Modern Radio Laboratories ® /Alfred P. Morgan Mash-up
 

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Making the coils.
Taking the mystery out of what wire goes where.

 
 
 
Elmer Osterhoudt and Alfred P. Morgan both give instructions on how to wind the coils and both are hard to comprehend. Morgan advises to just buy the coils, but gives instructions on how to wind a coil on a Bakelite coil form.

Elmer wants you to follow the diagram on the left, drawn in typical MRL lo-res 2D. Are you going to buy blank coil forms and wire and try to wind a coil based on this drawing? Better just buy the coils already made from MRL. This lousy drawing is marketing genius.

The superior drawing in Morgan's book is still confusing. Above, on the far right drawing, pins 1 and 2 on the coil are closest to you. The circle in Elmer's drawing is the socket. If you mentally tilt the circle and compare it with the Morgan socket, "P" is pin 1.
 
 

To complete the confusion, if you have an antique coil or tube socket, the numbers on the socket won't match up with the numbers in the drawing. If I had never seen a real coil, I'm not sure I could make one based on these drawings.

This explains the guys who say, "I built one of these when I was 10 years old. I've spent my carrier in electronics thanks to Alfred P. Morgan." These kids were winding radio coils from incomprehensible instructions. It is no wonder they were successful in life.


The coils are "standard" in the pin configuration. (The only reason I state this is that Morgan said to buy the coils, and if there wasn't some standard at the time, the radio wouldn't work.) A 1935 Bud Radio Products catalog states that their coils can be used in any circuit specifying a four prong coil, so this configuration dates back at least to 1935.

There are two small pins and two large pins. Logically, you'd think that the big coil would connect to the big pins and the small coil would connect to the small pins, but that's not the standard. Each coil is connected to a small pin and a large pin.

Elmer puts the tickler winding on the top, Alfred Morgan puts his on the bottom. It doesn't really matter as long as you wind the primary and tickler coils in the same direction.

The MRL coils wind upward, the Morgan coils wind downward. In other words, Elmer made a hole near the bottom of the coil form, pushed a wire through it and into pin 1. He then wound the coil up towards the top. Morgan did the opposite; he made a hole in the top of the coil form, pushed a wire through it and into pin 1, and then wound it towards the bottom.
 
A set of MRL coils.
 
A set of BUD coils from the 1920s.
 
Though both of these sets are color coded, there was no industry standard for the colors. The fact that the red (and possibly the green) coils are similar is either a coincidence, or Elmer adopted a color scheme from another manufacturer.
 
"OCTO" brand four pin coils. Red, blue, brown and green. The red and brown probably tune similarly to
the red and brown BUD coils, but not the blue and green. The tickler winding are on top, like the MRL coils.
                                                                                                                                                 
 Photo by Emil Sarlija

Here is MRL / Morgan comparison data for the AM Broadcast Band coils. Two coils are needed to cover the band.

 
 

Variable
Cap

Primary Coil turns Wire
Gauge
Tickler Coil turns Wire
Gauge
COIL 1          
MRL 10 - 140 84 28 14 28
Morgan 10 - 365 70 30 20 30
           
COIL 2          
MRL 10 - 140 170 34 25 34
Morgan 10 - 365 150 34 45 34
Notice Morgan uses less turns on the primary and more on the secondary, but he used a variable capacitor that is double the value Osterhoudt used. Osterhoudt tells us the first MRL coil will tune from 950 kHz to 2100 kHz and the second coil tunes from 436 kHz to 1000 kHz. The forms are 1.5" in diameter. 
 
 
The pin connections of the two coils (compare to the drawings). Wind both coils in the same direction. For example, if you wind the primary coil down from the top, starting on pin 1 and ending on pin 2, then also wind the tickler down from the top, starting on pin 3 and end on pin 4. Both coils must be wound in the same direction or the radio won't oscillate. Leave 1/8" between the two windings.

Markings in white are for the MRL coils.

 


  To wind the coil you need tension on the wire. This can be done by passing the wire through a hole in a clothespin (an idea from one of Elmer's handbooks).

Scrape the insulation from the end of the wire. Drill a hole in the top of the coil form, pass the wire through the clothespin, into the hole, and into pin 1.

Heat the pin with a soldering iron. Solder will wick into the hole, soldering the wire to the pin.

When the coil is wound, hold it in place with a piece of tape. Drill a hole, scrape the insulation from the end of the wire, and pass the wire through and into pin 2.
 

 
- Click on image -
After you wind the main coil, wind the tickler coil in the same manner. I found that trying to get the bottom tickler winding wire into pin 4 was almost impossible, due to how close the hole in the coil form was to the pin. Use this work-around:

After completing the coil, hold the wire in place with tape. Cut off enough wire to reach the pin, but add a few inches. Insert the wire into the hole in the coil form and cause it to exit the top. Scrape off the insulation at the appropriate spot. Next, insert a piece of coil wire into the pin. Push it up till it also exits the top of the form. Twist the two wires together and gently pull them out through the pin. Solder.
 
The completed coil. It tunes from below the broadcast band up to about 1100 kHz. Two coils are needed to cover the entire band.

 
The second coil, on the left. It tunes from around 900 kHz to 1750 kHz. Next to it is an MRL coil that covers about the same range. I try not to handle the MRL coils too much. The coil form on the left was made by Mike Peebles, so it too will one day become a rare item.

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