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 Post subject: AM PLL Transmitter kit
PostPosted: Aug Sun 12, 2018 5:26 am 
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A thread in the Clubhouse about kit transmitters about a month ago picqued my interest in the various basic kits on ebay: viewtopic.php?f=2&t=343483

After browsing several of them, I chose this one since it is tunable via DIPswitch over the AM range.
https://www.ebay.com/itm/531-to-1602-kH ... 3133708251

Assembly was a bit tedious since the board was silkscreened to denote resistors, capacitors, inductors, ICs and such, but no part designations. That info was in a few small pdf files I had to download and print. It all went well enough, but I found the traces hard to solder to. A light scrubbing to brighten the copper landing helped. I added sockets for the 5 chips and the crystal. Two crystals are included for both 10 kHz and 9 kHz channel spacing.

It didn't work. Sure enough, I'd made a couple placement errors. Still no go. A little resoldering didn't help. I tried the 9 kHz crystal and then it worked, but with poor range. The kit was short one 470pf capacitor. I found a mica in my parts drawers and later installed a more proper one I ordered along with a spare set of chips for it, in case any of those were related to the trouble.

Messed around awhile, then tried the other crystal again and it now worked on frequency. It seems my first attempt at soldering it was less than successful. That's when I decided to socket the crystal as well.

The "final," a single 2N3904 seemed a bit anemic, compared to the PN2222 pair with associated driver stage in the SStrans. It also ran very hot, but a heatsink seemed to tame that. The circuit draws about 60 mA.

Though now working well, range was still limited to the adjoining room of the house. The provided antenna wire was short even for Part 15, so I added on another hank I had laying around. That worked better! It now has similar range to my SStrans, the signal covers the house and outbuildings. Input level must be kept low or it will distort. Too bad, a little more gain really bumps up the signal's loudness and range.

It should be a good platform for a little experimenting. I tried a tuning cap across the antenna output, similar to that in the SStrans, but it only reduced signal strength slightly. I used my DX-150's meter to judge, it was placed just a few feet away with its RF gain reduced to put the meter pointer mid-scale.

So, after a bit of head-scratching and troubleshooting, I like it. Now to come up with an enclosure for it!
-Ed


Attachments:
AM PLL Transmitter.jpg
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AM PLL Schem.jpg [ 85.07 KiB | Viewed 3161 times ]

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Sun 12, 2018 5:53 am 
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Good post Ed, thx for publishing it. Very interesting.

You might try All Electronics for small plastic or metal enclosures for projects.

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Sun 12, 2018 2:41 pm 
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The AM is generated in the CD4053 and then the AM signal is fed to the 2n3904 transistor which is operating in class C!! It should be a linear amplifier in class AB or class A and not C! A class C amplifier is only suitable for FM and not AM.


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Sun 12, 2018 6:20 pm 
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Thanks fifties! I'll have a look around here for something suitable first before placing another order.

Dare, I appreciate your comments! The 2N3904 looked like the weak link. I don't know how to change it from class C to AB, any tips? I was going to try to copy the final from some other circuit and mock it up on a breadboard.

-Ed

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Sun 12, 2018 8:31 pm 
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Check this schematic
https://ibb.co/gCOpAp

Adjust R2 for about 3-5 ma of standing current with no input signal on the base. Then apply input signal and again measure the current. Total input DC power should not be more than 100 mW for FCC part 15 compatibility. The resistor and the diode turns on the transistor a little for class AB operation.


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Sun 12, 2018 8:32 pm 
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Location: Arlington, TX, USA
I'd have to see some scope traces to figure out how this thing generates an AM signal.Maybe some sort of PWM that gets amplified then low pass filtered? The final amplifier does not have modulation applied to it like most designs using a Class C final. I'm skeptical of this design producing a high quality signal.


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Sun 12, 2018 8:43 pm 
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Erich Loepke wrote:
I'd have to see some scope traces to figure out how this thing generates an AM signal.Maybe some sort of PWM that gets amplified then low pass filtered? The final amplifier does not have modulation applied to it like most designs using a Class C final. I'm skeptical of this design producing a high quality signal.


