KDKA AM Pocket Radio

On the weekend I got this pocket AM radio from a Saint Vincent DePaul thrift store for 99 cents. My guess is it was a free giveaway at the Pirates' stadium, one of those "First 1000 fans at the game get a free AM headphone radio!" game nights. Add a little beer sponsorship to help cover the costs and it made a nice little gift to get people out to PNC park.

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The radio is mono headphone only, no speaker. This was a surprise, since most pocket radios that look like this have speakers! In a way this is good, most listeners at a game would be listening on a headphone anyway to be able to hear play-by-play over the crowd, and it also saves battery power by having a lower power audio amp. The case is just molded plastic with no trimmings or textures.

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It looks like a simple and traditional superheterodyne, a classic circuit in all kinds of radios, and the detector appears to be a silicon diode, not germanium which is the better choice for a detector, with lower distortion. The ferrite bar in the loopstick antenna is not the smallest I've seen.

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The board is marked "TS 314". The tuner is on the top, tuning radio stations from 540 to 1600 khz. Circuit board is the usual phenolic, power is 3 volts from 2 AA cells. Vintage is unknown, and a friend thought it could be from the 1980s.

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KDKA is the flagship station of the Pittsburgh Pirates and operates at 50,000 watts at 1020 on the AM dial.

Regenerative Radio

How about a radio that uses a dozen parts, including one transistor and a battery? I built one last night, it's called a regenerative radio, meaning it uses radio frequency feedback to build up signal levels. This technique goes back 100 years when radio tubes were new, and circuits were simple, as a way to get the most out of one tube.

Regeneration works sort of like an open microphone in an auditorium, when you hear echo and squealing, and the PA operator has to adjust the mic level to stop the feedback. In a radio circuit, the feedback is almost instantaneous, and before the circuit breaks into outright oscillation, it can amplify a radio signal 100,000 times or more.

A regenerative radio needs a tuned circuit to select a radio frequency, an amplifier, a way to feed the amplifier's output back to the input in a controlled manner, and a way to demodulate the signal into sound that you can hear. I found the base circuit I wanted to use in a ham radio publication, looking for something simple, and without any audio transformers or hard-to-find germanium detection diodes needed.

modern-regen-schematic.gif

In my case a loopstick antenna coil from a pocket radio was used, and since I want to listen to the AM broadcast band, 530 to 1700 khz., it's just right. For the tickler I used the output coil that's already wound on the loopstick. Main tuning and regeneration capacitors are the kind used in pocket AM radio tuners, with two sections, 140 and 60 picofarads. The FET is an MPF 102, a very common j-fet for radio circuits. The RFC choke coil is measured at 4 millihenries, and off of a circuit board from an old TV. Many regenerative radio designs call for a 2.5 millihenry choke, and some use a resistor instead of the choke. For audio output, I just fed the source lead of the FET into the + side of a 10 uf electrolytic capacitor, and to some RCA jacks, feeding the line-in on a boombox as an amplifier. Many circuits feed a transistor or IC audio amp and speaker or headphones. Power is from a 9 volt battery.

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Boomer's regenerative tuner, click for a bigger picture

It worked right away, I had sound as soon as it was set up and the battery was connected, and I could hear local AM stations! On the strongest stations I'd estimate the audio output to be around 100 millivolts. Fidelity was unlike anything you hear out of mass produced AM radios today, with lots of upper end frequency response, not the usual telephone quality audio. In fact, the highs are too stong, and that's because stations use pre-emphasis on the treble range, similar to the boost put on vinyl records to reduce the level of surface noise. It works because the receiver pulls the treble range back down again to normal levels, reducing any noise that happens between the transmitter and receiver. I'd like to try an audio stage that has the complimentary de-emphasis built into it, along with amplification for weaker signals.

