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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Wed 15, 2018 1:31 pm 
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If I were you I would have used the pll section to drive a seperate 2n7000 based PA circuit built on a copper clad
board (so called ugly bug construction technique. It's easy and fast). I have tested the attached circuit. FM like sound quality at 1250 KHz and solid 100 meters range. I was driving the 2n7000 with a CD4060. Check the schematic. You can remove the CD4053 and then take a thin coax from pins 11, 12 and then use the sqaure wave signal to drive a 2n7000 PA with antenna matching circuit. Schematic here https://ibb.co/fyvXkp


Last edited by Dare4444 on Aug Wed 15, 2018 1:38 pm, edited 3 times in total.

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Wed 15, 2018 1:35 pm 
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or you can go the Vectronics way... Your choice. Reviewers said that Vectronics 1290 sounded nice too. It has a good output matching network.


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Thu 16, 2018 2:50 am 
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Well, rats. Chris Supply, the parts house I've gone to since the 70's had no inductors, pf caps or trimmers. Not much call for that sort of thing anymore. I go there maybe twice a year and way more often than not, they have what I need or a reasonable substitute. It's not like I've ever shopped there for chokes or trimmers prior to today, so I'll cut 'em some slack. Nobody there is retiring rich on my meager purchases. ;) The clerk took me to the flameproof resistors, which they had stocked up the wazoo. I don't think he'd heard of axial inductors before today. Trimmers and pica caps got a headshake, no.

For now, I guess I'll content myself with the 2N3904 class C to AB mod and make a parts list for the next time I make a Digikey order. Then I might try the Vectronics and/or 2N7000 mods.

-Ed

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Last edited by Ed in SoDak on Aug Thu 16, 2018 3:02 am, edited 1 time in total.

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Thu 16, 2018 2:57 am 
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Try ebay. China sellers. $5 for capacitors kit (1000 pieces).. resistor kit ... 1500 pieces for $6... Transistors and 2n7000, r
RF choke kits dirt cheap. All parts you will ever need.


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Thu 16, 2018 3:10 am 
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Thought of that, too. I like the assortments. I used to order from PolyPaks way back when. A lot of those oddball, unmarked parts are still to be found in my parts bins. :lol:

A resistor assortment I bought years ago still holds what I needed for this project and many others I've done. Same for my mica cap stash. Electronics Goldmine, All Electronics and other catalogs were my traditional experiment resource. Digikey is cheap enough, has the exact part and will mail cheap by first class and I have it in a few days. But they're not the best place for bargain assortments like the ebay asian parts sellers.

-Ed

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Thu 16, 2018 8:30 pm 
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Do you want the pll exciter to feed the 2n7000 or build a whole am transmitter circuit from scratch? I used 20 mhz crystal, one cd4060 to divide it by 16 or 1250 KHz and the output was fed to the 2n7000 PA circuit I posted above. Got FM like sound quality. Changing the frequency to 1500 KHz degraded the sound quality a little. Let me know how the class AB 2n3904 sounds after adding Vectronics output network to the transistor. The standing current should be 5 ma through the transistor for good linearity.


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Fri 17, 2018 4:12 am 
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Well, I want to give this kit-built a decent try-out before axing it or dissecting it much. I don't want to sell it short with so little testing done, but so far, meh. Perhaps I still haven't given it a decent source to broadcast, or at least one that better matches what this thing is looking for. It might digest a line-level input better than a low-Z headphone feed. But ultimately, it's more likely to be fed by the latter for what I'd intend to use it with.

Anyway, I want a baseline to better gauge if it's improved by a mod or not. It took some playing with each of my two SStrans before I was satisfied with them. Right now, I'm getting some heterodyning, so I think I should try it at another frequency. It's not nearly as nice in the higher audio range as the sound quality I get with the SStrans, which kinda set the Gold Standard for the commercially-produced type 15 broadcasters, and a shame it's not still being produced.

So the AB mod is next. Simple to do and I already have the parts breadboarded. Tonight I tried a flat screen tv audio source instead of my laptop, so I could compare the output directly to one of the SStrans on a 100kHz lower frequency. It was easy to tell which transmitter was tuned in. That may not be a fair comparison yet, as the SStrans is getting its audio from the RCA output directly off the dish receiver versus a cheap flatscreen's headphone jack that's feeding this Taiwan transmitter.

I shouldn't expect parity for one-quarter the cost and the far simpler circuit. If the AB mod also falls short, I'll make a parts order to do the network and 2N7000 mods. I probably won't start over from scratch. If I do, I'll go in a different direction than PLL and try out Mike Toon's LM386 circuit that's pretty popular. I have plenty of perfboards to move over any breadboard concoctions we manage to get working.

