Scope Shield

©Brooke Clarke, N6GCE

Much of the following operational and engineering information is from analyzing how the radio works and does not appear in any of the manuals.

Table of Contents

1.0 Introduction and General Information
    1.1 Equipment Description
    1.2 Photos
    1.3 Table of Specifications
2.0 Preparation for Use and Installation
3.0 Operating Instructions
    3.1 Introduction
    3.2 Controls, Indicators and Connectors
    3-3 Frequencies with Reduced Sensitivity
    3.4 Description of Radio Set Controls, Indicators and Connectors
    3.5 Antenna Connection
    3.6 DC Power
    3.7 Handset
    3.8 Changing Operating Frequency (Channel)
    3.9 Manual Single Channel Reprogramming
    3.10 Manual Split Channel Reprogramming
    3.11 Reading (uploading) Channel Data
    3.12 Filling (downloading) Channel Data
    3.13 Repeater Operation
    3.14 Datron World Communications Inc. MT1060MM
    3.15 Carry Bag
    3.16 150 Hz Tone Disable
    3.17 SPKR/MIC Mute Circuit
    3.18 Frequency Planning
    3.19  Antenna Planning
    3.20 Speakers
4.0 Theory of Operation
    4.1 Modes of Operation
5.0 Maintenance Instructions
6.0 Parts List
7.0 Reference Information
    7.1 Manuals
    7.2 Web Links
    7.3 NIIN Text File
    7.4 Related Radios
8.0 Experiments
    8.1AUDIO J5 Pins
    8.2 Working Cloning & Retransmission Cable Diagrams
    8.3 Drive Testing

1.0 Introduction and General Information

1.1 Equipment Description

1.1.1 Purpose of Equipment

The AN/PRC-128 is a short range, handheld tactical radio for use primarily by Air Force guards and special operations teams.  It was not intended for Army combat use.   AN/PRC-128 is a lightweight, militarized transceiver providing two-way, voice-communications. The radio covers the frequency range of 30-87.975 megahertz or 130 - 173.9875 Mhz depending on which RF module is installed. Its nominal range for reliable communications over rolling, slightly wooded terrain is 3,000 meters. The radio is capable of interoperating with the AN/VRC-12, AN/PRC-77, and SINCGARS families of radios in the VHF Low band fixed frequency mode.  AN/PRC-128 enables small unit leaders to adequately control the activities of subordinate elements in carrying out the unit's mission. AN/PRC-128 is a part of the Scope Shield Program with the following major components:

1.1.2 Characteristics, Capabilities and Features.

The physical and electrical characteristics of the radio set are given in 1.3 Table of Specifications.
Special capabilities and features of the radio set are as follows:
a.  Preset Frequency Channel Capability - The radio set is able to be externally programmed with - ten (10) preset f requency channels anywhere in the frequency range.
b.  Warning Tones - The radio set generates two separate warning tones; one indicates the battery is nearing its end of life and the other indicates a mismatch of the antenna and the operating frequency.
C.  Antenna Matching Switch - This thumbwheel switch selects the proper antenna matching network for the selected operating frequency.  The antenna warning tone is enabled if the switch position is incorrect.   In addition, the "50" position of the switch bypasses the antenna matching networks and provides a direct 50 ohm output for test use or any 50 Ohm external antenna,
d. Liquid Crystal Display - A lighted five-digit 7-segment display to indicate frequency, operational mode, and programming information.
e.  COMSEC Operation - Provides 16 kb VINSON compatible secure voice operation by simple attachment of the KYV-2() NSA approved COMSEC device between the radio set and battery.

The prior sentence is taken from the radio manuals, but the comments (see Links below) indicate that the radio is NOT capable of secure operation.  This may be because of some problem with the KYV-2A or maybe just some other reason like the KYV-2A modules were not available in the needed quantity.  The PRC-126 does not frequency hop like the SINGARS and other modern military radios.

When the battery voltage goes down to 10 volts the speaker will sound a low battery warning.  Also if the channel or antenna tuning rotary switches are misadjusted the antenna tuning warning tone will sound.  These may be heard by an enemy that is near by.

This radio is a member of the PRC-68 Family and uses the same frame-panel assembly as the PRC-126 and PRC-136 radios.