Check this out https://www.radiomuseum.org/forum/ein_e ... nnung.html

The schematics here use a CD4053 as well for AM generation. CD4053 is an electronic switch and works just like a diode. Single and double balanced diode
mixers are widely used in DSB and SSB transmitters.


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Sun 12, 2018 10:32 pm 
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Ok, I see what is going on. It uses the switch in the CD4053 to switch the modulating signal at the carrier frequency giving a series of pulses whose amplitude is equal to the instantaneous amplitude of the input signal (the audio signal). The output of the CD4053 is the same as an unfiltered modulated Class C amplifier. It's a different method of doing the same thing as the conventional modulated Class C amplifier. The "final" in the OP circuit is really only a simple on-off switch and is only there for signal boosting purposes, so it's not a linear amplifier. You could simply connect the filter to the CD4053 without the intervening stage and get similar results, but maybe a bit less signal out. If anything the "final" in the OP circuit is limiting performance because it really needs to be a linear amplifier at that point. It would need to be biased at least into Class AB territory for full modulation depth.


Funny that the first post in your link calls lack of crystal control a feature! "Frequency set with a knob, no DIP switches". Not the same as the circuit discussed above, however.


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Sun 12, 2018 10:54 pm 
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Great comments, I'm soaking it all in. I chose this one because of the DIP-switch tuning. My two SStrans have both been parked where I set them after construction. I want set-n-forget tuning, but not "rock-bound" to 1000kHz or one of the few other limited choices.

The kit I purchased does use a crystal, but only as a root or reference for its own method.

Edit: The lower cost variable cap transmitter from the earlier thread admitted to broadcasting harmonics well outside of AM BCB. That was the "feature" I was referring to.

Dare, thanks for clarifying class C and changing it to AB, and Erich also for your insights. I more or less inferred the 2N3904's switching status when reading the data sheet for it, plus how easily it seemed to saturate and distort.

Here's the theory of operation, copied from the auction:
-Ed
-=-=-=-=-=-=-=-=


Principle of circuit operation.
IC3 CD4060 is generated a reference signal for the PLL circuit using crystal frequency 9.216 MHz. The frequency of the crystal is divided by 1024 with the frequency divider within the IC. Frequency 9 KHz out on pin 14.
 
IC4 HEF4046, acting as a phase comparison and VCO. The phase comparison compares the phase of two signals, the reference signal from IC3 and the signal from IC5. The two signals have the same frequency but different phase. The phase difference is more or less, it will bring out some voltage we called the error voltage. We will apply this voltage to control the frequency of the VCO but before using it, we need to filter the high frequency signal with a RC low pass filter is connected to pin 9 of the IC. C24 and R9 define the VCO frequency range.

IC5 CD40103 acts as a frequency divider (÷ N) The frequency from the VCO Out (pin 4 of IC4)
We can set the divisor by switch SW1 for example need to broadcast at frequency 1,008 KHz set to divide to 111 frequencies out of IC5 it is equal to 9.08 KHz

SW1 switch must be set to binary code for example, if divide by “111” is decimal must be converted to binary the value is “01101111”

IC1 LM358 is an Op-Amp that amplifies the audio signal from low-power to high power to be suitable for use.

IC2 CD4053 acts as a mixer receiving audio signals to pin 1 and pin 13 mixed with radio waves received on pin 10 and pin 11 the signal is output on pin 14 and pin 15. The signal is strong enough to be used in the room. But we need to send more coverage, so we need to extend the signal.

Q1 is 2N3904 acts as RF power amp to amplify radio signal strength to increase the distance to broadcast. Before transmitting the signal must pass through a low pass filter. The frequency range 531 - 1602 KHz is a wide frequency range. We divided the frequency of 3 ranges It is necessary to select the correct frequency range.

The antenna can be used in many ways, but for home use you can use a 3-5 meter long power cord to act as an antenna and grounding helps to signal better.

