Tag Archives: Bob Colegrove

Guest Post: Remembering the Radio Shack TRFs

Many thanks to SWLing Post contributor, Bob Colegrove, for the following guest post:


Remembering the Radio Shack TRFs

As recalled by Bob Colegrove

There has always been an interest in DXing on the cheap.  At the same time, most of us don’t want to sacrifice any more capability than necessary.  In the late 1970s and the early 1980s, Radio Shack provided an attractive answer to this conundrum for medium wave DXers.  These were identified respectively by their Radio Shack stock numbers 12-655 and subsequently the 12-656A.  I remember them being very popular among National Radio Club members of the time.

These radios were commonly known by their sobriquet “TRF.”  Initially applied by Radio Shack itself, the term stuck.  TRF stands for tuned radio frequency receiver.  In the early days of radio, the term referred to the necessity for the operator to manually put an RF amplifier stage on frequency by adjusting the value of a variable capacitor or inductor.  As amplifier stages became cascaded in two or three stages, this became a real problem, as each stage had to produce the correct frequency before anything could be heard.  Eventually, designers hit on the idea of mechanically connecting all the RF stages together so tuning could be accomplished with a single knob.

Fast forward to the standard AM radios of a later generation.  Entry level (read cheap) radios were limited to two stages consisting of a converter and an oscillator.  This was standard design practice during the vacuum tube and transistor eras.  Better, more sensitive radios added a third stage, an RF amplifier operating ahead of the converter stage.  Obviously, this required more circuitry and, consequently, more expense.

Enter the Radio Shack TRFs.  The term TRF was a throwback to the days of the tuned radio frequency radios and referred specifically to Radio Shack’s addition of an extra RF amplifier ahead of the converter stage.  The TRFs were by no means the first radios to have this feature, but they were obviously marketed to folks who wanted longer than normal distance reception.  Further, the radios were AM only uncompromised by FM circuitry, which would have to be integrated into the design and provide a distraction at best and a performance compromise at worst.

I didn’t discover these treasures until late in their production cycle.  Consequently, my comments are mostly focused on the 12-656A.  In later times, the -656A retailed for $34.95.  On final clearance this dropped to $25, and I snapped up several for my friends and my own tinkering.  The internal layout was not especially good for repair or modification, but at least was well within this enthusiast’s capability.

The picture below shows the dial of the -656A.  The radio was designed prior to the expansion of the AM broadcast band to 1700 kHz; thus, it is only specified to cover 520 kHz through 1620 kHz.  Although I don’t recall ever having tried it, the circuitry could possibly be coaxed to 1700 kHz.  On one of my “hot rod” units, I replaced the silk-screened dial with a plain piece of Plexiglas backed with a hand-calibrated dial, which permitted accurate calibrations for 10-kHz channel identification.  The tuning knob and dial cord mechanism work very well right out of the box.

The controls are aligned along the right front side of the radio.  Below the tuning knob are tone and volume controls followed by an off/on switch.  The 655 is similar, except the tone switch is replaced by a slider potentiometer like the volume control.  In retrospect, the off/on switch should have been recessed into the front panel, as it is easy to accidentally turn on the radio with the protrusion of the switch.

The back of the cabinet (below) features standard 1/8” phone jacks for earphones (left) and an external antenna (right).  The TRFs may be powered by either four C cells or from 115-Vac mains.  Rather than having a separate 6-Vdc wall wart, the ac power supply components, cord and all, are contained inside the radio.  For storage, the power cord is simply wrapped up in its own compartment next to the battery compartment.

Below is a tinker’s view of the innards of a -656A.  Could the box have been made smaller?  Obviously, one could replace the internal ac power components with a wall wart.  Perhaps the C batteries could be replaced with AAs.  Problem is with the speaker.  The PC board hides a large drum for the tuning mechanism.  Given the smooth tuning and large dial, the tuning arrangement is not something I would compromise.  So, stacking the speaker with the PC board would require a much thicker box.