1.2 Photos

Front -
Back -
Top -
Bottom -


Top - Does NOT have the temperature sensing terminal
Bottom - Does have charging contacts
Front - Recomended chargers: 5-station 706841-802, 565604-801, -810, -803
Back - Recommended 130 mA for 6 hours between 40F and 100F batt temp
Manual Cover -
Low Band Module Top, Bottom -

1.3 Table of Specifications

General Specifications

Frequency Range 30.000 MHz - 87.975 MHz or
130 MHz - 173.9875 MHz 
depending on RF module
Available Channels 4640 in 12.5 kHz increments Low Band
3520 in 12.5 kHz increments VHF Hi Band
Minimum Channel Spacing 12.5 kHz
Preset Channels 10 can have seperate receive and transmitt frequency
Modulation Frequency Modulation (FM) 
Operating Temperature Range -45 deg F to +70 deg C (      F)
Moisture Resistence Watertight to a depth of three feet
Weight (battery, antenna included) 50 oz (1.42 kg)
Size 9.97" (253.2 mm) x 3.78" (96.0 mm) x 1.52" (38.6 mm)
Low Battery Tone Four 400 Hz beeps at six second intervals when Vbatt drops below preset voltage
Antenna Mismatch Tone 1000 Hz tone at 2 second intervals when selected frequency and ANT match switch position do not agree and the transmitter is disabled.
Communications range - long antenna & goose neck 1.86 miles (3000 meters)
Communications range - short (rubber duck) antenna 0.31 miles (500 meters)
Typical power consumption
  • 10 mA when in receive mode with the radio quiet

  •       (0.8 AH for NiCad batt / .01 = 80 hours of Rx only time) 
  • 46 mA when VOL is set at max and radio driving internal speaker (17 hrs of listen only time)
  • 470/290 mA when transmitting   (1.7/2.8 hrs of Tx only time  Hi/Lo band)
  • Transmitter Specifications

    Power Output 1 Watt
    Frequency Control Built-in Synthesizer
    Frequency Stability +/- 5 ppm max
    Spurious and harmonics radiation 43 dB below RF carrier level (second harmonic 40 dB)
    Modulation Deviation 4.5 kHz, limited at 7.5 kHz VHF LowBand
    3.5 kHz limited to 5 kHz VHF Hi Band
    (this is very close to commercial practice)
    Squelch Tone 148 - 152 Hz, 2.5 to 3.5 kHz deviation

    Receiver Specifications

    Adjacent Channel Rejection -60 dB
    Image Rejection -60 dB
    Sensitivity 0.5 microvolts for 12 dB SINAD
    Squelch sensitivity 0.3 microvolts
    Selectivity, +/- 25 kHz -50 dB VHF Low Bnad
    -60 dB VHF Hi Band
    Response to Spurious Signals -60 dB
    Audio Output less than 10 % distortion at 100 milliwatts

    SINAD = (Signal + Noise + Distortion) / (Noise + Distortion)
    Squelch although 150 Hz is sent during Low Band transmission the receive squelch operates based on signal not 150 Hz tone.

    2.0 Preparation for Use and Installation

    3.0 Operating Instructions

    3.1 Introduction

    Because of it's size the PRC-128 will not fit into a shirt or pants pocket (maybe a battle fatigue pants pocket).  Your choices are hold it in your hand all the time, OK for some uses, or use the carry bag connected to a military pistol belt and use the H-250 handset, or in quiet locations where you can hear the built in speaker, the M-80 Microphone.

    3.2 Controls, Indicators and Connectors

    Each operating control, indicator and connector on the radio set is identified in the photos above and described in paragraph 3.4.
    VOL Hint
    Note that except for front line use you should always turn the VOL control at least half way.  This is because if you just turn past the OFF position the speaker volume will be low any you will not hear the warning tones.  You want to hear the warning tones so that you will know that the ANT switch is in the correct position and that the battery is OK.  If you miss a warning tone because the VOL was set too low, you may spend some time trying to figure out why the radio is not working.

    3-3 Frequencies with Reduced Sensitivity

    The following frequencies have receiver sensitivity reduction.
    If they are selected during testing in Section 5, erroneous readings may result.
    If they are used in the field the reception range will be greatly reduced.
    ? list for VHF Low Band ? below is PRC-126 table?
    46075 50675 61425 71675 81925
    47075 54650 61450 76375 * 83925
      54675 64175 76400 *  
        64200 76425 *  
        64225 76450 *  
        66550 76475  
        66575 76500  
        69850 76800 *  
        69875 79350  

    * Frequencies with receiver sensitivity reduction below service condition limits. -- Potential Degraded Channels.

