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Hi Guys!
I wanted to post this by itself.
The 13.560 Part 15 transmitter
thread is so gigantically huge,
I thought this would be easier.Carl’s Big Talker, the transmitter
Hi Guys!
I wanted to post this by itself.
The 13.560 Part 15 transmitter
thread is so gigantically huge,
I thought this would be easier.Carl’s Big Talker, the transmitter
I’m speaking of was made by Carl-
but he had a lot of help from many
of you.I built the oscillator stage a year
or two ago, and it worked perfectly.This week (much later) I built the
buffer stage. There were two circuit
versions, one by Neil, and one by Phil B.,
if I’m not mistaken. One was more complicated
than the other. I built the more complicated
version just for the fun of it.It turns out, it doesn’t do anything, and it
doesn’t even draw any energy from the power
source. I guess the buffer is dead.Because of my eye thing, I used the lightest light
and the most mag I could.I was very careful. Everything was checked one
piece at a time. All resistors tested good.
The one diode was in the correct position. The
connections between components were checked with
an ohm meter. I did not check the one cap or
the one transistor, though. I’m not really
equipped to do that.So – my plan is to replace the transistor and
see if it will come alive. The transistor
was from Radio Shack. Maybe it was a bad one.
It was also an MPS2222A, which is not quite
what was on the schematic – but the parameters
are very close.A few other little things. To save money, I
used resistors from an assortment package. In
some cases, I had to combine some resistors
to get the values I needed, but there was
no value that was too far off the mark, as far
as I am concerned. I measured them all before
I put them in.There is one other oddity in this experiment.
I use rechargable batteries. I love them.
I have lots of them and they save me money.
A while back (again at Radio Shack) I bought
a couple of rechargable versions of the famous
9 volt square battery, which I think many
years ago was known as the 006P. (????)It turns out the the Radio Shack rechargable
9V is different than what we have seen and used
for many years. Most rechargable 9V batteries
where really 7.2 volts, which was fine. They run
for a while, you charged them up, and used them
again. (I used these batteries for my antique
transistor radios from the 1960s (I have two) for
a long time.)Then, when I had a few bucks, I thought I would try
one from Radio Shack. It’s specs are 9 volts at
100 mA. It’s not much. But I don’t run these
old radios for very long, so I got one. It turns
out, this 9 volts NiMH battery runs one of these
old radios for a long time. Longer than I expected.You are probably wondering why I am telling you this
and what is has to do with testing the 13.560 rig.Well, I used one of these batteries to test the
oscillator and it ran fine. Then I hooked the
buffer stage up and found it to be dead.Just for fun I tested the Radio Shack 9 V battery.
When I put it up to the meter, I was surprised
that the reading was 11.5 volts! I used a very
slow charger very conservatively. I don’t get it.
Now granted, this 11.5 volts was with no load on
the battery. But I put it on the oscillator
which is a small load – and the voltage was still
11.5 volts. The volt meter seems to be fine,
but I can test it some more. I wonder if I blew
the buffer stage with the higher voltage – but it
seems unlikely.So, I still consider this to be fun, it’s a mystery,
and I would like to solve it. If I can’t bring the
buffer to life, I will probably scrap it and build
the version that has less parts.If you guys have any comments, feel free to let me
know. I could also just dispense with the buffer
stage and go right for the osc. to the final amp,
but I don’t want to do that. I think the transmitter
will work better with a buffer.Best Wishes,
Bruce, DRS2
Total posts : 45366Try removing the diode. I am not sure of this, and when Neil sees this he will know what to say, but I believe the diode is needed to control the base of the final stage, and is not something the buffer alone can benefit from.
The 11.5V should be fine….. I think I’ve had my circuits up as high as 15V back when it was a two-stage transmitter called Pixie 2.
Total posts : 45366I will try that.
By the way, my voltmeter is not bad.
