http://technologyinterface.nmsu.edu/fall96/electronics/induct/induct.html
http://daycounter.com/Articles/How-To-Measure-Inductance.phtml
HTH ...
"The easiest way to increase the power to 100 mW is to replace R7 (510 ohms) with a 620 ohm 5% resistor. This will yield about 101 mW, but getting exactly 100 mW is not possible given standard resistor values and component tolerances."
What about replacing R7 with a 1kohm trimpot wired as a rheostat. Maybe with that, along with the other adjustments in the output network may help prevent problems while trying to peak the input power with minimum distortions etc.
Just a thought.
RFB
The trimpot idea is appealing. I will give it a try.
I was changing R7 to 620 ohms but I cannot find one of that value in my stash, so I used 680-ohms, which may be slightly high, but I'll order the 620.
Oh, no I won't. I'll put in a trimpot.
IMPORTANT CAUTION!!!
I just realized, DO NOT replace R7 with a trimpot. If you do, you will be able to short that circuit right to ground, which is NOT a good idea.
Here's how to do it. keep the 510-ohm resistor that's there, in series with that put a 200-ohm trimpot, so you can add the few ohms needed to reach 620-ohms. With this you might be able to get exactly 100mW.
Either approach would do. One just has to remember that the trimpot does have two ends and one of them goes to ground, thus not turn beyond a certain point on the trimpot.
The other method, using a "padding" resistor ahead of the rheostat, I would use a 300 ohm or higher, and insert the 200 or 300 ohm trim pot rheostat. This should give you a complete range covering the intended value of R7 plus above and below that value, while providing that buffer away from full ground potential.
Good luck!
RFB
Yes, the 300 padding resistor before the trimpot is good, because it would put the "sweet" setting more toward the center of the trimpot, always a neat way of having it.
Well, it might be a bit late for me to be replying to this 18 year-old thread, but thank you to @radio8z for this description of how he measured the DC input power to the final stage of his AMT3000. As the kit has not been available for a while now, I decided to scratch-build my own, and am close to finishing, with only the SSM2166 audio processor to wire up.
Noting Phil's and RFB's comments in this thread, I decided to make R7 variable, by putting a 200 ohm trimpot in series with a 470 ohm resistor. At the maximum value of about 658 ohms, calculated DC power input to the final was 107mW. Turning the trimpot to zero resistance, so the total value for R7 was 470 ohm, I calculated a power input of 100.4mW. Given the tolerances of the various resistors, and the accuracy of the BM235 DMM I was using, this is close enough for me.
I'm using the 82uH inductor to tune the 3 meter antenna lead at the top end of the band, and getting a sharp peak in field strength by adjusting C5. I'm thinking of seeing if I can fabricate a small board to replace the onboard lossy chokes with a more efficient 82uH inductor wound on a T130-2 toroid. That's an experiment for another day though. In the meantime, I still need to finish up the audio processor chip, and label the front and back panels.
It's fun having an AMT3000 again and so far, this clone I built seems to be working as well as the original. Thanks once again, for this post radio8z.
@rugster It's chilling that 18-years have passed since those discussions about the AMT3000's R7 input power (to the final RF amp). At that time I did the same thing you just did, namely put a variable pot to enable fine tuning of the transmitter's power. If I recall I put a 10K pot there, which was not as well designed as your way of doing it. I think I'll redo it based on your method.
@carl-blare 18 years. Where does the time go? Actually, 18 years ago, I wasn't even aware of the possibilities with Part 15 AM. I didn't have my first Part 15 transmitter, an AMT3000, until 2010, a mere 14 years ago. And here you all were, knee-deep in the stuff.
Being an incurable knob twiddler, I didn't want to risk accidentally shorting the emitter of that transistor to ground. I figured that if the ideal value for close to 100mW output was 620 ohms, then by using a 470 ohm resistor and 200 ohm trimpot in series, I'd be able to cover all the values that would give me DC power inputs each side of 100mW. Turns out that even with just 470 ohms, I am still getting 100.4mW. Taking into account the accuracy of my DMM (a Brymen BM235) and the tolerances of the resistors (all metal film 1% parts), I consider my calculation of 100.4mW to be pretty much on the nose.
Nevertheless, I still like to get it as close as possible, so I may change the fixed resistor to 390 ohms at some point, so I can get it just a smidgin closer!