    3.4 Description of Radio Set Controls, Indicators and Connectors

    Figure 3-1. Radio Set Controls, Indicators and Connectors
    Index No.
    ANT connector 
    Connects antenna (or 50 Ohm cable using 914598-801 adapter or my 68AA ) to radio to radio set.
    CHAN selector switch
    Selects one of 10 preset operating channels. 
    In programming mode, selects channel to be loaded.
    VOL/OFF switch
    Turns radio set on (clockwise)  or off (full counterclockwise). 
    Adjusts level of sound heard from radio speaker or handset earpiece.
    AUDIO connector
    Standard U-183/U style connector for use with external handset, or frequency transfer cable (cloning device or frequency programmer)
    PUSH TO TALK switch
    Enables radio set to transmit when pressed in normal operational mode.
    Selects transmit frequency when pressed just prior to SET  in manual programming mode. 
    Initiates output of all channel frequency data when pressed in FILL mode.
    Battery Connectors
    Connects battery to radio set.
    SVM connector & Plug
    J1 & P1
    Provides interconnection between SVM and radio set. 
    Jumper Plug allows radio to operate when SVM is not installed.
    SQ DSBL pushbutton switch
    Disables receiver squelch circuit while pressed (turns on speaker)
    The LCD will display either HI or LO depending on the RF/IF module that's installed.
    INCR pushbutton switch
    Used in manual programming mode to increment frequency digits. 
    Disables the Low Band 150 Hz squelch tone during transmit for maintenance testing on the PRC-128.
    SET pushbutton switch
    Used to initiate the manual programming mode, step through digits in display and LoAD new frequency information into a channel.  Programming mode is activated if SET button is pushed within 10 seconds of radio turn on.  Manual programming mode is deactivated 10 seconds after channel is programmed.  If the programming of additional channels starts within 10 seconds of completion of previous channel's programming, radio does not need to be turned off and on to retain activation of the programming mode. 

    Does turn on a backlight for the LCD.  But  you can only see it using Night Vision Devices.  With bare eyes you can not read the LCD with the back light turned on.

    FREQ display
    Five digit Liquid Crystal Display (LCD) used to indicate the selected preset channel frequency in MHz or the digits for tens of MHz and less for VHF Hi Band (the 1 is not displayed.).  If a split frequency channel, the display will show the Tx frequency while PTT is depressed and the Rx frequency when in receive mode. 
    To turn on the backlight depress the SET pushbutton. 
    The backlight is on continuously during the programming operation. 
    Other indications are as follows: 
    LoAD - In manual programming mode, indicates data previously displayed is now loaded in the selected preset channel memory location. 
    FILL - Indicates that a cloning device or remote programmer is connected and the radio set is in FILL mode and is ready to send or accept program data.
    Number in center of LCD in the range 1 to 10 indicates the channel number being uploaded or downloaded. 
    donE - appears after all channels have been uploaded. 
    HI or LO - when SQ DSBL is pressed (RF module band)
    Blinking display - when invalid frequency (between 174 and 188 MHz) is programmed in hi-band mode of operation.

    ?A3125 - appears after donE WHAT DOES THIS MEAN?

    ANT match switch
    Selects proper antenna matching network for the operating frequency selected or bypasses the antenna matching networks to provide a direct 50 Ohm output impedance.  Warning tone sounds in speaker if match is incorrect.  Corresponding frequencies for each switch position are as follows: 
    A  = 30.000 - 35.9875
    B  = 36.000 - 45.9875
    C  = 46.000 - 53.9875
    D  = 54.000 - 63.9875
    E  = 64.000 - 87.9875
    50 =130.000 -173.9875 &
    50 = 50 Ohms output 

    50 Ohm NOTE

    In order for the warning tone and transmit disable circuit to sense that a 50 Ohm antenna is in use: 

    A DC path to ground of 2,000 Ohms or less must be provided between the antenna terminal and ground when transmitting or receiving in the 50 Ohm output position.  There is a Dc return in my 68AA for this reason.

    If the antenna does not provide this DC path, an external DC path must be provided with an RF impedance greater than 1000 Ohms.

    Functions as a built-in speaker (receiver) and microphone (transmit) when external handset is not connected.
    When an external handset or headset is connected the internal speaker will go into mute mode.