The external antenna uses the standard approach of a small transfer coil wrapped around the ferrite bar for signal transfer.  Just as an aside, keep in mind for any long- or medium-wave radio having an external antenna coupled to the internal ferrite bar, the ferrite bar will remain active with the external antenna attached.  The external antenna is effective only to the extent that the phase and amplitude of its signal compliment or reduce that produced by the ferrite loop.

In addition to two intermediate frequency (IF) stages, the circuitry includes a 455 kHz ceramic filter in the base circuit of the first IF stage.  This provides very good selectivity.  Never satisfied the way things are, when I first got these radios, I duplicated a third IF stage in one of the units.  The result was a nice tight bandwidth still providing good audio.

The “improvements” in the -656A seem to be focused on the reduction of production costs.  I’ve already mentioned the tone switch for one.  Another example is the replacement of discrete components in the -655’s audio amplifier with a whopping 0.5-watt integrated circuit in the 656A.  Having no way to make a performance comparison, I will say my experience with the -656A is that it is still a very hot radio.

The TRFs filled a relatively minor marketing niche, namely DXing enthusiasts and perhaps a small number of expatriates who wanted to listen to broadcasts from their old hometowns.  The format evolution of medium wave broadcasting was already well on the way toward news-talk-ethnic broadcasting, and the appeal to rock ‘n rollers or virtually any music lovers just wasn’t there.  A sensitive radio had to include an FM band.  So, the TRFs faded into history sometime in the early ‘80s.

Enter the long-distance “superadios,” notably General Electric’s Superadios I, II, and III in the 1990s.  Radio Shack itself produced a clone-like Optimus 12-603.  The “super” by that time referred more to the audio quality than the sensitivity.  This later generation featured two higher-quality speakers packaged in a cabinet with somewhat better acoustics, separate base and treble controls, all driven by a higher-powered audio amplifier.

Having worked in the industry for a couple years as an assembler, I have never been convinced that inclusion of multiple band coverage does not result in some performance compromise.  Radios such as the TRFs have a special appeal to me.  The General Electric P780 is another well-regarded example of an AM-only, high-sensitivity, radio.  Maybe you have a favorite of your own.  If you’re into tinkering, and even if you’re not, a functional TRF or such radio can provide a lot of cheap entertainment.

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Bob’s Updated Passive, Resonant, Transformer-Coupled Loop Antenna for Shortwave

Figure 1. A Passive, Resonant, Transformer?Coupled Loop Antenna for Shortwave

Many thanks to SWLing Post contributor, Bob Colegrove, for the following guest post:


A Passive, Resonant, Transformer?Coupled Loop Antenna for Shortwave

By Bob Colegrove

Over the years I have resisted the level?of?effort necessary to construct and maintain outdoor antennas. Rather, I have focused on squeezing out all of the microvolts I could get inside the house. Many years ago I had access to a well?stocked engineering library, and used my advantage to gather information about the theory and development of loop antennas – a daunting undertaking for an English major. Ultimately, by adhering to a few basic rules, some of them dating back 100 years, I found quite acceptable performance can be had with an indoor passive antenna intersecting just a few square feet of electromagnetic energy.

Theory

There are a couple of advantages of resonant loops as opposed to non?resonant ones. The first is the fact that the signal dramatically increases when you reach the point of resonance. The second follows from the first in that resonance provides a natural bandpass which suppresses higher and lower frequencies. This gives the receiver a head start reducing intermodulation or other spurious responses. The downside of all this is that the resonant loop is, by design, a narrow?band antenna, which must be retuned every time the receiver frequency is changed by a few kHz. On the other hand, there is nothing quite as rewarding as the sight (S?meter) and sound you get when you peak up one of these antennas – you know when you are tuned in.