I'll post pictures of my build when done. Today, I hope to finish the final stage - the audio processor chip and the front panel pots and power LED. Then I just have to label the front and back panels. I an tempted to design custom black anodized aluminum front and rear panels but even for such small panels, it's expensive. I'll probably settle for good old Dymo labels instead.
I bought 3 different power supplies, which gave a wide variety of results with hum. When everything is finished, I plan to write it up as a blog post and will post the link here, if that's allowed. All of this is predicated on me not making a big boo-boo before it's completed, and effing the whole thing up. It can always happen......... 🤣
@rugster First off, your details are Greek to me but I'm surprised your almost finished, it seems like it was omly a day or two ago you said you might build one... I thought you meant maybe someday.. Wow.
Secondly, am I misunderstanding? are you saying you're getting 100mw to the input? - I thought that was next to impossible... that no transmitter design was that efficient. Not that I would know, but that's what I've always heard.
Anyway, Congratulations on your SStran clone. It's kind of exciting.
@richpowers 100mW DC input to the final stage - not output. You're correct that if there is 100mW DC input to the final stage, you're not going to get 100mW output, even with a very efficient amplifier. The AMT3000 final stage runs in class C which, if I remember correctly, is probably only around 60% efficient. On top of that, the small inductors in the antenna coupling are lossy, and then there's the incredible inefficiency of an electrically very short 3 meter antenna. It's a wonder anyone hears our transmissions at all!
I really hope I don't mess this thing up before I'm ready to show it off. Wish me luck please. Fingers crossed!
Yes, sometimes I can be glacially slow at doing things. I've been thinking of building an AMT3000 clone for a while now. I started thinking about it seriously, and ordering parts a few weeks ago. I probably didn't mention it here until all the parts had arrived and I was beginning to build. Building something like this from scratch takes a lot longer than assembling a kit. I start with a bare piece of copper-clad board, and have to figure out where to position all the different stages and individual components relative to each other - and have to make sure I can fit it all into the available space. You never get the perfect layout, but as long as I end up with one that works, and doesn't look too MacGyvered, then I'm happy!
@rugster "...The AMT3000 final stage runs in class C which, if I remember correctly, is probably only around 60% efficient..."
Yeah, that's more familiar with what I've heard. And aren't present day part 15 transmitters all Class C? - The only Class D part 15 transmitters I've ever known to exist were the SStran 5000 and the Info System transmitters in the early 1970s.
When you mention the audio chip processors.. These are the same chips used for processing in the SStran?.. this kind of goes back to the how hard could it be to build a DIY processor for part 15 AM and with asymmetrical limiting... which I dont think the SStran had built in.. is that something you could fo?
The SSTran AMT5000 was Class E. Theoretically, that class of amplifier has 80+% efficiency, but the one downside is that it can be difficult to tune. Particularly if you don't have an excellent ground.
That's why the AMT5000 showed so poorly in the AM Transmitter testing those 'other guys' did. Their ground was minimal and they had difficulties tuning the one Class E device they were testing.
The SSTran AMT5000 was Class E. Theoretically, that class of amplifier has 80+% efficiency,...
Right, Class E and so were the Info Systems transmitters Class E. I dont know why I thought Class D... I'm not clear on what Class is what, or how it's determined. But do understand a Class E transmitter has the most potential in efficiency, which is what was used in the 1970s
Slight update. I finished my scratch-build of an AMT3000 today, using the manuals and schematic that have been floating around online. After a tiring and harried night, in which I blew the final, and ended up replacing 5 transistors and some associated parts, it is working! I am still waiting on one part to arrive from Thailand, which will probably take another week or so. In the meantime, I am going to clean up some of the wiring, and label the front and back panel. There is a small amount of residual hum and some audio artifacts that seem to have been introduced by the audio processing IC. The processing chip was the last stage I built, and they weren't there before I added it. I think it has something to do with inadequate shielding on the wiring to the pots. I'm hoping that's all it is.
I wish I still had my "real" AMT3000 to compare it with. Initial impressions are that the modulation sounded quite clean when feeding the audio signal directly into the 3 transistor modulator stage, before I added the processor chip. Depending on how things go, I may even consider removing the processor stage and using outboard processing.
I'll post pictures (in a new and separate post) when the final part has arrived, and I've had a chance to clean it up a bit. Tentatively, I'm feeling quite happy with it though.