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Restoration of the
Stewart-Warner 301-A
Shortwave Converter

by YS1ECB February 1 2010

INTRODUCTION:

Having restored a 738 Silver Marshall Short Wave Converter from ground up, and after enjoying shortwave listening with it, I decided to venture into the world of the shortwave converters.
Some time in October of 2007, I bought a Stewart-Warner 301-A shortwave converter that looked like the perfect subject for restoration. The converter looked like shown in the images below. (Click on images to enlarge)

 

CABINET RESTORATION:

Firstly,since the cabinet's bottom was falling apart,I removed it and re-glued it.
Secondly I removed the old varnish by appling strips of Scotch Tape to the wood-see Fig3-Then by lifting these strips some varnish was removed easily.I removed the rest of it with sand paper.
Finally I polished the cabinet with very fine sand paper and applied tint with a soft cloth.
Process is shown in the images below. (Click on images to enlarge)

     
     

MECHANICAL REPAIRS:

The first problem was that the vernier system did not work,because the rubber cylinders that move the large toothed wheels were petrified and cracked.
To remedy this situation I disassembled completely the mechanism, and applied plenty of Teflon tape to both rubber cylinders. Then when I tested the mechanism the large toothed wheels made the appropriate grooves on their respective teflon covered cylinder. The repair is holding well until now Jan 2010. This process is shown in the images below.
(Click on images to enlarge)

   

PRELIMINARY TEST:

Now, that the vernier was working,came the test.
The converter's transformer provides only 2.5 V for the filaments of the 24-A and the 27.No B+ voltage is provided!! To provide the B+ I made a half wave power supply with a small transformer and a Pi filter. For this filter I used two 33Mf/160v capacitors with a 4.2 KOhms/ 1 W resistor in between both capacitors. This provided a B+ of 92.2 volts and a plate voltage for the 27 type oscillator of 73.2 V .Just perfect!! To see the location of the transformer and associated parts. See images below
(Click on images to enlarge)

   

For this test, the converter was coupled to the Lincoln radio that is used with the 738 Silver Marshall Converter. It worked with good sensitivity, but it showed a major problem : The band change toggle switches were making false contact ,and the variable condenser was very noisy. The variable condenser was fixed by removing both brushes,and cleaning them with an ink eraser rubber. Contact cleaner was applied to the brushes and ball bearings. Finally a drop of thin oil was poured into the ball bearings at both ends of the shaft , and the body of the condenser was strapped to the chassis, See Fig13 . The band change toggle switches are located to the left of the ANTENNA COIL. See image below. An attempt to restore these switches was made .Both were completely taken apart ; thoroughly cleaned and oiled. See image below. Unfortunately they worked well only a couple of month. Then, another solution was adopted.
(Click on images to enlarge)

   

SOLUTION OF THE BAND CHANGE PROBLEM:

Before addressing this problem, I must say that the 2 Megaohm grid leak bias resistor was found altered,and to bring it down to its correct value of 2 Megaohms,a 2.7 Megaohm resistor was connected in parallel with it. See image below.
(Click on image to enlarge)

The solution of the Band Change Problem was accomplished by strapping together three microswitches . To this bank of switches a foot was added in order to secure it to the chassis. See images below.
(Click on images to enlarge)

       

The next image shows that the Band Change Switch connects and disconnects three wires from ground. This is accomplished by a lever operating such a switch.
(Click on image to enlarge)

My solution to this lever effect was obtained by wrapping a green rubber lace around the same shaft and tightening it well. See image below.
(Click on image to enlarge)

The next image below shows that the three micro-switches are connected in parallel with the three wires of the original switch, which was left in place and still is operative. This preserves the originality of the Band Change Switch to a certain degree while correcting efficiently its intermittent operation.
(Click on image to enlarge)

INSTALATION AND OPERATION OF THE 301-A CONVERTER:

A small 5 tube AC-DC General Electric radio was chosen to operate with the 301-A converter. (Click on image to enlarge)

To efficiently couple the about 100Kc. IF signal from the converter to the broadcast receiver,inductive coupling was used.
The inductive coupling was achieved by using a 4 Cm. long BC band ferrite antenna.The ferrite core of the antenna was removed , and the coil was slipped over the ferrite antenna of the broadcast receiver ( the white General Electric radio ) . This process can be seen in the images below. (Click on images to enlarge)

   

The connection between the 301-A and the GE radio was accomplished by means of a 75 Cm. length of RG-58U coaxial cable.
The operating position of the 301-A converter can be seen in the image below. (Click on image to enlarge)

The operation of the 301-A converter is simple.
First tune the BC receiver to about 100Kc. If this frequency is in use by a broadcast station find a quiet spot ,a bit above or below the 100Kc. mark.
Second connect an antenna to the 301-A, and flip the Band Switch to its most clockwise position. This is the High Frequency Band ,which tunes from about 4.9 Mc. to about 14.3Mc.
Search for stations moving the tuning knob from about the 27 mark to about the 40 mark. This will cover the range from about 6.0 Mc. to about 7.3 Mc.
A second very active spot is between the 50 mark to about the 64 mark. This range covers from about 8.9 Mc to about 10.4 Mc.
Finally there is a third active spot from the 75 mark to the 92 mark, which covers from about 11.6 Mc. to about 13.6 Mc.
Please note that you will also find stations in other positions . The spots mentioned above are simply most active from my location in El Salvador.Also take note that every division of the dial is two units of the logging scale.
The knob located to the right is the antenna compensator, and it should be moved slowly for maximun gain. In my converter a 9 O'clock position fits most ranges.
The second position of the Band Switch is the Medium Wave Band it goes from about 1.8 Mc to about 4.6 Mc.I will let you discover the most active spots here,but I recomend the 74 mark which correspond to about 3.35 Mc , and you can listen here Radio Exterior de España.

FINAL REMARKS:

I use this converter with a single wire antenna, about 10 meter long and only about 1.5 meter above the roof.
I consider that its performance is very good given the antenna in use. I listen perfectly well to stations like: The Voice of Russia, Radio Croaccia, Radio Ucrania, Radio Vaticano. La Voz de los Andes, Radio Habana, NHK from Japan. Radio China Internacional, Radio Taiwan, The Voice of America, Family Radio, Radio Teheran. Radio Exterior de España, Radio Católica Mundial etc..etc..
I do think that the efficient inductive coupling used contributes a great deal to improve the overall sensitivity of the converter-BC radio system.
I am including the diagram and a short explanation on the converter found in Rider's Volume 2
(Click Here for the PDF).

Finally I want to thank my friend Germán and my friend Aaron Weed KC2NDA for their time, patience, and skill in up-loading this article to their respective web sites.
Also I want to thank my friend Tom Laszynski K8JRM for his constructive conversation on the subject as well as for his pictures on the 301-A converter of his own. Tom's converter is housed in a different cabinet than mine. My 301-A's cabinet is the more common one.