    3.5 Antenna Connection

    The threaded hole that the antenna mates with has a 5/16" X 24 thread the same as the PRC-68 & PRC-126 but different from the PRC-68B that has a 1/4" X 28 thread.

    3.5.1 Short Antenna

    The Short 19" tape measure antenna is used for communications to 500 meters on either the VHF Low band with the ANT switch set to the correct position (so there are no warning tones) or for use on VHF High band with the ANT switch set for 50.  Note that the short antenna is NOT a rubber duck, it's a 19" tape measure style.

    3.5.2 Long Antenna

    The36" tape measure antenna are used for communications out to 2,000 meters on VHF Low band only with the ANT switch set for the correct range (so there are no warning tones).

    3.5.3 50 Ohm Antenna

    When an external 50 Ohm antenna is used the Antenna adapter is used to convert from the special radio connection to a standard BNC(f) connector.  When this is done the antenna needs to present the radio with less than 2,000 Ohms DC resistance and the ANT switch set to the 50 position. Low Band External Antennas & Related Equipment

    The OE-254 antenna has a balun and appears as a DC short.  The RC-292 is a DC open and needs a choke added.  My 68AA antenna adapter will allow the simple use of the RC-292 and ham radio mobile whips.

    The Diplexer, VHF, CU-2194/URC has DC open radio connections and would need 2 each DC shorts to be used with 2 PRC-126 radios. Hi Band External Antennas & Related Equipment

    The OE-254 may work up to 174 MHz, some testing is needed here.

    3.6 DC Power

    3.7 Handset

    The H-250 Handset can be connected to the AUDIO connector.  This allows the Squad Radio to be carried in it's Carrying Bag attached to a standard pistol belt.  It also makes is much easier to manually program split Tx and Rx frequencies into a channel.

    There is a flat on the handset connector.  If this flat is positioned on and parallel with the PUSH TO TALK side of the radio it will mate with a small amount of rotation saving some time.  This can be easily done by feel.

    Other U-229 Audio accessories should also work with this radio such as the H-138/U, H-189/U handsets.

    3.8 Changing Operating Frequency (Channel)

    When the channel is changed the Antenna warning tone may sound.  If this happens just rotate the ANT coupler switch until a position is found where the tone stops.
    If the new frequency is lower than the previous one rotate the ANT switch toward A, if the new frequency is higher rotate toward E.

    3.9 Manual Single Frequency Channel Reprogramming

    3.10 Manual Split Tx & Rx Frequency Channel Reprogramming

    This is a capability that is not in any of the manuals.
    The field radios can use split Rx and Rx frequencies either for use with a repeater (You can combine two 6-pin Audio type Squad Radios radio sets using a repeater cable.) or to just seperate the Tx and Rx frequencies to make it more difficult for an eavsdropper.
    See the PRC-68B Repeater Information.

    First set the Receive Frequency as follows:
    Set the CHAN switch to the channel number to be reprogrammed
    Power Off for a few seconds
    Power On and within 10 seconds start the procedure

    Press SET to LoAD the receive frequency into the channel
    Now repeat the above bulleted steps for the transmit frequency To confirm that you have a split frequency channel read the LCD while in Rx mode then press PTT, the display will show the Tx frequency.
    Note that the PRC-128 can either transmit or receive it can not operate in duplex mode where the transmitter and receiver are both working at the same time.

    Very Different Split Frequencies

    If split frequencies that are very different are programmed, for example receive on 87.9 MHz and transmit on 30.0 are programmed, the antenna beep tone will sound until you turn the ANT knob to A.  The transmit frequency is what controls the ANT match function, not the receive frequency.
    When in Rx squelched or audio out modes the display will show 87900 and there will be no antenna beep tone.

    This is a very good way to handle this case as it allows split repeater operation where the Tx and Rx frequencies are in different antenna matching bands and also different filter bands.  The 30 to 88 MHz band is split into two bands 30 - 50.975 and 51 to 88 MHz with filters in both the transmit and receive signal paths.  By using a Tx frequency in one band and Rx frequency in the other band the repeater desensing will be greatly reduced.  The PRC-25 & PRC-77 retransmission systems require 50 feet of separation when a diplexer is not used.  It may be possible that two 6 pin type audio connector Squad Radios radios could be used in repeater mode and be quite close together without a diplexer.  The Magnavox repeater cable is six feet long.