Tuning only went up to somewhere in the 900 khz. area, so I'll need to remove the one of the capacitor sections, the 60 pf side. Regeneration is too strong as well, close to oscillation even at the lowest capacitance. That must be due to the loopstick's output coil being too close the tuning coil, so I'll make a tickler that can be slid along the coil and moved farther away. With refinements, I think this could make a nice hi-fi AM broadcast tuner for local stations.

References:

https://en.wikipedia.org/wiki/Regenerative_circuit

http://www.arrl.org/files/file/Technology/tis/info/pdf/9811qex026.pdf

Boomer

LPB TCU-30 carrier current coupler

Commercial equipment for carrier current broadcasting. Carrier current means that instead of sending your radio signal to an antenna, you place it on already existing wires, most commonly power lines that enter a building. From there the signal might feed up the power lines and into the power grid, to reach radios near the lines.

Carrier current was common with student radio stations on college and high school campuses, but it can also be used for community broadcasting in a neighborhood; you don't have to be a school to broadcast by carrier current. No license is required or available as it's considered a Part-15 radio service, in the same league with cordless phones and garage door openers.

Carrier current broadcasting is mostly done on the AM broadcast band and was used in the early days of radio as a kind of cable audio service. During World War II, radio transmissions by the public were forbidden, so amateur radio operators used carrier current to communicate with each other over power lines. Colleges experimented with their own stations in dorms and other areas, where they proved to be successful at reaching the close-in populations on a campus, so the practice spread to many schools in the USA after the 1940s.

In 1960, Low Power Broadcasting, or LPB was formed to serve the college radio market, building transmitters, power line couplers and other equipment for school broadcasting programs. They had a reputation for solid, high quality equipment for about 40 years, and virtually locked up the college market.

This is a review of their TCU-30 Transmitter Coupling Unit  (click on any photo to view it larger)

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Here's the mean looking 3 phase power cord. Note that this is for coupling the radio signal into the power grid, not to power the unit as it might imply! Typically the TCU-30 coupler unit, along with the transmitter, is mounted on a wall or in an equipment rack close to the power distribution panel for the building. This cord comes out of the bottom of the TCU, where it's plugged into a convenient 3 phase power receptacle, feeding all three phases, so that different parts of the building can be reached by the broadcast signal.

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SEMO is Southeast Missouri State University. So far I haven't been able to find any history on what AM broadcasting program they might have had, but the school's low power FM signal had been struggling to stay on the air. Rage FM, KDMC-LP went off air in 2014, SEMO surrendering the license to the FCC after 12 years of operation.

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The RF input connector, SO-239, receives the radio signal from the transmitter by coaxial cable. Will accept a signal from any AM band transmitter from 1 to 30 watts, with a 50 ohm output. Below 1 watt you might have trouble calibrating on the meter for best output, but it will still couple even a tenth of a watt signal to the line.

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Operating instructions sheet attached to the inside of the coupler's lid so it isn't lost, though some of these sheets will come unglued and flop down inside of the case.

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Cover off, view inside. There's not much in the way of open wiring; a double sided glass fiber circuit board handles all connections between the input cable and signal output to the lines.

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Closer view of the main board, pretty clean, no smoke or dust, so the unit was probably not operated in a boiler room like some of them were. Knobs are clean, even in the milled edges, so the coupler probably hasn't been used a whole lot, or it was adjusted and left that way.

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Multi-meter for combination of output wattage and VSWR. VSWR is Voltage Standing Wave Ratio, a measure of how much signal power is being reflected back from the power line it's connected to, and hence how much is getting out. Going by the chart, you adjust for a dip to the lowest VSWR, hopefully getting the meter into the green area. With careful tuning it might even be possible to reach the bottom of the scale, indicating that most of the power is getting into the line, and very little is being reflected. Guitarists might be drooling over those orange capacitors to the left of the meter! Those are the coupling caps to allow the RF to pass to the three phases of the line, while blocking most of the 60 hz AC voltage from feeding back into the tuner. The caps are rated at .1 microfarad at 1000 volts.