-Ed

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Fri 17, 2018 1:40 pm 
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Hi Ed,

Try playing a music video on YouTube with video quality set to 1080p and then feed the audio to the kit from your laptop. It is one of the best ways to test audio quality and fidelity.

The class AB mod should work. I have never tried the CD4053 for am modulation but it should sound good. You can remove the final transistor and the base to ground resistor and just connect a 6 ft wire directly to 4053 output to test the sound quality.


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Fri 17, 2018 1:51 pm 
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https://m.imgur.com/a/p8BDB

This is the circuit I was talking about.
HCF can be CD4060. The BD139 can be replaced with a 2n2219. This transistor isn't necessary if LM386N-4 is used.

I have the CD4060, 20 mhz xtal, 2n2219, and all the caps and resistors and I can send it to you. The RFCs and 2n7000

I can buy from ebay for you if you would like. I have an ebay account. The final variable capacitor can be replaced by a trimmer. But right now you try the AB mod and adding Vectronics output network and hopefully you'll have a good sounding transmitter with 100 meters range.


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Sat 18, 2018 8:50 am 
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Howdy Dare! I'm not moving nearly so fast. Plenty of irons going IRL with a year-long work project coming together, a bathroom makeover and road work in the district I'm a board member of. Then I get a bout of arthritis in my primary hand, just what I needed about now. :roll:

My transmitter project hasn't been parked, but to the casual observer, it might appear to be stationary. ;) Tonight I removed the 2N3904 and socketed its position, and pulled the associated few parts to ready it for the AB mod.

I have an extra CD4060, having ordered a complete set of spare ICs used in this kit. An early troubleshooting CMA that hasn't been needed - yet. What I lack are the trimmers and inductors one might choose, simply because I didn't pull and save those kinds of parts when I stripped a few gems out of things I was junking. But never fear, I've been an ebayer since forever. I appreciate your generous offer, but I'll make that plunge when I have a better idea about what I need or might use in the future. The past month or three I've been expanding the stocks to include more new-fangled ICs and other things I'll need to learn more about them.

Mostly to work on my vintage computers, this transmitter project just kinda popped up in the meantime and showed I still have a few gaps in things I need to acquire to play in this sandbox too. It was supposed to be more plug'n'play once built than it turned out, but I hope to still keep it pretty simple and not go wild with a buncha new designs.

I've been saving transmitter designs for years, but instead of a scratch-build, I took what I thought was gonna be the easy way out with this kit. Having gone this route, I'll take it to a conclusion before abandoning it.
-Ed

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Sat 18, 2018 7:44 pm 
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Cool! Looks like your hands are full. I designed an FM transmitter last night and worked on it for 8 hours. It's now ready for publication in electronics for you magazine. It's output is 100-150 mW and it would make it illegal here in USA but not in India where EFY magazine is published.


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Mon 20, 2018 5:10 am 
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How about using that pll to drive a 6888 instead of the xtal.

Bob


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Wed 22, 2018 7:25 pm 
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Howdy Bob, That might make an interesting hybrid, but not the direction I'm going. And likely a bit beyond my pay grade without some handholding, so I'm sticking with easy to do stuff.

As to the PLL, I've yet to play with the DIP-switch frequency selection very much to say for sure, but just idly switching the last few switches to a different position didn't seem to jump the frequency to a new spot. Broader changes work, I had it on 630 and 1580, and that's where my digital radio tuned it, though maybe 1570 worked better. I need to play with this more, but as I mentioned above, I tend to find a good spot and "park" there, so I don't need a ton of agility to begin with. I just don't want to be "married" to that one spot.

So the PLL portion sure looks like it could be adapted to other circuits. It happens to use several of the same chips found in the SStran circuit, which as we all know works great in that department.

-Ed

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Wed 22, 2018 7:55 pm 
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I've spent a little time breadboarding Dare's C to AB mod. Only 6 parts that I happened to have on hand. Mods to the transmitter amounted to socketing the transistor for easier modding, I removed one choke as it's replaced on the breadboard, lifted one leg of two resistors and tapped onto power and ground.

But then, things turned odd. It didn't work as planned. It didn't broadcast nearly as far and sound quality took a hit too. It seemed the ground wire was carrying most all the signal. I started checking my wiring and tried a different choke. Doing that while it was playing, I noticed it worked better with my fingers on the air-coil choke. This led to more messing around. I lifted the ground wire from the transmitter going to my breadboard, expecting it to quit working, and huh? It actually worked better, much cleaner-sounding. But still the signal preferred the ground wire going to my fuse panel. Next I started moving the antenna wire to other places and finally, on the choke leading to the 2N3904 Base, it seemed to work properly with audio comparable to my SStran. I have no clue why this is the case, but if it works wired up "wrong," I'm not sure if I care! Plenty more things to try and learn the reasons.