There is nothing new about the loop antenna described here. It’s just the distillation of the information I was able to collect and apply. There are a number of recurring points throughout the literature, one of which is the equation for “effective height” of a loop antenna. It basically comes down to the “NA product,” where N is the number of turns in the loop and A is the area they bound. In other words, provide the coil with as much inductance as possible.

Unfortunately, for resonant loops, the maximum coil size diminishes with frequency.
With this limitation on inductance, the challenge becomes minimizing unusable capacitance in the resonant frequency formula in order to get the highest inductance?to?capacitance (L/C) ratio possible. Some of the unusable capacitance is built into the coil itself in the form of distributed capacitance, or self?capacitance between the coil turns. This cannot be totally eliminated, but can be minimized by winding the coil as a flat spiral rather than a solenoid, and keeping the turns well separated.

The second trick is with the variable capacitor. Even with the plates fully open, there is residual capacitance on the order of 10 to 20 picofarads which can’t be used for tuning purposes. A simple solution is to insert a capacitor in series, about 1?4 the maximum value of the variable capacitor. This effectively decreases the minimum capacity and extends the upper frequency range. In order to restore the full operating range of the variable capacitor, the fixed capacitor can be bypassed with a ‘band switch.’ With the series capacitor shorted, the variable capacitor operates at its normal range and extends coverage to the lower frequencies. Continue reading

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Bob Colegrove on “The Joys and Challenges of Tuning Analog Radios”

Many thanks to SWLing Post contributor, Bob Colegrove, who recently shared this excellent article and has kindly allowed me to share it here in the the Post. Bob prefaced it by saying, “Being a retired technical writer, I started the attached article some time ago for my own amusement, but it quickly got out of hand.

“Got out of hand” in a very good way, Bob!

An excerpt from Bob’s article.

I love how this piece takes us through receiver history and explains, in detail, the mechanics and innovations. It’s also a very accessible piece that both the beginner and seasoned radio enthusiast can appreciate.

But don’t take my word for it, download it and enjoy!

Click here to download The Joys and Challenges of Tuning Analog Radios as a PDF.

Thank you again, Bob. This is a most enjoyable and informative read! This was obviously a labor of love. Thanks for sharing it with our radio community!

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AM Radio History: 80th Anniversary of the “Havana Treaty,”

Many thanks to SWLing Post contributor, Bob Colegrove, who writes:

Hi Thomas,

I came across this article on Wikipedia. It is a few days late, but thought it might be of interest to others. The link is

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

Briefly, this past Monday, was the 80th anniversary of the implementation of the “Havana Treaty,” which was actually signed on December 13, 1937, and finally implemented 80 years ago on March 29, 1941. It provided for reorganization of the “AM” medium wave band into frequency allocations for clear channel, regional and local stations.

AM radio was the Internet of its day. The invention of the telegraph notwithstanding, radio provided widespread, instant communication, albeit one way, to a vast population reaching hundreds of miles from the transmission source. It extended to the most rural parts of the country adding “A battery” and “B battery” to the lexicon.

The initial licensing process had been done with very little planning and forethought using 96 channels between 550 and 1500 kHz. The reorganization was the culmination of the need for some order to reduce mutual station interference and provide more reliable service to listeners. It involved frequency changes for about 1000 stations in several countries. March 29, 1941 was informally known as “moving day.”

The Wikipedia article details the changes made at that time and goes on to describe subsequent expansions of the AM broadcast band.

Fascinating! Thank you for sharing this bit of radio history with us, Bob!

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DIY: How to build a Passive Resonant Transformer-Coupled Loop Antenna for HF reception

We recently posted a tutorial on building a simple Noise-Cancelling Passive Loop (NCPL) antenna. This prompted SWLing Post contributor, Bob Colegrove, to share his excellent article on building a Passive, Resonant, Transformer-Coupled Loop (PRTCL) Antenna:


A Passive, Resonant, Transformer-Coupled Loop Antenna for Shortwave

By Bob Colegrove

Over the years I have resisted the level-of-effort necessary to construct and maintain outdoor antennas.  Rather, I have focused on squeezing out all of the microvolts I could get inside the house. Many years ago I had access to a well-stocked engineering library, and used my advantage to gather information about the theory and development of loop antennas – a daunting undertaking for an English major.  Ultimately, by adhering to a few basic rules, some of them dating back 100 years, I found quite acceptable performance can be had with an indoor passive antenna intersecting just a few square feet of electromagnetic energy.

