Introducing the

Radio Shack Special

For a detailed view, click here

I have browsed the world wide web many months in search of a one transistor FM receiver. I have seen a couple... but they were always attached to some sort of added device., such as another IC or another transistor for amplication in the receiver itself.. Through my continued quest of searching for that 'too good to be true' one transistor, I happened to run across a super-regenerative receiver, by Charles Kitchin, famous for his vast knowledge of regenerative designs. I printed out the schematic and made it. It turned out extremely well...for a two transistor, three coil unit.

With much experimentation and 'un-design'...as I call it, I began taking away a single component of the 'newly-made' receiver...one by one a component came off. Down to one transistor and two coils and 8 other components...it finally stopped. I then put component number 12 back. I must say that without sacrificing or comprimising for sensitivity, selectivity and audio output...this little gem is still shining it's light as bright as it can. I then continued to experiment and re-design a small SOURCE Coil for the receiver. Standing no taller than 1/2 of an inch and 8mm's in diameter, this homemade coil is doing its' job just fine. I also made a homemade fixed capacitor in the 12 component count. Substitution came around again and replaced the telescopic antenna with a 10 inch 22 gauge magnet wire. Not too bad for a fully functional FM receiver. Of course, to beef up the audio, an LM386 circuitry was added to the output of the receiver. In fact, the LM386 component count rivals that of the receiver itself.

Large View of the Receivers' 12 Component Count

And so became the 'birth' of The Radio Shack Special...the name given to it because all of the components may be purchased at your local or nearby Radio Shack store. Although Radio Shack does not carry a typical 6 lead AM/FM variable (tuning) capaitor for tuning in to all the FM stations, I have managed to come up with a simple little way of going from station to station by just stretching and sqeezing the DRAIN coil with a plastic TV alignment tool (RS Part # 64-2223). If you would prefer to use a variable (tuning) capacitor, you can salvage this device from an AM/FM tuning radio. The PCB has been designed to fit both of these methods.

All the experimentation that one can find is here in this remarkable receiver...increasing sensitivity...finding other areas of interest in the VHF spectrum...altering the uH's of the SOURCE coil for better signal clarity when spanning other parts of the VHF spectrum...adjusting antenna heights for the best overall reception...adding taps to the DRAIN Coil to further increase sensitivity...putting in a larger audio amplifier to drive a loud speaker ...increasing or decreasing the DRAIN/SOURCE capacitor as still another way to acheive added sensitivity...enclosing the entire unit in a metal case for that personal touch...the list goes on and on.

My end goal was two-fold...1) To make an extremely simple basic FM receiver with just one transistor and 2) To get hold of all the components at your local or nearby Radio Shack store, thereby surrendering this to anyone who is willing to learn a bundle of knowledge through experimentation and hence, laying down all the tools needed for a splendid in-depth project.

As time went on I started posting the little jewel on many different electronic forums...as you probably already know. The first person to make this was a fellow (now friend) named Pedro. He not only made it...but designed a beautiful looking enclosure to the finished project. When Pedro completed his project, this is what he had to say about it:

you don't need to build complex technicalities ...such as slope or phase detectors required for fm reception. The amazing simplicity of this super regenerative receiver brings you the pleasure of hearing your favourite FM stations and at the same time the satisfying sense of personal achievements.

...Pedro

To see Pedro's Project, just click HERE

RADIO SHACK SPECIAL

Introduction...........................................................................................................Page 1

Get All of the Parts First..................................................................................Page 2

Making the Homemade Coils..........................................................................Page 3

Construction of the Printed Circuit Board (PCB)...................................Page 4

The Ordered Way of Soldering the Components.....................................Page 5

The First 'Turn-On'.............................................................................................Page 6

The Radio Shack Special Schematic............................................................Page 7

Page 1

Introduction

In this exciting project, not only will you have a very unique one transistor FM recevier, but also be in-store for making home-made air-core coils and a home-made fixed capacitor. And even more than that, when you finish 'your' project, your journey has just started. With your now-working FM receiver, you can start experimenting with many wonderful things that I had mentioned. Once you have mastered all these things...you will surely have a better grasp on the whole concept of what 'hands on' experience truly is. It is a very rewarding experience when you have gone into the unknown...taken roads that you have never taken before...roads that lead you to knowledge of things that you would have never thought possible...and all on your own!

I did send an e-mail to Charles Kitchin and told him of the 'un-design' I had given to one of his designs. In fact, he had said that back in the earier years, there was a certain TV company that had used that same 'un-design' method. If you would like to see just where the birth of the Radio Shack Special came from, take a trip now to see Charles' original schematic of his super-regenerative receiver. Just click here.

