While rummaging around in my junk pile I came across some historical items from the early days of my part 15 escapades.
While rummaging around in my junk pile I came across some historical items from the early days of my part 15 escapades.
This is a photo of my second FM transmitter which I built in about 1959 from plans published in an electronics magazine. (My first FM tx was a tuner from an old Admiral console Hi Fi where I tapped the local oscillator to feed a roof top antenna and applied audio to the grid. My cousin, three air miles away, could hear it loud and clear. But I digress....)
And a bottom view: 
This was a rather simple circuit but it had a strange feedback path from the emitter to the the coil consisting of a series RC. The specified transistor was a 2N499 which was very expensive ($5 in 1959) and which I managed to destroy. Fortunately, I found that a cheaper 2N708 worked as a replacement. Most of the parts were from scrap radios and television that I sometimes retrieved from the community dump. The rather messed up coil resulted from the rather labor intensive tweaking needed to get the thing to operate in band.
I never connected this transmitter to an outside antenna since a short length of wire would cover a range of several hundred feet.
Later, I built the third transmitter shown here: 
In this improved version I omitted the C to E feedback and it worked much better. It turned out that the C to E capacitance of the transistor was sufficient for the feedback needed for oscillation. Curiously, the author of the article stated that he did not know how the circuit produced FM with audio which led me to believe that he didn't know much about electronics (and neither did I) but his article was good enough for me to get started.
At the time my main interest was in part 15 AM using my Knight Kit broadcaster since few people had FM receivers back then but this was still interesting and fun to experiment with.
I have a photo of my "studio" somewhere and if I can find it I will share this other piece of history.
Neil
It was really fun to see your old FM circuits
and hear the story.
I know what you mean about tuning the coil
by stretching it or compressing it. It is not
easy to get an oscillator like that on channel.
It's funny, but, I know there were some FM
tube transmitter circuits in those days, too.
I think some of them got out pretty well.
i have to go right now, but I will probably have
some more comments for you about this.
Best Wishes,
Bruce, DOGGRADIO STUDIO 2
Yes, those early circuits are wonderful to see, dating from a time when FM was still somewhat of a novelty unknown to most people, and transistors were entering the scene.
You guys got me scrounging in the files for this re-print from Electronics Illustrated, 1964, an article by Harry Kolbe titled LICENSE-FREE FM TRANSMITTER, which was the first FM transmitter I built, a very neat looking little item with 3-tubes and a VU meter.
The 6C10 tube was short and wide with 3 distinct sections inside, all dedicated to audio chain, from input to modulator/meter driver, the 6AQ8 contained two sections, modulator and oscillator.
There were no other RF stages after the oscillator.
6X4 rectifier tube.
The sound quality was professional with the 75kHz pre-emphasis, and although the author claims 500-600 feet, I didn't get much range and always thought I'd made an error, so I had several years of fun going over and over every detail, eventually experimenting with modifications, but it never went very far.
Now, of course, we would add a buffer-driver and a final stage and gimmick with it to ride the legal limit....
Neil, I would be interested in knowing what
kind of receiver your friend used to pick up
your FM transmission. There were no FM
radios in our house at all until 1964, when
my Dad got a Magnavox stereo record player.
It had a tube FM tuner (no AM) - and it was just mono.
It sounded great, but had bad selectivity and
sensitivity. Years later, in an attempt to get
it to work better, I used to point an electric
fan into it somehow. I don't quite remember
how I did this. I think there were ventilation
holes on the top, and I just put the fan on
top and pointed it down into the circuitry.
When I did this, the FM sensitivity improved somewhat.
in about 1969 or 1970, we had an FM Part 15 station
that used one of the Radio Shack transistorized
kits. It consisted of an audio preamp and an
FM BCB oscillator.
At one point, in order to try to get more range out
of this little circuit, some of the guys actually put
the transmitter up on the house roof, and we fed audio
and supply voltage up there.
It was summer, and when the sun came over to that
side of the house, the transmitter would start to move
to the next channel, and then the next. (I don't remember
if it went up or down in frequency.)
One guy would have to go up on the roof to put the little
transmitter back on channel. The only way to do that
was to compress or expand the oscillator coil.
There was a fairly good borrowed FM tuner down in the
cellar where the station was. It had a calibrated dial that
we had sort of marked off to make it even better.