I tested 2 regular alkaline 9 volt
batteries that had been sitting around
for a while. One tested at 8.7 V. The
other was at 8.3.I have two of these rechargable 9 volts
NiMH batteries. Turns out they are
170 mA/hr, still not much – but I thought
they had less energy. Anyway, I just tested
the two I have. One is 11.5 volts, and the
other it 11.1 V. Weird.I agree with you. I don’t think the higher
voltage would hurt the circuit, but I’m glad
I got your opinion on it anyway.Best Wishes,
Bruce, DRS2
Total posts : 45366Bruce, I think you are using the buffer I suggested. I didn’t build it but the simulation showed it works. Nonetheless, simulations are sometimes flawed but I think it worked for Carl.
If I recall the buffer draws quite a bit of current. This is because I tried to keep the output impedance low to minimize the loading effect of the final.
The diode in the circuit is intended to control the DC voltage across the coupling capacitor. Without this what can happen is the base emitter diode of the transistor will act as a Zener diode of about 5 volts. This will also control the cap voltage but usually requires a higher input signal voltage. No harm in trying the circuit without the diode.
If memory serves, Carl reported Phil’s circuit also works so it seems you have two options to try. The 2N2222 should work fine at this frequency but I have found experimentally that the 2N3904 works a slight bit better at RF.
Neil
Total posts : 45366I’ll take the diode out, and – I have
a bunch of 2N3904s sitting around here
somewhere. So I’ll try those, too.Well – at least I know I can build
oscillators. You probably remember
I tried to amplitude modulate the
oscillator by itself. The result
was an FM signal that got even
wider when I loaded the oscillator
down with a transmitting antenna.It did go about 1000 feet down the
street, though.Radio. Great stuff!
Thanks again,
Bruce, DRS2
Total posts : 45366Bruce, one of the diseases of young engineers and of some of us older ones is not knowing when to finalize a design and “ship it”. But, as we work on our own projects there is nothing to stop us from experimenting and tweaking until our circuit is perfect. For me this is part of the the fun of hobby electronics, that is to try to achieve the best circuit possible. Along the way it is remarkable what one can learn by doing this.
Until the advent of forums such as this it was difficult to share information with others. Now we can do so and we don’t have to wait until things are perfect. The “blow by blow” reports you and Carl and some others have posted are a great learning experience for all of us.
Neil
Total posts : 45366Yes Neil, the blow by blows have included blowing up many parts along the way.
I guess I didn’t get bold when the Big Talker was invented, because I took all the clues from you (Neil) and Phil B, plus I think Ermi helped me wire the AC transformer correctly, and other good advice.
But I did not try changing anything or adding my own twist, because this was really my introduction to solid state design.
In long past I tinkered a lot with tube RF, that’s when I went wild and tried crazy things, like grid dips, big variacs with high voltage and tubes cranked up to blue and orange colors.
The only way to get a transistor to glow is to put an LED behind it.
Total posts : 45366“If I recall the buffer draws quite a bit of current. This is because I tried to keep the output impedance low to minimize the loading effect of the final.”
Is this buffer stage designed around unity gain and just providing the isolation or does it do both isolation and amplifying?
If amplification is needed, I suggest putting a two stage buffer circuit. One section is purely isolation and unity gain for the oscillator, the 2nd stage provides gain and inversion if needed depending upon the 2nd stage’s configuration of the active element.
Just tossing thoughts.
RFB
Total posts : 45366Removing the diode didn’t make the
buffer operate.That’s OK. I have a plan. I’m going
to replace the MPS2222A with a 2N3904,
as you suggested, Neil.For me, it’s still fun even if it doesn’t
work. (Seriously.)So here we go. I’ll let you guys know
what happens. And then we can all figure
out and learn about a bunch of stuff.
