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Home :: PRC-47 Variable Tune Modification

PRC-47 Variable Tune Modification

By Major Breckinridge S. Smith USAF (Retired)
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Please Note: Army Radio Sales Co. and this Article's authors are not responsible for any damages or personal injury whatsoever, that may occur as a result of information provided here. This Article is published in good faith and as far as we can tell accurate. Make sure you understand the instructions before starting. Modifications to military radio sets may invalidate the suppliers warranty and reduce the re-sale value of the radio.

Variable Frequency Control For The PRC-47
A simple mod requiring one 100K pot and 20 inches of wire. Anyone can accomplish this mod that has a soldering iron. This mod is stable and has been tested on several radios.

Thanks to Ray KA3EKH for collaboration on this mod, it was his idea to direct me to re-attack the temperature compensation circuit.

The PRC-47 transceiver can only be adjusted in 1 Kc steps, this modification will allow you to swing the oscillator 400 to 500 cycles on 80 meters, 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. See block diagram below.

Terms Used:
* Pot = Potentiometer.
* Osc= Oscillator.
* Mod= Modification.

Master Oscillator Block Diagram


Modification Points

Circuit Diagram

The mod consists of two changes in the circuit of the R.F. Oscillator module. All you have to do is to lift the end of a resistor (ARROW) and solder on a wire, and put a wire jumper across a small cap C-5 (ARROW) in the Oscillator module. By lifting the wire you are disconnecting the temperature compensation circuit for the crystal oscillator, you will then provide a voltage via a 100K pot mounted on the front panel of the radio to swing C-12 a crystal varicap. By soldering a wire across C-5, which is a "series" cap you are effectively removing it from the circuit and thus increasing the operating value of varicap C-12 making its value higher and more effective.

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.

Remove The Foam, Correct Side

This Is The Wrong Side

The Old Foam

The First Target, 470K Resistor
The first target "Red Arrow" is the end of R-6 a 470K resistor. (Yellow-Violet-Yellow)

Unsolder the end shown by the "Red Arrow" and lift the resistor end. Do not unsolder any other wires on this terminal. The "Yellow arrow" points to C-5. The resistor is now lifted free.

Connect a 5 inch wire to the free end and then cover the resistor and wire connection with heat shrink for insulation and strain relief.

Moving on, the 2nd target , "Yellow Arrow" ,C-5 a 1000 pf ceramic cap. You should be able to see the value printed on the side.


Un-Soldering The 470K Resistor

Covering The Resistor & Wire

The Second Target

Soldering The Wire Across The Cap
Solder a wire across the cap leads. A lead cut from a resistor makes a nice wire.

Find the connector which is in the bottom of the oscillator module. You are going to use a spare pin to supply the variable voltage from a panel mounted pot. 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 and the mod functioned OK.

Diagram of the module connector located in the bottom of the module, see diagram below.

See the picture of  the connector J9 underneath the chassis showing the spare pins available. I choose pin four, the second pin over from the large connectors A2 and A3. It all ready has my Violet wire connected. This is a good choice to prevent shorting out the connection. This picture is for information only you will connect the pin later.

The Connector

Module Connector Diagram

Connector J9

R6 and C5 Photo
Photo showing the red wire connected to R-6 and the short across C-5. Photo also depicts how the master Osc. is trimmed for frequency, No trimming was necessary on any of the radios tested. Just leave the trim alone. Leave The Trimmer Alone!

Replace the foam with a new piece or else carefully pick up all of the little pieces of foam that you removed previously and glue it all back. Replace the cover and 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.


Replace The Foam

Replacing The Cover

Removing The Fuse Holder

Ex-Fuse Hole Now For The Pot
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.

See the picture of the rear of the pot mounted in the hole, Violet the centre 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 for the other wires.


Connect 3 Wires To The Pot

Install The Pot

Rear Of The Pot

Next target area. 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.

Wire Strain Relief

Next Target Area, See Arrows

Snaking The Wires

Test Jig
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 centre wire of your pot to pin 4 of J-9, see diagram below. Put a piece of heat shrink on the wires. Its usually easier to put the heat shrink on the wire first and then solder the wire.


Connecting The Red Wire

Connecting The Centre Wire To J9

Module Connector Diagram

Knob Installed
All there is to do is to install the knob. To check calibration and find the 12 o'clock or centre position for your pot tune in a time standard, I like CHU on 7335 Kcs.

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