    3.11 Reading (up loading) Channel Data

    In order to use the channel assignments already in a radio to program other radios the first step is to copy the channel data into the cloning device.  This is done by: Also see seperate web page for Audio Connectors & Cloning - Fill - Retransmission for more information on the PRC-126.

    3.12 Filling (down loading) Channel Data

    This can be done using a cloning device or remote programming devicewhich could be either a keypad or laptop computer.  The procedure is:

    Channel Reprogramming Using the Frequency Transfer (Cloning) Cable (from PRC-68B manual)

    This procedure explains how to load preset channel frequency information from a master radio set into a second radio set using the optional frequency transfer cable.

    a. Refer to paragraphs 3.9 and 3.10 as required and load the desired frequencies into the designated master radio set.
    b. Turn the OFF/VOL control on both radio sets to OFF.
    c. Remove handsets (if used) from both radio set AUDIO connectors and connect frequency transfer cable between the two AUDIO connectors.
    d. Turn both radio sets on. Both radio displays will indicate FILL and both will have squelch defeated (receiver noise present in SPKR/LMIC).
    e. Press the master radio  PUSH TO TALK button to start the data transfer.
    f. Frequency transfer is complete when both radios become squelched and the master radio set displays a frequency.
    g. Turn the OFF/VOL control on both radio sets to OFF.
    h. Remove frequency transfer cable and install handsets (if used).
    i. Turn both radio sets on and check channel 6 to verify frequency transfer.

    The implication of this is that the cloning is done with a cable as opposed to using a cloning device that holds that frequency data.  Since the data protocols are different for different radios, only the same model number radio can be cloned using a cable.

    This is not quite true, if a radio using 12.5 kHz channel spacing (like the prc-128) is cloned to a PRC-126 then the PRC-126 will work with those 12.5  kHz channels even though they can not be manually programmed.
    Using a computer does allow the channel data from one Squad Radio to be read and then down loaded into a different model or similar model Squad Radio.

    The cloing cable is symetrical.  That means that the radio whose PTT is pressed becomes the master radio.

    3.13 Retransmission Operation

    The following is from the PRC-68B manual, but should also work for the newer Squad Radios

    Two radio sets can be configured as a retransmission station using the optional repeater  cable.
    Use the following procedures to set up the two radio sets in the repeater configuration.

    a. Turn both radio sets on. Ensure antennas are installed on both radio sets.
    b. Set the designated receive radio set CHAN switch to the repeater input frequency.
        This frequency must be the same as the field radio sets transmit offset frequency.
    c. Set the receive radio set OFF/VOL control to midrange.
    d. Set the designated transmit radio set CHAN switch to the repeater output frequency.
        This frequency must be the same as the field radio sets' receive frequency.
    e. Set the transmit radio set OFF/VOL control to minimum volume but not OFF.
    (Note that J2 pin 24 is a switch that senses when the VOL control is at minimum.
    This must the Repeater Tx mode.)
    f. Connect the repeater interface cable between the two radio set AUDIO connectors.
    g. For maximum repeater station range, locate the repeater station at as high a point as possible, free from any physical obstructions.

    29 Aug 2011 - an eBay acution for a PRC-128 (High Band) shows the following frequency programming:

    PRC-128 high band channel table
    146.588 -5


    Desense occurs when a receiver has a strong signal coming into the radio that causes the AGC to set the receiver gain to it's minimum value even though the strong signal is not at the radios receive frequency.  The further from the received frequency the less of a problem this is.  If the strong signal is in the stop band of the receivers input filter then it is not a problem.  Since some of the filters in the PRC-68 family of radios may be low pass it would be best to make the transmit frequency of the repeater much higher than the receive frequency.  That would mean that the field radios that would use the repeater would have their transmit frequency lower than their receive frequency.

    For example setup the repeater with a Tx frequency of 89.9 MHz and Rx at 30.0 MHz.  Now the repeater receiver will have very good rejection of the transmitter.
    The field radios would be set for Rx on 89.9 and Tx on 30.0.  The field radios do not have a desense problem because they are far from the repeater.

    This is different from Squelch Capture.

    3.15 Carry Bag

    Because of it's size the PRC-128 will not fit into a shirt or pants pocket (maybe a battle fatigue pants pocket).  Your choices are hold it in your hand all the time, OK for some uses, or use the carry bag connected to a military pistol belt and use the H-250 handset.

