasn't real fine wire. I was going to rebuild the output coil because I planned on pairing up the transmitter with the adjustable generator that Macrohenry made the circuit boards for. At least I think it was him.


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 Post subject: Re: AM transmitter mostly alive
PostPosted: Feb Sun 18, 2018 4:41 am 
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I'm thinking of using an AM radio loop as it will be easier to wind turns and tweak. I'll report back. The modulation is very weird. Could it be that the tap in the toroid is so low that it can't "pull it down"?

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 Post subject: Re: AM transmitter mostly alive
PostPosted: Feb Tue 20, 2018 11:34 am 
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I got it working! Turns out there was two issues.

One was that I had put the audio onto the bus where all three transistors connect. :oops: :oops: :oops: Modulation now reaches 100% though still needs to be driven somewhat hard, I was using the headphone output of some amplified speakers as straight from my laptop it wasn't sufficient to go past 15% mod. After this I was already having much better range. The input cap on that bus was messing up the tuning as connecting it to my laptop the xmitter would begin acting strange

Second is that the variable capacitor is somewhat strange. They are labeled CBM223 and no info is available online save for some strange auction sites. I thought these we're identical dual gangs and then I realized one side is 20pf and another is around 120 pf. I switched to the 120pf and tuning is much better.

I'll do some range checks tomorrow, easily covers the areas in my room where before I couldn't hear it though.

I'm thinking of building a pre- amp circuit. The site says you can increase gain on the audio amp by reducing the resistance of the 100 ohm resistor on it's emitter. Instead I might add a small monitor speaker and amp to the box so I can check for the presence of incoming audio. From there I can drive the xmitter. Still not sure. I have a transformer I can use for this project but it's not enough to drive an audio amp. Most it can do is probably 100mA.

The transmitter is putting out about 75V p-p on the output. Modulated to 100% using a 1khz tone it goes up to about 130Vpp. :shock:

Current consumption is about 30mA constant and 300mA peak at full modulation.

Look at that lovely waveform!

Image

Only thing it lags is more audio quality. Comparing it to other stations though i think the limit is just what AM can't manage. Here you can hear a bit of audio coming from it.

https://youtu.be/ksoqqnPv5f4

EDIT:

I just did a range test. It covers the whole house!! :D

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 Post subject: Re: AM transmitter mostly alive
PostPosted: Feb Tue 20, 2018 7:28 pm 
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tube42 wrote:
Only thing it lags is more audio quality. Comparing it to other stations though i think the limit is just what AM can't manage. Here you can hear a bit of audio coming from it. https://youtu.be/ksoqqnPv5f4
D


Good news you got it working. Nice screenshot of modulation.

Assuming your recording is accurate, this ckt is capable of better audio. Chasing it was challenging for me. IIRC the best audio was not possible at maximum RF output. It may be that max Q was limiting bandwidth.

Also, the input to the xmitter may be sensitive to impedance mismatches. One thing that often works to reintroduce highs is to put a normal load on the source devices, such as a 64 ohm resistor for devices meant to power headphones. Then I take the audio off that, even if it needs a preamplifier.


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 Post subject: Re: AM transmitter mostly alive: FIXED!
PostPosted: Feb Tue 20, 2018 8:47 pm 
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I don't remember who it was, but someone on these forums said this circuit is sensitive to loading. That can explain distortion with normal audio if the antenna is sharply tuned (Q too high).


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 Post subject: Re: AM transmitter mostly alive: FIXED!
PostPosted: Feb Tue 20, 2018 9:43 pm 
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Forgot to mention in the video there's some scratching from the cam mic not the transmitter. Most of what seems to be missing is low end. Listening to other AM stations the difference isn't too big. I might wind up another toroid with multiple taps.

I'm working on my power supply for this and the cabinet. Will update with progress pictures.

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 Post subject: Re: AM transmitter mostly alive: FIXED!
PostPosted: Feb Wed 21, 2018 5:59 am 
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Hi all. AM is capable of wider frequency response than fm stereo. The limits are usually in the receiver and can also be in the transmitter if the Q of the matching circuit is too high causing sideband cutting. Run your transmitter through a proof of performance test to see if that is a factor.


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 Post subject: Re: AM transmitter mostly alive: FIXED!
PostPosted: Feb Thu 22, 2018 3:08 pm 
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I’m glad to hear that you got it working.


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 Post subject: Re: AM transmitter mostly alive: FIXED!
PostPosted: Feb Thu 22, 2018 8:01 pm 
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Nice work!

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 Post subject: Re: AM transmitter mostly alive: FIXED!
PostPosted: Feb Sat 24, 2018 4:13 am 
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Almost finished with this project. Just need to mount a speaker.

I have been playing around with it. I notice it has a hum on the transmit audio (possibly on the monitor amp audio too but can't tell). Turning the plug around reduces this hum a bit. Hanging a clip lead hooked to ground makes this go away a little. Hooking said clip lead to earth ground reduces my signal output. This could be cuz the transmitter is indoors with brick and mortar walls. When I hook the transmitter up to my CB antenna on the roof the signal goes very high but can't be peaked with the vari cap. This however produces best signal all around. Peak for this is at the very end of the vari caps range. I'm hooking up just a clip lead up to the center of the CB antenna. With this configuration hum comes in very loud but so does everything else. If I then hook a clip lead to ground (just a clip hooked to a ground point of the xmitter) the hum goes away almost completely but the lead must be positioned just right. Wizardry.

Ground loop hum is so frustrating. My power transformer is close to the rest of the transmitter circuits including the toroid but it happened even with the PS far away.

Edit: May have spoke to soon when I said it wasn't due to the transformer. If I ground the frame of the transformer most of the hum goes away. I'll make a permanent connection there and also add some sort of shielding between it and the toroid. I will try to change the orientation of the toroid transformer since i think their magnetic fields are in directly in line.

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 Post subject: Re: AM transmitter mostly alive: FIXED!
PostPosted: Feb Wed 28, 2018 3:28 am 
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Finished product. No I am not a carpenter. The wood I used was damaged and old but seemed more interesting to use it rather than a black project box. My dremel helped a lot. I don't think I'll tweak anyore for higher audio fidelity. The difference to what I hear on the dial isn't huge. Almost no hum after grounding transformer core. The rest I can't figure out where it comes from. Range is excellent. Goes all the way around our house and onto our neighbors.

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 Post subject: Re: AM transmitter mostly alive: FIXED! FINISHED!
PostPosted: Feb Wed 28, 2018 3:55 am 
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Looks like a transmitter to me! I like the way the serrated knob barely protrudes through the slot. Congrats and enjoy!


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