Theory

There are a couple of advantages of resonant loops as opposed to non-resonant ones.  The first is the fact that the signal dramatically increases when you reach the point of resonance.  The second follows from the first in that resonance provides a natural bandpass which suppresses higher and lower frequencies.  This gives the receiver a head start reducing intermodulation or other spurious responses. The downside of all this is that the resonant loop is, by design, a narrow-band antenna, which must be retuned every time the receiver frequency is changed by a few kHz.  On the other hand, there is nothing quite as rewarding as the sight (S-meter) and sound you get when you peak up one of these antennas – you know when you are tuned in.

There is nothing new about the loop antenna described here.  It’s just the distillation of the information I was able to collect and apply.  There are a number of recurring points throughout the literature, one of which is the equation for “effective height” of a loop antenna.  It basically comes down to the “NA product,” where N is the number of turns in the loop and A is the area they bound. In other words, provide the coil with as much inductance as possible.  Unfortunately, for resonant loops, the maximum coil size diminishes with frequency.

With this limitation on inductance, the challenge becomes minimizing unusable capacitance in the resonant frequency formula in order to get the highest inductance-to-capacitance (L/C) ratio possible.  Some of the unusable capacitance is built into the coil itself in the form of distributed capacitance, or self-capacitance between the coil turns. This cannot be totally eliminated, but can be minimized by winding the coil as a flat spiral rather than a solenoid, and keeping the turns well separated.

The second trick is with the variable capacitor.  Even with the plates fully open, there is residual capacitance on the order of 10 to 20 picofarads which can’t be used for tuning purposes.  A simple solution is to insert a capacitor in series, about ¼ the maximum value of the variable capacitor. This effectively decreases the minimum capacity and extends the upper frequency range.  In order to restore the full operating range of the variable capacitor, the fixed capacitor can be bypassed with a ‘band switch.’ With the series capacitor shorted, the variable capacitor operates at its normal range and extends coverage to the lower frequencies.

Construction

I have constructed similar loops covering long wave, medium wave, and shortwave all the way up to about 23 MHz.  I wanted to optimize this loop for the most active portion of the shortwave spectrum. Consequently, it covers approximately 2.6 to 12.3 MHz.  See Figure 1.

Figure 1.  A Passive, Resonant, Transformer-Coupled Loop Antenna for Shortwave

Figure 2 is a schematic diagram of the antenna.  Cd (in red) is the distributed capacitance of the primary coil, L1.  This is not tunable capacitance, but it still contributes to the resonance; likewise, the 15 pf minimum capacitance of C1.  By adding C2, the minimum total capacitance can be lowered to greatly increase the upper range of the antenna. S1 is the ‘band switch.’  It shorts out the series capacitor, restoring the maximum low frequency.

Figure 2.  Schematic Diagram

Frame – The frame is made from 3/8”-square basswood or poplar dowel (see Specialized Parts).  Two pieces, each 36” long, have been predrilled at ½” intervals to accommodate the primary and secondary coil wire (think of a tennis racket).  It is a good idea to drill holes along the length of each dowel – more than you will need. You may decide to change things later on, and drilling holes in an assembled antenna is not easy.  Also the two dowels are notched in their centers to fit together. See Figure 3 and Figure 4. The clear plastic disk in Figure 4 is a packing disk from a spindle of CDs; it is cemented to the square dowels, and used to hold them at right angles.  Any rigid, light-weight material will do.