    Bag Photo showing a PRC-128 in the main pouch with H-250 attached and a spare battery in the front pouch.  There is a place on one side to hold both antennas and the goose neck and on the other side a flap to clip the Handset.

    The bag back has two standard military pistol belt clips and will not attach to a standard civilian belt.  The bottom of the main pouch has a snap that when opened will allow for the extra length of the Secure Voice Module.

    The original Operator's manual (4' x 5 3/4") would also fit into the main pouch with the radio.
    This is the same part number carry bag as is used on the PRC-126.

    3.16 150 Hz Tone Disable

    If the INC pushbutton is pressed at the same time as PUSH TO TALK the 150 Hz tone that is normally transmitted on Low Band will be disabled, but only as long as INC is held down.  You can hear this if you listen to a receiver while pressing and releasing the INC button.  This function was included to aid testing the PRC-128, not for an operational reason that I know of.  The 150 Hz tone is needed by the PRC-25 and PRC-77 radios for their tone squelch to work.

    Note that there is no need for a 150 Hz tone on VHF high band becasue there are no prior radios in that band that need the 150 Hz tone.

    3.17 SPKR/MIC Mute Circuit

    This is the circuit that puts a DC voltage on the handset speaker in order to determine if there is an external handset.  If it finds that the resistance of the handset is below some value, then it mutes the internal speaker.

    This circuit also acts as a digital data receiver and sends the data to the micro controler.  In this function the mute function may not think there is an external handset and allows the internal speaker to operate.  The squelch is opened for data transmission by the microcontroller.

    3.18 Frequency Planning

    When deciding on which frequencies to use in the range of the PRC-128 consideration should be given to a number of factors.
    You may want someone to hear your transmission that has only a commercial receiver.
    You may not want someone with a commercial receiver to eavesdrop on your transmissions. Ham 2 meters 144 -148 MHz

    Another factor is the use of loop antennas for Direction Finding.  The popular US models AT-339/PRC (38 - 55.4 MHz) and AT-748/PRC  (30 to 76 MHz) range and do not completly cover 30 to 88 MHz.  This may be a reason to use or not use the 76 to 88 MHz range.  There is no equivalent to these lops for VHF high band.

    Compatibility with the PRC-68 would be improved if for example the PRC-126 used CHAN 5 for 51.0 MHz.  That way both radios would have the same channel number for the same frequency.  Note that the PRC-68 MUST have 51.0 on CHAN 5.

    3.19  Antenna Planning

    When these radios are going to be used for an operation, some thought should be given to the antennas for each radio whether used as a base, mobil, or repeater/retransmission radio.  There is a tradeoff.  using the ducky results in short range which may be a good thing depending on the circumstances.

    If you are looking for the maximum range, without going to amplifiers, then the best antennas and siting will be needed.

    3.20 Speakers

    It is possible to use the military speakers like the LS-454 with the PRC-xxx series radios, but it would be awkward to transmit by using the radio's built in microphone.  The amplifier speakers AM-4979/GR and AM-6747/V have vehicle DC power inputs and a pass through connector setup that allows connecting either a handset like the H-250 or more practical a microphone like the M-80. The AM-4979/GR is specified for +28 VDC operation, but inside there is a 1N2976 12 Volt Zener diode, so it looks like it would work just fine on a 13.6 VDC standard car supply.  A wire may need to be added from the 13.6 supply to pin E on the squad radio AUDIO connector to allow the radio to run off of car power.

    4.0 Theory of Operation

    4.1 Modes of Operation

    The radio can be in a number of different modes of operation.  The function of the controls and indicators changes depending on the mode of operation.  The Modes are:

    AUDIO J5 Functions

    See Chapter 8 Experiments on the AUDIO J5 Pins and Audio Connectors & Cloning - Fill - Retransmission web page for more information.

    TSEC/KYV-2A  Secure Voice Module     SVM

    5.0 Maintenance Instructions

    The radios in the PRC-68B series will display digits on the LCD with just the frame and AF/SYNTH module, the RF/IF mdule is not required for this.  The Cloning cable will transfer the frequency set from one radio to another of the same kind with just the frame and AF/SYNTH module.

    For repair advice you can contact Jim Karlow, KA8TUR  He might do a repair, but it is a low priority.