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Impedance selection toroid, a ferrite donut core wrapped with coils of wire having many taps, which are selected by the dial switch above to match the RF impedance of the power line, which varies widely, and is often pretty low, hence the selections for 72 ohms down to 1.1 ohms.

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Capacitance decade, in series with the signal to the line and used to tune out the reactance, or resistance of the line to the radio signal. Dipped mica capacitors are in a matrix that can be adjusted in fine steps for more or less capacitance.

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5 wire multifilar coupling toroid

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TCU-30 coupler's specification sheet. It can handle up to 30 watts!

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LPB equipment like this coupler is usually found on the used/recycler market after schools close their broadcasting programs. Since LPB equipment was engineered well and carefully built, many of these units are still in good condition or can be repaired easily to help you to get on the air. There are guides available to help you understand how to hook it up, rules to go by and how to stay safe while using it, since after all you're connecting to AC lines.

Boomer

Who's Who in PA radio 1927

From the estate of Peter Granba's trash can comes this Who's Who In Radio publication, 1927 Pennsylvania Edition, created as an historical document.

In the Preface, W. E. Johnson writes,

"I have tried to make this as complete a history of every prominent radio person and organization in the state of Pennsylvania, as possible."

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Who's Who In (Pennsylvania) Radio with 8CRK, 1927

8.5 x 11 sheets folded in half, thicker paper cover, 65 pages, illustrated with line-art drawings.

Sections include:

  • Broadcasting stations
  • Radio Artists and Orchestras
  • Radio Editors
  • Manufacturers and Jobbers
  • Radio Dealers
  • Amateur Operators

There are 34 Pennsylvania AM broadcasting stations listed, most running between 10 and 500 watts. All four of the Pittsburgh stations profiled in this book are still on the air and serving listeners' radios, which seems incredible 89 years later!

KDKA 970, now 1020 with a general talk format. They are a designated 'clear channel' broadcaster with 50 kilowatts.

KQV 1090, now 1410 and an All News format, which it's kept since the 1970s. Before that they broadcast rock and pop music. 500 watts then, 5,000 now.

WJAS 1090, now 1320. In 1926 it was sharing air time with KQV on 1090. Looking at their schedules, one station would broadcast for as little as a half hour, leave the air, then the other would sign on 30 minutes later, and they'd alternate throughout the day. Had a lengthy broadcast schedule during prime-time hours at night, 8-11 pm.

In the 1970s, WJAS jumped on the rock and roll bandwagon and became WKTQ, '13Q', with hot pop, contests and giveaways. By the early 1980s, most pop music and its younger listeners had moved over to the FM band, so the station got the WJAS call letters back, starting a 'Music Of Your Life' adult standards format, and picked up some of the veteran TV and radio personalities from around town, like Bill Cardille and Jack Bogut. In 2014 WJAS picked up a conservative talk format that had been dropped by an FM station. 500 watts then, 7,000 now.

WCAE 650, now 1250. Was WEAE, then WTAE with adult contemporary music and variety. It went to talk, and then Radio Disney as WDDZ, running AM HD radio for a while. 500 watts then, now 5000.

Most stations started with a live, variety format and local news, and weren't run by radio companies, but as a sideline or promotion for another business, such as a local store or hotel. A hotel could conveniently be used as a performance space for musicians and other acts that would then be broadcast.

Here's a complete scan Who's Who In Radio so you can take a journey down memory lane, thanks to Peter Granba's trash can! He's an Amateur Radio operator (ham) in this book, but I like the pride he feels for his own radio station, writing "Radio Station 8CRK, Ambridge Pa." in the front and back of the book, then signing his name.

Whos-Who-In-Radio-1927.pdf

Note, if the pdf file looks out of focus or pixelated in your browser, please download it and use the viewer on your computer, thanks.

Boomer

Radio 1946

Radio - Design, Production Operation, a technical magazine from 1946

I've always liked collecting old radio books and magazines, from kid's science project books to Popular Electronics.

