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39 Articles

Retrotechtacular: The Spirit Of Radio

May 5, 2015 by 11 Comments

Many of us still tune in to terrestrial radio for one reason or another, be it baseball games, talk radio, or classic rock. But do you know how the sound is transmitted to your receiver? This week, our spotlight shines upon a short film produced by KYW Radio that serves as a cheerful introduction to the mysteries of amplitude modulation (AM) radio transmission as they were in 1940.

Sound vibrations enter a microphone and are converted to electrical current, or an audio waveform. The wave is amplified and sent several miles away to the transmitting station. During this trip, the signal loses power and so is amplified at the transmitting station in several stages. This audio wave can’t be transmitted by itself, though; it needs to catch a ride on a high-frequency carrier wave. This wave is generated on-site with a huge crystal oscillator, then subjected to its own series of amplifications prior to broadcast.

The final step is the amplitude modulation itself. Here, the changing amplitude of the original audio wave is used to modulate that of the high-frequency carrier wave. Now the signal is ready to be sent to the tower. Any receiver tuned in to the carrier frequency and in range of the signal will capture the carrier wave. Within the reciever, these currents are converted back to the vibrations that our ears know and love.

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Vacuum Tube

Vacuum Tube Repair After A Spectacular Failure

January 17, 2015 by 33 Comments

[Eric] has an Atwater Kent 55C AM radio from the early 1900’s. He’s been trying to restore the radio to proper working condition. His most recent pain has been with the rectifier tube. The tube is supposed to have a complete vacuum inside, but that’s not the case here. When the tube is powered up, it glows a beautiful violet color. It may look pretty, but that’s indicative that gas has leaked into the tube. It needed to be replaced.

[Eric] had a tube that would serve as a good replacement, but it’s plug didn’t fit the socket properly. He was going to have to use this old broken tube to make an adapter. Rather than just tearing the old tube apart, he decided to have some fun with it first. He hooked it up to a variac, an ammeter, and a volt meter. Then he slowly increased the voltage to see what would happen. The result was visually stunning.

The tube starts out with the same violet/blue glowing [Eric] experienced previously. As the voltage increases, it gets more and more intense. Eventually we start to see some green colors mixing in with the violets. [Eric’s] reaction to this unexpected result is priceless. As the tube gets increasingly hot, the anode starts glowing an orange-red color. Finally, the filament starts to crackle like a sparkler before the tube just gives up and completely fails.

After the light show, [Eric] moves on to replacing the tube. He begins by tapping on the old tube’s socket with the end of a screwdriver. After much tapping, the glass starts to come lose from the socket. After a bit of wiggling and twisting the tube finally came free from the socket. [Eric] luckily had an unused octal socket that fit perfectly inside of the old socket. All he needed to do to build his adapter was to connect the four pins from the old adapter to the proper pins on the octal socket. Piece of cake.

…Or so [Eric] thought. After testing some new tubes with a tube tester, he realized he had soldered all four pins incorrectly. On top of that, he had super glued the adapter together. He eventually got the two pieces apart. This time he removed all of the unused pins from the octal socket so he wouldn’t get it wrong. Another run on the tube tester confirmed that everything looked good. After plugging the tube into the radio, it worked just as expected

If you need fabrication rather than repair, we’ve got you covered there as well. Check out [Charles Alexanian’s] process for making new vacuum tubes in his garage. Now if you just have too darn many of them around, you can always decorate your pad with ’em.

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Dusty Junk-bin Downconverter Receives FM On An AM Radio

October 6, 2014 by 10 Comments

This amateur radio hack is not for the faint of heart! With only three transistors (and a drawer-full of passive parts), [Peter Parker, vk3ye] is able to use a broken-looking AM car radio to receive FM radio signals (YouTube link) on 2 meters, an entirely different band.

There are two things going on here. First, a home-made frequency downconverter shifts the 147 MHz signal down to the 1 MHz neighborhood where the AM radio can deal with it. Then, the AM radio is tuned just slightly off the right frequency and the FM signal is slope detected.

The downconverter consists of a local tuned oscillator and a mixer. The local oscillator generates an approximate 146 MHz signal from an 18 MHz crystal, accounting for two of the three transistors. Then this 146 MHz signal and the approximately 147 MHz signal that he wants to listen to are multiplied together (mixed) using the third transistor.

If you’re not up on your radio theory, a frequency mixer takes in two signals at different frequencies and produces an output signal that has various sums and differences of the two input signals in it. It’s this 147 MHz – 146 MHz = 1 MHz FM signal, right in the middle of the AM radio band’s frequency range, that’s passed on to the AM radio.