Figure 3. Square Dowel Showing 1?2” Hole Spacing and Lacing of Secondary Coil


Figure 4. Cross Members Notched and Square Dowel Reinforcement

Primary Coil – With a coil size 36” in diameter, you likely won’t be able to get more than two turns of wire to resonate at frequencies up to 12 MHz.  This takes into account the precautions described above to minimize unusable capacitance.  AWG 22 stranded, insulated wire was used to lace this coil; ensure the dowels remain at right angles with one another.  Note that one set of holes in the dowel is skipped between the first and second turn.

Tuning Capacitor – Almost any salvaged variable capacitor can be made to work.  For a typical 2-gang unit, the gangs can be connected in series through the common rotor sections and metal frame with the stator terminals of each gang used as the outer terminals.  This will create a lower minimum capacitance as described above.

For the antenna described here, a single-gang, 365-pf capacitor (see Specialized Parts) was used with a fixed mica capacitor in series.  The minimum capacitance of the variable capacitor is nominally 15 pf. Figure 5 shows the capacitor assembly for the primary circuit. Components are mounted on a perforated circuit board, which, in turn, is mounted to the bottom of the vertical square dowel.  A portion of the base can be seen at the rear. A large diameter tuning knob is suggested, as the peak tuning for a properly constructed loop will be very sharp and require a delicate touch. As an option, I have used a planetary reduction mechanism on other antennas to give an 8:1 ratio with the capacitor shaft.

You may notice at high frequencies that the antenna is somewhat unstable with body contact of the knob or around the tuning capacitor.  This is because the resonant circuit is operating at a very high L/C ratio with capacitance at just a few picofarads. Body capacitance will tend to detune the antenna.  It may be useful to extend the knob 2 or 3 inches from the tuning capacitor with an insulated shaft.

Figure 5.  Capacitor Assembly

Secondary Coil – The secondary coil operates at low impedance to feed the lead-in.  There are two extremes governing the size of the secondary coil. A coil which is too small will not pick up much of the magnetic field generated by the primary circuit at resonance.  On the other hand, a secondary coil which is too large will overcouple or load the primary circuit. This will reduce the Q, or sharpness of the tuning.

The secondary coil is 16” diagonal at the largest turn and consists of 7 turns of AWG 20 buss wire.  Buss wire was used so the coil can easily be tapped after the 1st, 2nd, 3rd, 4th, and 6th turn.  The 7th turn is not currently used.  A tapped coil will provide better impedance matches to the lead-in when the antenna is used through a wide frequency range.  The taps are selected with a rotary switch. The taps are connected so that the outer turns are used first, and inner turns connected as needed.  It is important that unused turns remain unconnected (free) rather than shorted. See Figure 6.

Figure 6.  Secondary Coil Switch

Lead-in – A twisted pair of AWG 22 stranded wire is used as the lead-in.  This will be more flexible than coax. The lead-in should be kept as short as possible and twisted tightly so it will not pick up any signal by itself.  This is important at shortwave frequencies. A twisted pair can be fabricated from two lengths of wire with one set of ends anchored in a vise, while the remaining ends are twisted in the chuck of a hand drill.  Most portable radios are equipped with a standard 1/8” phone jack at the external antenna connection point.  So, this antenna is terminated with a 1/8” phone plug.

Base – There is nothing special about the base.  Your only guidance should be to make it as stable as possible.  Since the frame is light, most of the weight will be at the bottom with the capacitor assembly and other parts.  That helps stability. This antenna uses a 5” plastic jar lid for the bottom. Keep the base small, as the antenna will likely be operated on a desk or table.

Operation

The antenna is intended to operate in close proximity to the radio, such as on a desk or table.  There must be sufficient space to rotate the loop laterally. As described, this antenna has a range of 2.6 MHz through 12.3 MHz with a band overlap around 8 MHz.  Depending on your selection of capacitors, your range and overlap may be slightly different.