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Mon 13, 2018 9:14 pm 
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Did u try out the modification to class AB? Don't forget to add the 10 ohm emitter resistor. It prevents thermal runaway. Is audio better after the mod?


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Mon 13, 2018 11:23 pm 
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Not yet, but I sure did study it and compare to the existing circuit. My "me" time came late and I only messed with checking out some wallwarts as candidates for the DC supply. So far I've been using an auto jump pack as a pure v 12DC source. The wallwarts I've tried with unloaded voltage nearest to 12v do not provide what the transmitter is looking for and I get some noise but no signal. I have an LM317T regulator I will install at some point to rein in over-volted wallwarts.

Depending a bit on the radio used, the signal does seem less than stellar, just sort of distorted, but another radio might sound clean. I've only tested with my laptop streaming Antioch old time radio, which seems a bit bass-heavy in its audio. Hardly representative to judge audio quality yet.

I'll get after it and will post the results. Subjective though they may be without an o'scope.

Thanks for looking after my thread and how I'm making out with it!
-Ed

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Tue 14, 2018 12:45 am 
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Load the output with two 100 ohm resistors in parallel. Audio quality should improve. Also connect a wire antenna to the output along with the 50 ohm resistor load.


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Tue 14, 2018 8:23 am 
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I had a 51 ohm resistor so tried that instead of two 100 ohm paralleled across antenna and ground. Still using Antioch OTR streamed to a couple different portable transistors. So any change is subtle, but seems slightly better with it.

After a hunt though the parts stash, I have the AB circuit mocked up on a breadboard but hit a snag on the 100 uH choke. I think I found an air-core that might work or a couple wirewound on ferrite cores to try. The existing circuit uses a 47uH in that position, would that work well enough? Sounds a bit small.

Basically, I plan to substitute this AB portion for the 2N3904 section of the transmitter, taking the input section over to the breadboard and returning it to the circuit where it feeds into the LC network. That way I only have to lift a leg on a few parts and not modify the circuit board itself to accommodate the extra parts.

Busy days with road work being done on our district and a trip to the city tomorrow. But maybe I'd have time to hit the parts house for the choke.
-Ed

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Tue 14, 2018 1:34 pm 
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At 1.5 MHz the inductive reactance of 47 uH is 440 ohm. Maybe it's good enough. Don't lower the frequency. Higher value choke will offer a high resistance to RF and prevent power leak. The audio does not sound good as according to my observation the output is designed for a 50 ohm load and not a Hi-Z wire antenna so adding a dummy load improved sound quality. You can add the mod for class AB operation and then copy the output network from Vectronics 1290 AM transmitter. That will match the final collector to a 6 feet wire antenna.


Last edited by Dare4444 on Aug Tue 14, 2018 2:27 pm, edited 3 times in total.

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Tue 14, 2018 1:36 pm 
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Schematic of 1290 Vectronics AMTX. Copy the output network and add it to your kit. I suggest replacing the output 2222 with 2n3904 as the latter will give a slightly higher output power


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Tue 14, 2018 9:27 pm 
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Dare4444 wrote:
Schematic of 1290 Vectronics AMTX.

Image
Is this the complete print? Where does the base of the Transistor connect to?

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Tue 14, 2018 10:22 pm 
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I was hoping it goes here.

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Tue 14, 2018 10:24 pm 
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Complete schematic

https://ibb.co/eBEWAp


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Tue 14, 2018 11:22 pm 
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Interesting to note, the Vectronics Tx does use class c modulation.

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Wed 15, 2018 8:05 am 
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Thanks again Dare! My anticipated city trip was delayed due to crappy weather, so I spent part of the day helping to clear a long-neglected and plugged 30' long 18" culvert across the main road in our district. I played a minor role today, in prior years I've taken that job on by myself before it became overwhelming, and nobody else even tried after that till now. Good to see it flow again!

The advantage to the delay in my trip is I can buy the parts to built the Vectronics network.

Heh, I just misspelled "network" above as "notwork." I hope that's not an omen! :lol: It'll be built on a breadboard anyway, so hopefully no lives will be lost.

-Ed

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