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I enjoy reading the scans done by other radio freaks trying to preserve history, so here's a contribution from me, scanned several years ago. It's a trade magazine called Radio, with 56 pages, written for engineers and technical people. One could say it's like a 1940s version of Radio World.

I believe that this magazine came from the trash of Peter Granba here in Pittsburgh. In the 1980s there was a big curb side trash pile on a main road nearby, and the trove was so good that I asked my dad to drive me there so that I could pick up some of the radio-related items. Granba was apparently a Ham radio operator and electronics experimenter.

April 1946 contents include:

  • Cover: Western Electric's 'Clover-Leaf' FM broadcasting antenna
  • Inductive Tuned Loop Circuits
  • IF Amplifier Stability Factors
  • Electro-Mechanical Analogy In Acoustic Design
  • AM Transmitter Design
  • D-F For Static Pulses
  • Recent Radio Inventions
  • Bridged T And H Attenuators, Diode Conduction
  • Loss Due To Shunt Or Series Resistance
  • Capacitor Machine, Co-ax And TV, FM With Non-Linear L

In 1946, television and FM seemed to be well on their way, with the FM antenna tower on the cover, and high power FM, VHF, UHF and microwave tubes for airplane instrument landing being detailed. One advertisement is for the Eimac 3X2500A3 external anode tube, capable of 3500 watts output from 88-108 mhz, the modern FM band.

My favorite article is Modern AM Transmitter Design starting on page 30, which is about the latest 250 watt broadcast transmitter, as used in many communities across the country. It's fan-less, so it can be placed right in the studio, uses motorized final amplifier tuning, a high level plate modulator, and has a frequency response from 30 to 10,000 hertz.

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250 watt AM band transmitter

Get the full draft quality but readable PDF scan here:

Radio-Design-Production-Operation-1946.pdf

Boomer

Crosley radio

The story of my Crosley Model 125 'Litlfella' AM table radio, 1931-32

One day long ago I was hiking in a wooded area a few miles from my house, and came across a dilapidated little barn. One side of it was open and I peeked in and saw a big old radio sitting on a shelf, or a pile of wood. As you'd imagine, everything was gray with dust and webs, but I could tell what it was, I'd seen radios like it in old movies, where curved-top sets like that were part of the decor in parlor rooms.

I don't know why, but I left for home and came back later on my bike. I was around 11 or 12, and went everywhere on my one speed bike, so maybe I thought I couldn't carry a 17 pound radio all the way home without wheels. I sat it on the bike's seat and wheeled it home.

The Crosley had a couple of issues, the cloth behind the speaker opening was all torn up, though the speaker cone looked okay. A bigger problem was cracks in the '80' rectifier tube's glass envelope, which meant the radio couldn't possibly play without a replacement tube. My theory at the time was that since the power cord had been stored in the back of the radio, the plug had hit the tube, but I'll never know for sure.

For a speaker cloth, I went to mom's fabric box, where she kept lots of carefully folded scraps for patches to mend clothes with. I got something I thought would look good in the radio, took out the speaker and put the cloth in front of it. It looked a little like Hawaii, but I thought it was okay at the time, and I'm glad I put it in, since it's protected the speaker cone from damage over the years.

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After that I stored the radio on the bar top in the basement, waiting until I could get an 80 tube for it. At the time I wasn't connected with the radio scene or collectors, so I didn't know how to get a tube, just thinking antique stores might have it, or I'd find another radio that had a good 80, but I never did. Years later I was at a radio gathering and made friends with someone who had an 80 tube, and he offered to send it to me free of charge! Thanks Workingman.

The new 80 brought the radio to life, the tubes lit and it had some hum and a scratchy volume control, but in spite of that the tuner was able to pull in a few local stations, weak but listenable. I put the radio away again and thought that sometime later I could restore it more. I was writing with a friend at Hobby Broadcasting's blog about his old radio restorations, and he mentioned that radio companies have made replacement speaker cloth for these old radios more recently, and asked what kind model of Crosley it was. I didn't know and went to check, and thought I might as well do a photo shoot while I was at it, inspiring this article.