    The TS-3354 was designed for testing the PRC-68 but can be used with all the Squad Radios that operate in the VHF Low Band.  When using the TS-3354 Test Set with a Squad Radio, the antenna coupler switch on the Squad radio needs to be set for 50 Ohms (or "H" on the PRC-68B) to match the test set.  The beep tone will remind you to do this.


    This is a more modern version of the TS-3354 for use with the a wide family of VHF Low Band radios, not just the Squad Radios.  This test set contains a frequency counter, power meter for forward and reverse and a comb generator to test receiver squelch as well as a field strength meter so that the PRC-68 antenna circuits can be adjusted.


    What test set is used for the VHF High band (130 - 174 MHz)?

    PS magazine -

    These are PRC-126 Tips, but apply to the PRC-128 also.

    October 96 PS 527 pg 36
    AN/PRC-126 . . . Take Care of the Li'l Guy

    July     97  PS 536   pg 60
    AN/PRC-126 Packing
    The performed packing, NSN 5330-00-942-5120, in the bottom of your AN/PRC-126 radio set wears out and sometimes falls out.  Poor packing or no packing causes battery problems.  Check to see if the packing is there, worn or compressed.  If it's not there, or if it is worn or compressed so that the battery does not fit snugly, replace it.  It's item 7 in Fig C-1 of TM 11-5820-1025-24&P.

    March 98  PS 544   pg 61
    AN/PRC-126 Parts
    Here are four NSNs that PRC-126 radio users need.
    Antenna matching knob
    Battery Housing
    Carrying pouch

    Audio/Power Test Adapter

    This has the following parts: The above circuit diagram is from TO 31R2-4_810-3 for the PRC-128.

    Tool Kit TK-101/G

    Contains the spanner wrench for the antenna connector.

    6.0 Parts List

    Part No.
    Receiver Transmitter 5895-01-256-9639 C-1 80058 RT-1547/PRC-126
    Carrying pouch 5820-01-255-4068 C-1 2 37695 349924-1
    Lanyard 5985-00-933-2454    
    SMB 522304
    Ant Short 19" tape measure   C-1 3 37695  518026-1
    Ant AS3575 Long  36" Tape    C-1 4 37695 721153-3
    Antenna Connector Adapter with BNC(f)       37695 914598-801
    Handset 5965-00-043-3463 C-1 5 80058 H-250/U 
    Synth/AF Module       37695 721599-801
    Low Band Module       37695 721597-801
    High Band Module       37695 721598-801
    Scope Shield Radio Operating Instructions      
    Battery       37697 548103-804
    Simple battery Charger
    Bren-Tronics model BB-388A/U 6140-01-419-8190        
    AP-388/U Battery to Charger adapter (holds 2 batt) 5940-01-427-8601        
    PP-8444/U, Universal Portable Charger (UPC) costs $562.52 new 
    TM 11-6130-489-13&P (075017.pdf) is Restricted on ETM
    Squelch Adjusting Tool 5120-01-096-9410 C-2 2 37695 808234-1
    Seal, Nonmetalic ST 5330-01-218-8312 C-2 3 37695 345110-1
    Module Cover 5820-01-255-4070 C-2 4 37695 918267-804
    Dust Cap Assembly 5340-01-276-5783 C-2 7 37695 817455-801
    Knob, Channel 5355-01-283-6567 C-3 1 37695 517448-1
    Knob, Volume 5355-01-283-6568 C-3 2 37695 517448-2
    Connector, AUDIO S935-01-321-8082  C-3 3 37695 186137-5
    Switch, Pushbutton 5930-01-208-2271 C-3 4 81073 39-351BLK
    Push To Talk Button 5930-01-324-3265 C-3 5 37695 165803-1
    Jumper Plug, SVM 5935-01-099-0005 C-3 6 98278 095-9003-0024
    Frequency, Window Glass
    C-3 7 37695 349619-3
    Knob, Antenna Match 5310-00-368-4467
    C-3 8 36659 516502-2 
    Connector, Antenna 5985-01-325-1800 C-3 9 37695 187055-1 
    Power-Audio Adapter 5820-01-276-0380 
    C-4 1 37695 421378-801
    Coupler, Transmission (ANT to BNC(f)) 5985-01-097-7337 C-4 2 37695 914598-801
    Silicon Compound, 2 oz 6850-00-177-5094        
    Lint Free Cloth, yard  7920-00-924-5700        
    Secure Voice Module     SVM
    PCG-68 -
    Programmable Code Generator
    CSD-68 fill gun (Code Source Device?)          
    AN/GRM-114A Test Set 
    This is an IFR Communications Service Monitor Model 1000S and appears to have no PRC-xxx customization TM 11-6625-3016-14 (051046.pdf) and others are on line at ETM
    Vehicular mount/amplifier OF-185  5820-01-301-6301        
    Tool Kit TK-101/G 5180-00-064-6178        
    TS-3951/PRM-34 6225-01-094-5646        
    TS-3354 6625-01-091-3157        
    Frequency Transfer Cable (Cloning cable) 5995-01-201-1391       568698-801
    Repeater cable  none       568698-802
    OE-254()/GRC Antenna Group 30 to 88 MHz 5985-01-063-1574        
    RC-292 antenna: 
    30-36.5, 36.5-50.5 or 50.5-79.95 MHz (not 80 - 88)
    H-250/U Handset 5965-00-043-3463  5965-01-247-4723        
    H-138/U Handset          
    H-189/GR Handset 5965-00-069-8886         
    Ear Transducer (ear mic)
    Earphone Transducer (EM-200)