The problem was - how did the guy on the roof and the
guy looking at the FM tuner in the cellar communicate
so the rig could be put back on channel? We didn't have
any CB walkie talkies or anything like that.
It took 4 guys. The guy at the tuner would yell to a guy
on the other side of the cellar where the ping pong table
was. He would go to the back hatchway, and yell to a
guy in the back yard. This guy would yell up to the guy
on the roof.
As you can imagine, this was very difficult to do. I don't
know how we did it at all, but we usually got the little
oscillator rig back on channel.
We were all learning then and it was so much fun!
Carl, do you still have the little tube transmitter that
you built? And how far did it's signal travel? Did it
cover the inside of your house, at least?
Somewhere, floating around the web, there used to be
a one tube FM transmitter circuit - a free running oscillator.
I think it got out pretty far. But you would never catch me
using one of those today.
I will probably have some more comments. This is a
great topic. I love the Part 15 history.
Best Wishes,
Bruce, DOGGRADIO STUDIO 2
Now that I have a C.Crane transmitter to compare it to, the old tube unit had about the same amount of strength inside the house. Rather puny.
Like I said, I tried various ways of boosting the signal, but it was wimpy.
I have a few parts of it in a box. I have not saved old equipment, because with a recording business I had a sea of equipment all the time and I resisted the temptation of also becoming a museum of old equipment.
The article is here, when my copier starts working you can remind me to post it.
Bruce, if I recall correctly my cousin had just upgraded to a component audio system at the time. The Admiral I referenced was his which he had given to me for parts. I believe his tuner was an EICO HFT-90 built from a kit. These tuners were among the best at the time and I built several for people to earn some $$. My first personal tuner was a Lafayette FM tuner which was in a little black box. Interestingly, the Lafayette used the same pre-assembled tuner RF module as the EICO. The tuner stage used a plastic coil form which was slug tuned for the coils and with time the heat from the tubes in the Lafayette warped the form until the tuner was no longer usable. The EICO didn't have this problem.
One of the EICOs I built came home to roost when the business I built it for closed when the owner retired. He called me and asked if I wanted it. I said yes and used it for the remainder of my college days and for the next 15 years in my home system. I added an IC based stereo demux. and it performed wonderfully until I replace my old stuff with a Kenwood tuner/amp.
One thing I really liked about the EICO was the tuning indicator. It was a tube which produced an exclamation point of green light which served as both the tuning indicator and the dial position indicator. It used a DM70 tube for this and I bought a few an put them in my old tube type AM radios as tuning indicators.
I now have three EICOs, two of which I built...the one mentioned above and one that I built for my parents, and one I purchased at a hamfest. I bought the third one for less than the price of the tubes so I have parts if needed for the other two which I occasionally use. Despite their age and years of use neither of the first two have ever needed any tube replacement.
Carl, except for some brief recording work I did in college for pay I have never been an audio pro so I didn't accumulate enough equipment to be concerned about storing old things around here when they are pulled. That's why I have some interesting (to me) junk to play with from time to time.
It is kind of fun to look back at old electronics and to hear the stories of others. There are remarkable similarities in the experiences of those who grew up with electronics in the tube and transistor days.
Neil
Even though I haven't carried the weight of keeping the old stuff, I think of it with a lot of nostalgia, including the three different models of Knight Kit AM transmitters, two different "1st" FM transmitters, a high power AM I built, tested, and unbuilt because it seemed to be heard "everywhere" which made me very paranoid. I only wanted to turn up variacs to the point where grids dipped and plates glowed blue & orange.
I had a suite of Eico kits including an AM & FM tuner and a 50-Watt amplifier. Part of the FM tuner is still down there.
Well, the memories could go on and on and then I might start trying to find substitute replacements for all the old stuff so as to re-live the good old days. Got to accept the world as it is, somewhat... wait a minute... this is drifting into philosophy. Got to remain focused....
Niel, it's early here in Connecticut, and
the sun isn't up yet. But I will be back
with comments about the equipment you mentioned.
Back in the late 1960s, I was dying for a good
FM DX receiver, and it would be many years
before I acquired one. I remember wanting
an Eico, or a Lafayette FM tuner - badly.
I'll be back with more comments soon.
Best Wishes,
Bruce, DOGGRADIO STUDIO 2