  1. Tune the receiver to a desired frequency.
  2. Set the band switch on the antenna to the corresponding band.
  3. Tune the antenna capacitor to resonance (peak signal).
  4. Rotate the secondary switch to the position of maximum signal strength.  Begin with the fewest turns (generally one) in the secondary.
  5. It may be necessary to repeak the primary circuit.

Repeat the procedure to test operation of the upper or lower band.

Unlike similar loops for long and medium wave reception, this antenna is not especially responsive to direction for peak or null signal reception.  However, you will find it very useful to reduce or possibly eliminate locally produced noise. Simply rotate the antenna on its base.

Modification

The basic concept for this antenna can easily be extended to higher or lower frequencies.  Removal of the inner turn of the primary will significantly raise the upper frequency; whereas, adding turns will increase the lower range.  Note that the lacing of the primary coil skips one set of holes in the square dowels between the first and second turn. This minimizes distributed capacitance between turns.  This separation should be maintained if additional turns are added to lower the operational frequency.

Specialized Parts

Some sources for square wood dowel and single-gang 365 pf variable capacitors are listed below.  The author does not endorse any of them. Prices for similar capacitors vary widely.

Square wood dowel:

Variable capacitor (365 pf):


Bob, thank you so much for sharing this excellent, detailed tutorial. Although I don’t have the exact same variable capacitor, I have all of the other components to make this antenna. I will have to put this on my Social DX bucket list! Thank you again!


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Guest Post: The National Association of Armchair Adventurers (NAAA)

Many thanks to SWLing Post contributor, Bob Colegrove, who shares the following guest post:


National Association of Armchair Adventurers (NAAA)

as recalled by Bob Colegrove

Those of you who were into SWLing in the late ‘50s or early ‘60s may remember the NAAA.  It was an engaging promotional effort by the National Radio Company to generate interest in shortwave listening.  This was done through ads run in magazines such as Popular Electronics, and displays in stores.  Yes, shortwave radios were once sold in brick and mortar stores.  The very proper gentleman depicted below, replete with khaki uniform, pith helmet, jackboots and calabash pipe is none other than Sir Oswald Davenport, Chairman of the Board, and a consummate SWLer of his time who never left the comfort of his own armchair in order to tour the world as it was virtually possible to do at that time.  For those too young to appreciate the connection, a “davenport” was a genericized trademark meaning couch or sofa in today’s parlance.

As the ad indicates, for fifty cents you were admitted to the Association.  This included a nice booklet containing an introduction to shortwave listening by Jack Gould, radio-TV critic for the New York Times, a sampler of stations ‘currently’ broadcasting around the world, log sheets and a large certificate of membership suitable for framing and signed by Sir Oswald himself.

The certificate reads, “this is to certify that (name) is a privileged member of the National Association of Armchair Adventurers… and is hereby entitled to explore the four corners of the earth, to sail the seven seas, to cross, in the comfort of a favorite chair, the six continents and to visit freely without passport, the 260 countries of the world.  Permission is also granted to eavesdrop, whenever possible, upon aircraft and satellites in outer space, ships at sea, the activities of local police and fire departments and the conversations of radio amateurs throughout the world. This experience in international espionage entitles all members to pose with authority as experts on world affairs and to expound at large on the solution of all problems.  Membership and participation in all privileges is authorized for a lifetime of pleasure, or as long as said member is the owner of a National Shortwave Receiver.”

The radio depicted in the ad was the then new National NC-60 Special, a 5-tube ac/dc, entry level receiver, which was a direct competitor of the Hallicrafters S-38E.  This writer recalls standing in front of the two mentioned receivers on display at the local ham shop in downtown Indianapolis in 1959 trying to decide on which radio to invest his hard-won sixty dollars.  The salesman wasn’t pushy, it came down to a coin flip, and the Halli came home with me. Although I’ve never regretted it, I still wonder what exotic mysteries lay behind the NC-60’s dial. Although my pen-and-ink name has long since faded, the NAAA certificate still hangs prominently in the shack.

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