(I sound like Beaver Cleaver.)“Hi Mrs. Cleaver, that sure is a nice
dress you’re wearing today!”Oh No! It’s Eddie Haskel. I’d better
get out of here before he starts messing
around and breaks my oscillator!Bruce, DRS2
Total posts : 45366So I dropped a 2N3904 into the buffer
circuit. Probably way too fast. It
will probably have to be soldered again
later.I put a slightly used 9 volt battery on the
oscillator. I touched a 20 dollar CB/ham
field strength meter antenna to the RF out
from the oscillator. I got a reading of
about half a scale.Then I hooked ground and RF out from the
oscillator to the buffer amp.Then I put the buffer amp on a DIFFERENT
power supply (a variable bench supply.)I attached the cheap CB/ham field strength
meter to the RF out on the buffer.Then I varied the DC voltage from zero to
15 volts. How interesting! Around 7 volts
the buffer was putting out much much more
RF than what was going into it. (Way past
pinning the meter.) Below
and above 7 volts the RF out when down.
Above 7 volts the current drain went way
up, but the RF went down. I do realize
that the condition were the most RF is
coming out of the buffer is not necessarily
where we want to be.I know there is a tremendous amount of
interesting things going on here.I intent to look into this further. This
is very cool.Best Wishes,
Bruce, DRS2
Total posts : 45366Wow, MICRO1700 Doggradio Studio 2,
The Big Talker shortwave transmitter we built uses a single DC+ source for the oscillator and buffer combined.
With your very interesting experience, you have me wondering what would happen with this transmitter if we varied the voltage supply feeding each stage…
It’s a “must do” experiment.
Total posts : 45366It sure is interesting!
I have a scope, but I’m sure
it won’t go up to 13 MHz. It
would be very interesting to
look at the waveforms involved.So it’s on to the final stage
now.I’ll run that on another supply
and see what happens.If it wasn’t for you I wouldn’t
be able to do this at all!Thanks again!
Bruce, DRS2
Total posts : 45366RFB queried: Is this buffer stage designed around unity gain and just providing the isolation or does it do both isolation and amplifying?
I am going by memory here since I don’t have the schematic handy but the buffer was designed to amplify. The gain is approximately the collector resistance divided by the emitter resistance reduced by the load of the final base circuit. My attempt was to drive the low impedance final base circuit hard so that saturation is ensured providing class C operation.
It is common for the output from a buffer to be coupled to the final base by means of an impedance transformer in which case the current draw of the buffer can be reduced but this adds complexity which I tried to avoid.
Bruce, it is a rare scope that will not work up to 20 MHz. If this is the case with yours a 20 MHz scope will show the waveforms and though they might be a bit low pass filtered at 13 MHz useful information can be seen. Lab scopes are usually marked on the front panel with the max. frequency information, else you can search the net for the specs from the model number.
Neil
Total posts : 45366Thank you for the info. I
will try to figure out where
the scope is and drag it out.Do you know that I was originally
an EE major? The detached retinas
I had previously had me terrified
that I was going to go blind again.It was all just the wrong time for
everything.And eventually there were many more
eye operations.Anyway, I love this RF stuff. If I could
I would take some courses, but things
are too busy now.I can just see how interesting this all
is and how many things are happening
at once.I’m going onto the final amp. And an
the meantime, I will be searching for
the scope.Thanks again, Neil!
Bruce, DRS2
Total posts : 45366I’m putting the final amp/modulator parts
on the board.I have a question about the modulation
transformer. You guys may have answered
this before, but I can’t find the answer.Sorry about that.
I have the Atlas Sound LT-72 transformer. Mine
has 2 bare copper wires coming out of it which
I believe to be the 8 ohm side. Do you guys
know if this is true? These bare wires are really
fragile. I don’t know why they did it this way.If that is true, I think I can figure the rest out.
This transformer has 8 additional wires of
different colors, and they don’t seem to
comply with some wiring diagrams I have
seen on the net. I may have to do this by
trail and error.So I guess I’m back to my original question.
Are the bare wires the 8 ohm side? Did I get
a weird goofy transformer?Do you guys have any ideas?
Thank you in advance.
Best Wishes,
Bruce, DRS2
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