Wednesday, December 16, 2015

My First Shortwave Receiver

       Everyone will always remember the first receiver that took them into the shortwave bands. I found mine in storage at my folk's house this week, retrieved it and brought it home.  I thought it would be interesting to share what got me started.
       It was a homebrew receiver built from parts that had been part of lab experiments that came with a radio-tv repair correspondence course that my father had taken in the early 1950's.  It was, in fact, watching him solder and work on assignments that first stirred my interest in radio.
       After he completed the course, the materials, including parts and printed lessons were stored away in a closet and when he figured I was old enough to know what to do with them,  I was given the boxes of goodies. I was eleven years old at the time and already DX-ing with table radios and had begun poking around in them, trying to improve performance and hoping to get something going that would tune the short wave bands.
       Among the boxes of parts and papers there were chassis with holes punched and chapters on designs for various circuits used in superhet receivers.  It was a matter of putting the pieces together to get the receiver transferred from sections of lessons to being an operating radio.  There was one experiment that detailed making an H.F Converter to tune from about 6-18 MHz that would convert those signals to be tuned in by a regular broadcast receiver tuned to just above the high end of the broadcast band. That design became the front end of my first receiver to tune higher than the 160 meter amateur band ( I had previously gotten a standard broadcast receiver to go that high by pulling out the tuning slugs and loosening the trimmer capacitors as far as they would go).
       With the help of the head tech and owner of Lawson's Radio and TV repair shop I managed to get the thing assembled and working. I am not sure how many times I had the chassis in the basket of my bicycle for the two mile ride from our house to his shop, but with his help and parts from his parts stash and junk box,  the radio became a working reality.
       The basic radio was a six tube superhet that tuned the broadcast band with one RF stage and one IF stage.  It had an AC/DC power supply with a double section capacitor and choke filter.  The RF stage was modified with a double tuned input aligned to 1650 kHz with another fixed tuned circuit on its output.  The HF converter consisted of a 12K8 pentagrid converter circuit with its own filament transformer supply that lifted B+ voltage from the basic radio.  The converter had its own two gang variable capacitor with the original radio set up fix-tuned to 1650 kHz.

Front and top view of the chassis of the receiver. The main broadcast band tuning capacitor is behind the large drum to the left with the HF converter variable capacitor that provided tuning for the short wave range is on the right. other controls volume, tone and power on-off switch.

Top view of receiver chassis At top left is the edgewise S-meter.  This was actually a milliammeter reading plate current of the 455 kHz IF amplifier stage.  The stronger the signal, the greater AGC voltage applied to the stage and resulting in lower  plate current for the 12SK7 IF amplifier tube.  Thus the S-meter actually read backwards.  The stronger the signal, the lower the plate current indication. To make the meter read correctly,  it was simply mounted upside down! The glass tube to the upper right is the 35L6 audio output tube.  The metal tube to the upper left is the 12SQ7 triode-dual diode tube that served as detector, agc and first AF amplifier.  The glass tube at mid rear of the chassis is the 12SK7 455 kHz amplifier.  The metal tube behind the upper variable capacitor is the 12SA7 Second converter stage. The metal tube below the upper variable capacitor is the 12SK7 RF amplifier for the original BCB set redesigned with fixed coil tuning that became the 1650 khz IF amplifier stage. The glass tube at the rear of the main chassis is the 35Z4 rectifier tube. The metal tube with the grid cap connection above the lower variable capacitor is the 12K8 first converter. The lower variable capacitor tuned the HF converter and was the one used for selecting the shortwave frequency desired.

Under chassis wiring of the receiver. The HF converter chassis is to the left.

Close detail of the upper part of the HF converter stage showing the 12K8 Pentagrid Converter tube, the variable capacitor and the input tuning coil.

Closer top view showing the chassis top near the tuning capacitor for the base BCB portion of the receiver. The coil to the left of the larger variable capacitor is part of the fix-tuned 1650 khz IF circuit.  The two silver cans are the electrolytic power supply filters. The clips on the rear of the chassis are the connections for antenna and ground.  The small transformer on the rear of the HF converter chassis is for filament voltage for the 12K8 converter stage. The other six filaments are in series and operate like a conventional  AC/DC receiver.
Under chassis detail of the HF Converter stage. The large coil is the oscillator coil for the 12K8 pentagrid converter. The small coil to the upper left is one of three tuned circuits between the converter and the rest of the receiver tuned to 1650 khz.

Detail of IF amplifier stage showing the home made IF transformers. Windings were taken from discarded IF transformer cans, slipped over wood dowels and mounted horizontally to allow them to be placed farther apart than originally made to provide looser coupling in an attempt to get better selectivity.  Fixed regeneration was also introduced into the stage to improve selectivity of the stage. Tuning was accomplished with mica trimmer capacitors below the coil and reachable by screwdriver through holes in the chassis from below.

The receiver was not the highest performing beast on the planet,  but it did provide me with a method of listening in on the hitherto un available shortwave  frequencies.  It served for about three years as a way to explore my new world.  I learned a lot about receivers from the project.  In later years, I learned a lot that could have been done to improve it, including such things as converting the AC/DC supply to a transformer supply, which would have required another small chassis to handled the transformer. Voltage regulation could have improved stability.  A manually tuned preselector  could have eliminated the aggravating images that appeared on some places on the dial.  The addition of a 1650 khz crystal between the two tuned circuits between the converter and the rest of the receiver could have greatly improved selectivity. The addition of a beat frequency oscillator could have improved cw reception that was obtained in the original by simply increasing the IF regeneration until the 455 khz stage went into oscillation.  And a Q-multiplier added to the 455 khz IF could have finished the job of making the tuning really sharp.  But that is probably just day dreaming " what-if's"...though to this day I wonder how it would have turned out had I known that at the time and actually made it happen.  In any event,  this receiver which probably looks a little crude to most was the source of many great hours of listening and adventures in the wilds of the short waves.


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