Next, the AM radio slope detects the frequency-modulated (FM) signal as if it were amplitude modulated (AM). This works as follows: FM radio encodes audio as changes in frequency, while AM radios encode the audio signal in the amplitude, or volume, of the radio signal. Instead of tracking the changing frequency as an FM radio would, slope detectors stick on a single frequency that’s tuned just slightly off from the FM carrier frequency. As the FM signal gets closer to or farther away from this fixed frequency, the received signal gets louder or quieter, and FM is detected as AM.

At 5:23, [vk3ye] steps through the circuit diagram. As he mentions, these are old tricks from circa 50 years ago, but it’s very nice to see a junk-box hack working so well with so few parts and receiving (very) high frequency FM on an old AM car radio. A circuit like this could make a versatile front end for an SDR setup. It makes us want to warm up the soldering iron.

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Retrotechtacular: Fundamentals Of AM And FM Radio Communication

September 24, 2013 by 20 Comments

How radios send and receive information can seem magical to the uninformed. For some people, this week’s Retrotechtacular video, “Frequency Modulation – Part 1 Basic Principles”, from the US Army Department of Defense 1964 will be a great refresher, and for others it will be their first introduction into the wonderful world of radio communications.

The stated objective is to teach why FM radio communication reduces interference which normally afflicts AM radio communications. Fundamentals of AM and FM is a better description, however, because the first part of the video nicely teaches the principles of AM and FM radio communications. It isn’t until later in the clip that it delves into interference, advantages of FM modulation, and detailed functioning of FM radio. The delivery is slow at times and admittedly long, yet the pace is perfect for a young ham to follow along with plenty of time to soak in the knowledge. If you’re still on the fence about becoming a ham here’s some words or encouragement.

Though the video isn’t aimed at ham radio users it does address core knowledge needed by amateur radio hobbyists. Amateur radio is full of many exciting communication technologies and you should have a clear understanding of AM and FM communication methodologies before getting on Grandpa’s information super highway. Once you have your ham license (aka ticket) you have privileges to create and test amazing ham related hacks, like [Lior] implementing full programmable control of a Baofeng UV5R ham radio using an Arduino.

Join us after the break to watch the video.

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AM Tube Radio Restored And Given MP3 Playback Too

May 2, 2013 by 46 Comments

mp3-antique-radio

This AM radio looks a bit like it did coming out of the factory. But there are a lot of changes under the hood and that faceplate is a completely new addition. The project really is a restoration with some augmentation and [Michael Ross] did a great job of documenting the project.

The Kenyon radio was built in 1946 and uses vacuum tubes for the amplifier. Considering its age this was in relatively good shape and the first thing that [Michael] set out to do was to get the electronics working again. It involved replacing the messy collection of capacitors inside. He then cleaned up the tubes, checking for any problems, and put the electronics back together to find they work great!

He cleaned up the chassis and gave it a new coat of finish. The original dial plate was missing so he built a wood frame to match a dial scale he ordered. The bell-shaped brass cover hides the light that illuminates the dial.

He could have stopped there but how much do people really listen to AM radio these days? To make sure he would actually use the thing he added an Arduino with an MP3 shield. It patches into the antenna port via a relay, injecting modern tunes into the old amplifier circuit. Catch a glimpse of the final project in the video after the break.

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AM Chiptunes Played By A Modified Antenna Analyzer

October 26, 2012 by 5 Comments

Believe it or not, this VK5JST aerial analyzer kit is going to rickroll you. [Erich] wanted to see if he could use the device in a different way. His adventure led him to use it to feed different tones to an AM radio, producing the all too familiar [Rick Astley] offering.

There’s a fair bit of math that goes into getting the correct signals to generate a given pitch. But it basically boils down to patching into the hardware early in the RF generation. This way an audio signal can be rolled into the carrier frequency. Since this kit uses a PicAXE microcontroller with available source code it is rather easy to add audio input to tweak what the chip is putting out. But there is also some hardware tinkering to be done. Read more about that at the article linked above, and don’t forget to check out the bottom of that page to hear the final results.

AM Singer: A Tiny AM Transmitter

July 30, 2011 by 32 Comments


[Simon Orr] wrote in to tell us about his AM transmitter prototype that he plans to put into production in a few months. The build is based on an “Easy AM Transmitter” featured in this Instructables article.

Interestingly enough, this device is capable of transmitting in the 100KHz to 480KHz frequencies. The AM band goes from 520 KHz to 1610KHz, so in order to hear this signal, one must actually tune the radio to twice the emitted frequency. This allows one to tune into the harmonic frequency and receive a signal in this range.

Using the harmonic frequency to transmit is an interesting concept by itself. Additionally, the idea that one could build this device with or without the kit in the future should appeal to experienced hackers and those just starting out alike. Check out the “AM Singer” prototype video after the break. Continue reading “AM Singer: A Tiny AM Transmitter” →