Throughout this website, you will also find other nice projects. Some by me and some by my 'newly-founded' friend...Pedro. We have put all these wonderful projects out to you in hopes that...just like fishing, if you have never experienced the joy of catching your first fish...you will never know what its' like. The same goes for these projects... we have all the 'bait' here for you to take...but it will be left up to you for the taking. I still routinely go and post my Radio Shack Special to as many radio/ham/electronic forums as I can..not to mention always updating the website from comments and/or advice by recent visitors. I am again, glad to see that you have managed to get here. Pedro would concur with my feelings. With all that said, continue the voyage into the wonderful world of the single and proud MPF102 transistor.

Most of the time it is the simple things that catch our attention...realizing later that much was learned from it.

...your friend, Patrick

Page 2

Get All Of The Parts First

Let us begin the project...start by getting all of the parts needed as shown below. Once you have them, go ahead and proceed.

* 2 is only needed if not using a variable (tuning) capacitor.

Page 3

Making The Homemade Devices

The following two devices must be strictly made according to plan. It is these devices, that if not made to exact trueness, will warrant a 'no-go' situation, when it comes time to turn the unit on for the first time. I will go into great detail in how to make each one of these items. I will begin with the SOURCE Coil....then the DRAIN Coil. Let us begin:

The SOURCE Coil

You will need 4 items to make this coil. 1)A drill 2)A 1/32" bit 3)a round non-metallic form, whose measurement in diameter is exactly 8mm's. I used a round wooden pencil, in which the diameter was exactly 8mm's. 4)30 gauge enamel-coated (red) magnet wire:

Begin by drilling a 1/32" hole somewhere toward the center of the non-metallic form. Take the end of the 30 gauge magnet wire and place through the hole of the form. Let is stick out 1/2 inch and then make a 90 degree bend with it. This will keep the wire in place while you are making your wraps around the form...make 26 wraps. Make tightly wounded- wraps of wire around the pencil. Make sure they are good and tight. When you get to your 26th wrap, take the drill and make another hole right beside the last wrap. Then cut the magnet wire and place thru this second hole. Make sure the wire extends out of the hole 2 inches. Then make a 90 degree bend in the wire so as the wire can hold itself in place. The coil should be good and tight and will not come undone. Trim both ends of the wire as shown in the picture below. The picture is what is should look like just before soldering into place on the PCB. Once you have finished making it, place it aside until it is time to solder it on the PCB.

Figure 1

The DRAIN Coil

You will need two items to make this coil: 1) 22 gauge magnet wire 2) 5/16" standard threaded bolt. Cut 15 inches of 22 gauge manget wire off the spool. Take the 5/16 inch standard threaded bolt and wind 7 complete wraps of the magnet wire into the threads of the said bolt, if you decide to use a variable (tuning) capacitor wit the receiver. If you decide NOT to use a variable (tuning) capacitor with the receiver, wind 9 complete wraps of the magnet wire into the threads of the said bolt. Gently unscrew the 'now-made' coil from the threaded bolt. Be careful not to alter the 'spacing' of the newly made coil. It is from this freshly-formed coil, will you be 'right' on the FM band. Now from the top of the coil to the bottom of the two legs....it should be right at 3/4 of an inch. The picture below is what is should look like before soldering to PCB. Once you have finished making it, place it aside until it is time to solder it on the PCB.

Figure 2

Page 4

The Printed Circuit Board (PCB)

The Front Side

The Back Side

Understanding the Above Two Pictures

The drawings pictured above is a replica of the PCB needed. When printed out, it should be within 2 mm's on both the width and length. If not, send the document to a Paint or Graphics Program and sqeeze or stretch the picture, until a printed out copy will give you the exact measurements required. Since this project deals with VHF, the electrical routing has to be 'mimmicked' as closely as possible. How well you achieve the final 'replica' of the pictured PCB will be a determining factor in getting the receiver to function or not. In the world of electronics, at one time or another, one has usually been through the ritual of making a PCB. I will skip the actual making of the PCB, in the understanding that most would know how to accomplish a 'replica' of it. If you are new to the idea of making PCB's, I have made a complete section on this, as all of the items needed may be purchased at your local Radio Shack store. I go thru a step-by-step procedure on how I make my PCB. Just click here for The Complete Makings of the Radio Shack Special PCB Using Dry Transfers.

