The AM Forum

This particular Part 15 "AM Broadcaster" project fascinated me when as a kid, I read the original article in ELECTRONICS ILLUSTRATED. It's a 12AY7 driving a 12AX7, rf output coupled to AC house wiring. But I imagined modifying it for use with a real aerial and being the neighborhood DJ.

Sorry about the slow QSL.NET download. Anyone with a faster server who wishes to "adopt" these files is most welcome to.

(I skipped page 3 because it was just an advertisement)

http://www.qsl.net/wb1gfh/bcst1.jpeg
http://www.qsl.net/wb1gfh/bcst2.jpeg
http://www.qsl.net/wb1gfh/bcst4.jpeg
http://www.qsl.net/wb1gfh/bcst5.jpeg

Interesting. Looks like a modulated oscillator though. Would be pretty unstable and wouldn't take much audio without FMing.

Here's a better design.

http://www.amwindow.org/misc/gif/xmitter3.gif

:D.......Yeah it's funny they never mentioned FM'ing and drift in those EI articles. But for a kid playing around wth a crystal mike and turntable, the "phono oscillator" projects were just great. Kinda like the "keyed oscillator" cw xmtrs. Ideal if you were just starting out homebrewing.

My first toy transmitters were homebrew, using parts salvaged from broadcast radios.  My first attempt was an unmodulated oscillator.  Later, tried to modulate it, but it sounded crappy.  I ended up with a MOPA that sounded pretty good.  Don't quite know what kind of modulation I used, just built an AF amplifier stage, and kept poking audio around in the PA stage till I came up with something that modulated the carrier.  As I recall, it was in the control grid circuit, so I probably accidentally applied some kind of grid modulation.  With 30' or 40' of antenna, I could get good enough a signal that I could get on my bike with a portable radio and ride about a mile before it faded out.  

I would sometimes spin  records or just relay some other station from a nearby receiver.  Using original broadcast receiver tuning capacitor and oscillator coil, it would cover the entire BC band with one tuning knob, and track well enough to put out about the same strength signal on the bottom end and on the top end.

There was an old guy down the road who used to warn me: "The F.I.C.C. is gonna get you."

I also lusted for the little broadcaster when I was a kid with no money. About 10 years ago, I built it up. What a major disappointment. Drift, hum, and a range of about 20 feet. Forget about turning your house wiring into a giant antenna. It just didn't shake down that way. I could get about 20% modulation before the FMing made the distortion unbearable. I can email you photos if you're interested.

73,
Darrell

I loved the project-view illustrations in EI and PE of the 60's. As a kid, I used to copy them. I had a talent for art, and got quite good at imitating this style of drawing. Later I talked my way into Raytheon on the basis of these sketches and got a job creating 3-D exploded views (or Illustrated Parts Breakdowns as they were known). As I recall my first assignment was Navy Aegis radar system subassemblies. My boss handed me a stack of 2-D blueprints to work from. I didn't know how to read prints, but since I told him I did, I had to make good. So I secretly stuffed the wad of (classified) blueprints into my bag and took them home to study. With the help of a library book, I figured out how to interpret them. I'm sure if I tried that stunt today, it'd probably land me in Gitmo.  :D

Nah, Lewisburg is more like it.

Some of the hand-drawn art work was amazing. Even with CAD I don't think most mags these days beat it.

My drafting professor in college did the schematics and drawings for the Collins S-lines.  The #1 priority as set forth by Art Collins was readability and logical flow of the circuitry.  Modern CAD skizmatics and drawings don't hold a candle.

73 John

Here's another one I found stuck in the gigabytes of crap on my computer. Modulated oscillator and no power transformer.

(http://www.amwindow.org/misc/jpg/515.jpg)

Here you go. ;)

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=58295&item=7523554681&rd=1

My homebrew ones always used series string filaments, from the old "All-American Fivers."  Never tried a power transformer version.  One that I recall used 50L6 rf output, 35Z5 halfwave rectifier, 12SQ7 (triode section) rf oscillator, another 12SQ7 mic preamp, and some other 12-volt tube (don't remember which one) as control grid modulator.  One side of 110v line was connected directly to the chassis, and the thing had no cabinet.  I even used it outdoors a few times!

I bought mine from Lafayette Electronics Catalog, I think it was $4.95 or something like that. It had two tubes in a small bent steel chassis. I learned about hot chassis from that thing, when I tried to use it to rebroadcast audio from my grounded Knight Kit Star Roamer, my first kit project. As soon as I grabbed the headphone output and plugged it into the phone jack on the Lafayette, fireworks occured.

Another kid and I both had our radio stations going, a block apart. We could barely hear it at the other end, on a good day, but had a lot of fun playing 45s and speaking over a mike. This would have been in the late 1960s I think. Same time Neil Armstrong was walking on the moon.

Yes I had the Lafayette phono oscillator too. Ya can't beat a hot chassis and exposed AC connections for fun! I started drawing up schematics on how to feed the signal into the power lines when I realized I'd probably end up killing myself so I abandoned the idea in favor of stuffing a bunch of cherry bombs into a cardboard toilet paper tube and calling it a rocket.

Here's pics of a nice old RCA  one tube phono oscillator..............Larry


(http://home.comcast.net/~gerboid/rcaosc.jpg)