And oh yeah, I've noticed heterodyning in the receiver all along. There's nearby nighttime channels, but not on my chosen 1570-1580. All the heterodying magically disappeared and I had full quieting of adjacent-channel noise when I removed the ground and relocated the antenna to where it supposedly shouldn't be. :?: :?:

I'd have to admit it's due to some mistake I've made or maybe the transmitter still has errors or bad solder joints.

My attempts at measuring Base current also seemed to be a fail. I'm using the recommended 470k and haven't tried a different resistor yet. I tried reading DC current from ground to the Base, both across and in series between the choke and the 2N3904 Base. Quiescent current seemed high, like 80mA or even 160mA and dropped say 10% when applying a strong signal. Which all seemed way out of whack with what Dare described. But current did vary up/down with the audio, so something was happening. Shrugging my shoulders as to what I was doing wrong, I plodded on with the above "discoveries." ;)
-Ed


Attachments:
C2ABmod.jpg
C2ABmod.jpg [ 42.22 KiB | Viewed 1461 times ]
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Dare C to AB.jpg [ 19.76 KiB | Viewed 1461 times ]

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Wed 22, 2018 9:32 pm 
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2 things I noticed. First is it a typo or did you actually use a 470k resistor? The penciled schematic show it as 4.7k.

Secondly, I may be wrong and 444 can verify but I think what he meant was 5 ma quiescent current at the output of the transistor and not the base.


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Wed 22, 2018 9:56 pm 
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I was thinking about this circuit, and unless I'm mistaken the output of the multiplexer will go from 0V to the supply rail at 100% modulation. If that's so, you won't be able to amplify it any further from the same supply voltage. The best you could do in that case is use an emitter follower to provide more current but the same amplitude. Trying to use a Class C amplifier to boost the modulated signal was a pretty stupid mistake by the seller of this kit, and just isn't going to work. It has to be a linear amplifier of some kind to work as expected.


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Wed 22, 2018 10:46 pm 
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Forumuser, 470K was from memory. (what memory?) I accidentally did it right and have a 4.7k in-circuit. Came across a 5k pot, so I think I'll sub that in.

Thanks for the heads-up on measuring the current. Makes more sense now that I think about it. And isn't Base voltage, not current, what one normally checks when troubleshooting? It's all part of my multi-faceted mission: To maybe finally "get it" after all these beers. Er, years I mean!

Erich, Way up above in the thread 35Z5 notes the Vectronics uses Class C. Re-reading the SStran docs, so does that successful design. So it can't be all bad. The Sstran uses a Darlington pair for the PA.

The purpose of my little mod, provided by Dare444 and shown above, was to shuttle off the Class C PA to a breadboard, where I implemented his Class AB final in its stead.

As I perhaps learn a little more, I may go back to the original circuit and give it another go.
-Ed

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Wed 22, 2018 11:01 pm 
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Ed in SoDak wrote:

Erich, Way up above in the thread 35Z5 notes the Vectronics uses Class C. Re-reading the SStran docs, so does that successful design. So it can't be all bad. The Sstran uses a Darlington pair for the PA.

-Ed


Yes they do, but there is one BIG difference: The Vectronics is a conventional high level modulated Class C amplifier with its supply voltage being the audio signal itself, it doesn't amplify an already modulated AM signal. The SSTRAN uses something called a Wenzel modulator; it looks like a differential amplifier with a modulated current source. Again, not trying to amplify an already modulated signal. The circuit being discussed here unless I'm totally mistaken is using a Class C amplifier to amplify the already modulated signal. It won't work any better than trying to use a Class C amplifier to amplify an audio signal and drive a speaker with it. Sorry, but this thing looks like something thrown together from a few different ideas (the original circuit from the German language site used neither a PLL nor a buffer amplifier). That buffer amplifier needs to be a linear amplifier.


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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Wed 29, 2018 5:28 pm 
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Well, I can't knock success. The AB stage that Dare came up with is working, however oddly I may have it wired up. It now has similar or better range than my two SStran and the audio sounds fine. It still overloads too easily, I imagine I can improve that by reducing the op-amp gain or by adding a pot to the input.

Thanks for all the comments and help!
-Ed

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 Post subject: Re: AM PLL Transmitter kit
PostPosted: Aug Wed 29, 2018 7:38 pm 
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The AB is stage is working? Awesome. That's all you need. By quiescent current I meant the current flowing through the collector without RF signal applied. The rfc should be big enough so that the rf on the base sees a 'high' impedance into the resistor/diode biasing circuit. In4148 and 4.7k should give you 5-10 ma of quiescent collector current.


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