Once you have made the PCB, you may proceed to the next page.

Page 5

The Ordered Way of Soldering the Components

Now comes the time to start soldering all of the components on the PCB. But first, you must decide now if you:

1) Will NOT USE a Variable (Tuning) Capacitor for the project.............

2) Will USE a Variable (Tuning) Capacitor for the project.......................

Choose the appropriate section below for deciding to USE or NOT USE the variable (tuning) capacitor:

Not Using a Variable (Tuning) Capacitor

You must understand, that when NOT USING a variable (tuning) capacitor, two components in the unit will need to be altered and a cross-over wire will be installed on the finished PCB. The two components that need to be altered are the DRAIN Coil and the DRAIN Capacitor. Let's take the DRAIN Coil first...you will be using the homemade DRAIN Coil that will have the '9' complete wraps of the 22 gauge magnet wire. Refer to Page 3 for the complete specifications on the DRAIN Coil.

Next will be the DRAIN Capacitor...you will be using one 4.7pF capacitor for the DRAIN Capacitor.

And last, you will be soldering in a 'cross-over' wire between the two connections where the Varible (Tuning) Capacitor would normally go. Item #18, on the "Component Placement Guide" drawing, is where the cross-over wire should be soldered. From the 'plus' side to the 'neg' side. The wire should be straight and lying flat on the PCB when the soldering has been finished. Even though no soldering has taken place yet, do solder in the 'cross-over' wire now.

Once you have understood all these these concerning NOT USING a Variable (Tuning) Capacitor, you may proceed to the section of "The Ordered Way of Soldering in All of the Components". Just skip over the next section...which is USING a Variable (Tuning) Capacitor...

Using A Variable (Tuning) Capacitor

You must understand, when USING a Variable (Tuning) Capacitor, two components in the unit will need to be altered. They are the DRAIN Coil and the DRAIN Capacitor. Let's stake the DRAIN Coil first...this coil will need have the '7' complete wraps of the 22 gauge magnet wire. Refe to Page 3 for the complete specifications on the DRAIN Coil.

Next will be the DRAIN Capacitor...you will need three 4.7pF capacitors in parallel. Put all three together and then twist all of the three legs on one side, then twist all three legs on the other side. Solder these 'two' sets of legs together. They are all now in parallel. This will bring up the value to 14.1pF's. This will be the required value, togehter in series with the Variable (Tuning) Capacitor, to acheive a full span of the FM broadcasting stations. Once you have made the coil and made the three parallel capacitors...put these two items aside and solder them into place later in the 'Ordered Way of Soldering in all of the Components'.

The Ordered Way of Soldering the Components (cont'd)

You shall start to solder all of the components to the PCB. You shall solder in an ordered way so as not to get caught in a 'cornered' spot when soldering components on the PCB. Below are two drawings: the Component Placement Guide and the Component Identification and Value Guide. These two diagrams will guide you as to where each component (and it's value) will go on the PCB. Below the two diagrams will begin the ordered way of soldering.

Component Placement Guide

Component Identification and Value Guide

1) Now that the PCB is finished and ready, start by placing the soldering in the low-profile 8 pin socket on the back side of the PCB. Once that is done, you may now insert the LM386 in the 8-pin socket. You are now through with the back side of the PCB.

2) Flip the PCB to the top side and start by making your support for the PCB right now. Use the two mounting holes for securement. It may be placed on an ordinary piece of thin (1/8" thick) wood, like paneling wood. Let it rest about 1/2 of an inch high from the wood. Any way you would like to mount it would be OK. Just don't use a metal structure, as this could have a definate effect on the overall performance. Later, once you have the receiver working well, you may then experiment, by enclosing it into a metal box of some sort for that 'professioinal' look. Again, be watchful for differences in performance when bringing metal close to the unit.

3) Next, solder in the MPF102...then the 2 potentiometers.

4) Then, solder in the SOURCE Coil and the DRAIN Coil. (Remember the DRAIN Coil should be the correct one, according to whether or not you will use a Variable (Tuning) Capacitor.

5) You will start soldering the rest of the components (except for the tuning capacitor, the switch, the audio jack and the 9 volt battery), starting from the extreme 'right' side of the PCB...that is, the audio section will be the starting point...as you solder each component in, keep moving left, until you get to the last component, which should be the DRAIN/SOUCE Capacitor and the DRAIN Capacitor (Remember the DRAIN Capacitor should be the correct one in relation to USING or NOT USING a Variable Tuning Capacitor), which is on extreme 'left' side of the PCB. This order should be met, otherwise you will find yourself cornered and having no way to insert the soldering gun tip to solder certain components.