It's interesting to think about the history of this radio, how it probably sat in someone's parlor and was the main source of electronic information a household would have at the time. Television wouldn't come to Pittsburgh for another 16 years yet, 1948.

Related links:

Here's a small gallery with more pictures of my Crosley:

http://boomerthedog.net/crosley/crosley-model-125.html

Someone has restored their 125, different cabinet, looks to be the same chassis:

http://mcclellans.com/Crosley125LitlfellaRestoration.htm

The 125's schematic:

http://www.nostalgiaair.org/PagesByModel/413/M0043413.pdf

Hobby Broadcasting's blog

http://hobbybroadcasting.blogspot.com/2014/02/philco-87-restoration-blog-thread.html

Boomer

Nice packing comrade!

I'm on the way to building an AM broadcast band synthesized PLL oscillator. What that means is I'll be able to choose a frequency between 530 and 1700 khz, set some little switches, and the oscillator will put out a stable signal at the chosen frequency. This small signal will feed a power amplifier to boost the signal, with a modulator attached to give it sound, making a small broadcast transmitter, an advanced version of the Radio Shack AM kit that I had.

I started to order parts for this project last month, and I was surprised at how these chips came packed. These are CD4060 oscillator/divider ICs, and are CMOS chips, which should be kept away from static electricity, yet these are packed in what appears to be a plain poly bag! They're probably okay, with protection diodes on their inputs, but I'll put the tea kettle on and make sure to open the bags in the presence of steam, to kill any static that might be generated, then put them into anti-static bags or foam. It's the dead of winter here, very dry air and the worst time to be handling static-sensitive parts.

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Boomer

AM Radio Broadcasting Kit

With Radio Shack on the brink of RIP, I thought it would be a good idea to remember them by writing about one of my favorite kits.

I like to build small transmitter circuits for the AM broadcast band, something that started when I was 12 with a Science Fair AM Broadcaster kit, just like the one in this ad.

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from radiogeekheaven.com

DJs were hot stuff on the radio; I liked listening to them and wanted to do it too! This was a pretty easy transmitter to build, the most important thing was to get the transistor leads into the correct holes, and wrap them on the springs on the underside of the kit's deck, which in its own way was a double sided circuit board.

The transmitter worked well enough, my 'city grade' coverage radius was the next room and down one floor, but it was fringe outside the house, using my pocket radio as field intensity meter. At this rate, I'd never be able to build a fanbase or any quarter hour listening, the signal didn't even reach to the neighbor's house! Ultimately I was able to reach several houses down the street, but that required higher level engineering, like wrapping the regulation green antenna wire that came with the kit around our rotary telephone, a trick I'd learned from building crystal receivers to get them to receive more stations, with the phone and its line acting as a longer antenna.

That kit was a lot of fun, and a great intro to broadcasting for a younger pup, but it suffered from a problem that all of the simple AM transmitter kits did then, poor frequency stability. The very same tuner knob system that let you dial in any open frequency in your area was its biggest shortcoming too, it would drift over onto other stations causing beat tones, and when sound was applied to the microphone, the circuit would produce both AM AND FM, since the audio was affecting the frequency too. Tuning across the signal with an analog dial radio, it was bright, loud and clear on each side, with a 'hole' right in the middle, where the sound was weak and distorted. I didn't understand that my transmitter was 'FMing', and that I was hearing it through 'slope detection', but with the split signal I thought I'd created my own version of AM stereo!

I built other oscillator transmitters like this, including a couple of tube circuits, by re-wiring the guts of old AM table radios, but all of them had problems with drift and FM, and didn't sound like other stations on the dial. I knew that I wanted something better, but didn't know how to get it. Later I found out about quartz crystals from things like walkie-talkies, but didn't know how to apply that idea to the AM broadcast band to get a transmitter to lock on frequency.

Next I want to get into circuits I'm trying today, for both full tuning across the AM band, and high stability at the same time, stay tuned!

Boomer