Mission: Provide variable frequency control for the PRC-47

 

Revised 2/08

Lots of folks have accomplished this simple modification.

This modification was first published in 2003 but had an error in resistors selected and this error was caught by IKK4YNG, Paolo and also Jim KF7A was involved because he discovered that the could reverse bias the original modification and get a plus or minus swing. That should have been my clue that I had selected the wrong resistor on the wrong side of the varicap. Anyway the modification worked fine but this new revision is better and and the good news is I discovered that you could obtain a better frequency swing by reducing the shunt capacitor on the varicap.On the radios that I have made this modification the swing was sufficient for the new channels on 60 meters. I have never made any mistakes in the past with of my modifications and this is the first. And if you believe that you will believe anything.

 

Overview: A simple mod requiring one 100K pot and 20 inches of wire. Anyone can accomplish this mod that has a soldering iron. The on board regulated power supply is utilized for the voltage.

Credit: Thanks to Ray KA3EKH for collaboration on this mod, it was his idea to direct me to look at the temperature compensation circuit.
Background. The PRC-47 transceiver can only be adjusted in 1 Kc steps, this modification will allow you to swing the oscillator 400 to 600 cycles on 80 meters depending on your set , and as you increase the operating frequency the swing becomes larger, up to 2 Kcs at the top end. You will find that this will help you zero beat just about all signals heard.
Your target is the Master Oscillator . This oscillator feeds the entire frequency chain.

  CLICK to Enlarge.  The mod consists of small reversible changes in the circuit of the R.F. Oscillator module. All you have to do is to lift and disconnect the end of a resistor R-6 (RED ARROW) and solder on a wire to the lifted end of R-6, and if necessary lift the end of a small cap C-9 (Blue Arrow) in the Oscillator module. By lifting the resistor and disconnecting it from the temperature compensating network of the module you will then be able to provide a voltage via a 100K Potentiometer mounted on the front panel of the radio to R-6 in order to swing C-12 a diode varicap. The voltage for the potentiometer will come from the set's internal regulated supply. Yes you are disconnecting the temperature compensation circuit but unless you are going to operate in extreme climates I don't think you will need it.

By lifting one end of C-9, which is in paralleled with the varicap you are effectively removing it from the circuit and thus increasing the operating value of varicap C-12 making its capacitance higher and more effective.

Note: Some sets do not have C-9, other sets had 2 or 3 capacitor's in parallel as C-9 as determined at the factory. C-9 is composed of a single or several brown micas.

Note: As a suggestion I would do the R-6 mod first, test the unit and note the frequency swing and if more swing is needed then lift the end of C-9(s)

 

Stay with me, this is a simple modification I promise.

 

   


Target Module, last one in the right rear. Marking on the top is Oscillator R.F.


   Unscrew the module retaining screws from the bottom of the radio, remove the module and then remove the module cover, the cover slides off.
   Take out the foam. Be sure and remove foam from the correct side. This is the right side.
   THIS IS THE WRONG SIDE
 The first target R-6 (Red Arrow) a 470K resistor.(Yellow violet yellow) going to connection post near the top of the module.You need to lift the post end of R-6 free of the post. In the photo the post is marked by a RED ARROW. Notice that on this particular set that the body of R6 was buried in silicon. IMPORTANT: When you lift the end of R-6 from the post do not remove any other wires on this post. You may have to cut away a portion of the silicon covering R-6.

  Click to enlarge.  Unsolder and lift the end of R-6 clear of the post (tie point) and connect a short piece of wire to the freed end of R-6. This wire run to the bottom of the module for connection to an unused pin. In the photo R-6 is shown with a red wire attached.

Do not unsolder any other wires on the R-6 terminal post.

As a suggestion I would do the R-6 mod first, note the frequency swing and if more swing is needed then lift the end of C-9(s)

If Caps C-9 are in place on your set then lift the end or ends if multiple caps. Leave any other connections to any of the posts on the posts , do not remove them.

 

   THIS IS A PICTURE OF THE CONNECTOR IN THE BOTTOM OF THE OSCILLATOR MODULE. The oscillator module plugs into J-9 on the main chassis. You are going to use a spare pin to supply the variable voltage from a panel mounted pot fed through this module pin to R-6

  Select a spare pin on the module connector and connect the new wire from the free end of R-6 to this pin. This way you will be able to provide a variable voltage to the module for frequency control and still be able to plug and unplug the module without any extra wires. I used pin 4, its a nice even number and easy to get to.

I used some type 77 ferrite beads to de-couple the wire but ran tests without the beads, but for best construction practices I highly recommend using the beads.

   Diagram of the module connector located in the bottom of the module,  Pin 4 is between pin 3 and pin 5.
   This is a shot of J9 underneath the chassis showing the spare pins available. I choose pin four, the second pin over from the large connectors A2 and A3. In this picture my voltage control wire(white arrow)has all ready been connected. This picture is for information only you will connect the pin later.
  



      Replace the foam with a new piece.
   



      Replace cover, trim foam as necessary.

   Remove the 115 volt fuse holder(originally suggested by Dennis Starks), this will create a hole, the hole will be used for mounting the frequency control pot.

 

   



    Heat shrink the unsoldered leads and secure with a cable tie.

 

   Connect 3 wires to the pot. Use a lock washer to keep the pot from moving once it is installed. I used a 100 K pot but 50K , 250K will work, play with it.
 



      A handy shop tool is an extra hand.
   In this picture of the rear of the pot mounted in the hole, Violet the center tap supplies the variable voltage to pin 4 of J9(Osc module socket), the left red wire will go to the 20 volt voltage source and the far right wire (black) is grounded to the chassis.

   A small solder lug is used for the ground wire of the pot and as a strain relief the other wires.

 

 

    Next target areas. Top arrow points to the voltage distribution strip. The bottom arrow points to J-9 the oscillator chassis socket.
   Snake the wires from the pot down and behind the front panel, secure with a solder lug and cord or cable tie, and then run through the chassis grommet.
   My test jig that sat on my bench for a month. Amazingly stable. Really neat installation which is typical of my work.
   Connect the RED end wire of the pot that you installed to terminal 4 of TB-4,this point is going to supply regulated 20 volts DC from the radio's power supply. The bottom of the picture is the rear of the radio. On some radios TB-4 was not marked on the chassis.
    



      Connect the center wire of your pot to pin 4 of J-9.
   Pin 4 is next to pin 3.      Pin 3 is next to pin 2.      Pin 2 is next to pin 1.
   Put a piece of heat shrink on the wire. Its usually easier to put the heat shrink on the wire first and then solder the wire.

  

  The knob is installed.    To check calibration and find the 12 o'clock or center position for your pot tune in a time standard,     I like CHU on 7335 Kcs*

      Notes: Mhz and Khz are not recognized on this site, only American terms are used.
 

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                          For information on ferrite beads see:

                          http://www.palomar-engineers.com/