6)You have now completed putting all of the components on the PCB, except for the Variable (Tuning) Capacitor, the ON/OFF Switch, the Audio Jack and the 9-Volt Battery. If you had decided to NOT USE the Variable (Tuning) Capacitor, go ahead and secure the ON/OFF Switch, The Audio Jack and the 9-Volt Battery into position. You may use that '5-Minute' JB Weld for securement of the first three devices and for the battery seucrement, you can use a small piece of velcrose. You are now ready for Page 6, as the receiver is now ready for its' 1st 'turn-on'. If you had decided to USE the Variable (Tuning) Capacitor, go ahead and secure the first three of the said components, but DO NOT secure the Variable (Tuning) Capacitor, until you find out what '2' leads, coming from the variable capacitor, are the correct ones. The capacitor typically has 6 leads coming from its' base. You are to find out, by temporarily hooking up the capacitor in-line with the circuitry. To understand this variable capacitor and how one can spot the 'two' correct leads, just click HERE.

Below is a picture of the finished look of the Radio Shack Special. Remember, when it comes time for the 1st 'turn-on', DO NOT secure the variable tuning capacitor (if you had decided to use it) to the PCB; once you have found the correct 'two' leads coming from it, you can then JB Weld it in place. If you had decided NOT TO USE the variable tuning capacitor, the picture below is what it should look like...although without the variable tuning capacitor.

The 'Finished' Look

Take a good overall look of the PCB. Look and inspect for solder droppings, electrical routing 'breaks' or bad soldering on components. This will be your final inspection before turning the unit on.

Page 6

The First 'Turn-On'

Here we go...

With a 'fresh' 9-volt battery in place and switch to OFF and a 'standing tall' 10 inch 22 gauge magnet wire in place for the antenna...set the unit accordingly...make sure the 1K potentiometer is turned all the way to the right...make sure the 47K potentiometer is turned all the way to the left, then turn 1/4 the way to the right.

If you had decided to NOT USE a Variable (Tuning) Capacitor, schroll down to the 'purple' text and continue.

If you had decided to USE a Variable (Tuning) Capacitor, with everything soldered into place and all the connections made from the variable capacitor, the audio jack , the switch and the battery. you may now proceed with the 'purple' text.

With the headphones on the ear's and with your potentiometer tool (the tool needed to adjust both potentiometers) in hand, turn the receiver ON!

One of two things will happen...

You will either receive good clean static...

Or you will be on an FM Radio Station...

If everything went according to plan...it should have come on. For the ones that had decided NOT TO USE the variable tuning capacitor, sqeeze and stretch the DRAIN Coil to adjust for different stations. For theones that had decided TO USE the variable tuning capacitor, turn the plastic knob on the shaft of the variable tuning capacitor and adjust for different stations.

Now take your potentiometer tool and begin to turn the 1K pot to the left, slowly, until you hear the static increase...as you keep turning the static will then decrease again and then the unit will go out. When it does, then turn back to the right until you hear that loud static again...that is where you want the pot to remain. There is a certain area where the static gets the loudest, right before it goes out...try to find that spot. Once you have it, you may know breeze through the stations and also using your volume control (47K pot).

There you have it, my friend! I hope everything went well and that you are experiencing the many benefits from the...

Radio Shack Special!

Troubleshotting the Radio Shack Special

Along the way of making it, you may unhesitantly e-mail me for whatever concern or advice or question you make have concerning the project. If you have come this far, and it doesn't perform at all...or for that matter 'well', it would seem there could be another culprit lying in the circuitry there to challenge you a little more. Re-look at everything you went through...correct polarity on capacitors...correct circuitry placement...droppings of solder in between two separate electrical routings...a break in an electrical routing. Also, touch the .1 fixed capacitor on the receiver output to see if you hear a hummmm in your headphones. This would let you know if the audio section is up and running. If you do not hear a humm when you touch this capacitor, increase the volume and try again. Still if no humm, then your audio section will have to be re-looked at again...Do please send me a line ifa culprit is still there...two heads are always better then one!

Page 7

Below is the schematic layout for the Radio Shack Special. Although one needs to pay strict adherence to following the exact routing of all the electrical circuitry as shown on the PCB routing, which is shown further up on this webpage, this schematic is intended just to show how the components are arranged in relation to one another.

...and let the project begin!

...your friend, Patrick

You may e-mail me at...mailto:braincambre500@yahoo.com