        37695 588088-1
    H-157 AIC modified
    no NSN
    no NSN
    Vehicular Adapter OF-185/PRC
    30 -88 or 130 - 174 MHz depending on amp/fil
    5820-01-301-6301       901602-801
    Diplexer, VHF, CU-2194/URC
    limited upper freq of 76 MHz
    Radio ports are DC open - not good with PRC-xx 
          51859 755115A0000
    Mofified Radio Shack 19-345 Simplex Repeater Controller          

    SIGNAL TECHNOLOGY CORP (formerly Eaton microwave products div, formerly Addition Labs?)
    SUNNYVALE, CA 94086

    7.0 Reference Information

    7.1 Manuals

    Note that Scope Shield is an Air Force program so the manuals are not Army Tms but rather Air Force TOs.

    MX-63-127 Scope Shield Hand Held Radio Operator's Instructions, Radio Set AN/PRC-128, 15 July 1988, 20 pgs (4" x 5 3/4")
    TO 31R2-4-810-1 Operating Instructions for Radio PRC-128
    TO 31R2-4-811-1 Operatiog Instructions for OF-185 RF Power Amplifier & Power Supply/Audio Assembly
    TO 31R2-4-810-3 Technical Manual, Depot Radio Set AN/PRC-128, RF Power Amplifier, and Adapter Group OF-185

    Magnavox presales sheet for Ancillaries Cables - showing the cloning and repeater (retransmission) calbes with no bumps, just cables.

    7.2 Web Links

    AN/PRC-68 Legacy by Alan D. Tasker, WA1NYR
    PRC Data series by Dennis Starks
    U.S. Military Portable Radios by By Alan Tasker, WA1NYR
    Collecting Military Radios by Ralph Hogan WB4TUR
    The Boneyard Radio Price Guide -
    The PRC-25 Story by Dennis Starks
    PRC-68-B by megaman
    AN/PRC-126, Radio Set at Fort Monmouth
    Non Tactical Portables at Army Radios web site
    Communications  Security and Related Equipment by Frederick W. Chesson -
    Additional Comments by Dennis Starks on the Army Radio web page -
    BB-388A/U, PP-8444A/U Charger, BA-5588U, 1588U, BB-588U
    Datron World Communications MT1060MM 35 W RF booster amplifier, power conditioner, and power supply.
    Audio Connectors & Cloning - Fill - Retransmission -
    Military Radio Specifications - Radio Set AN/PRC-126 (RT-1547) -
    Military Image Files - just a small gif image
    *Crossing Linear Danger Areas* - how PRC-126 is deployed in a Squad
    Soldiers of Fortune Ltd. - U.S. surplus equipment donated to the Bosnian mission include: 732 AN/PRC-126 handheld radios plus batteries, 1,600 AN/PRC-77 manpack radios with batteries
    Ranger Training Brigade - Ranger Handbook  TOCChapter Seven Communications - some operation and maintenance info
    Center for Army Lessons Learned - Communications Equipment - "AN/PRC-126 Squad Radio worked well, especially in MOUT. Minor criticisms noted the audio signal for low battery and other functions which jeopardized position security and the need for an ear piece and whisper mike." - In Praise of Checkpoints - Squad use of PRC-126 and reporting check points - How to Turn Company Morters into a Combat Multiplier - Other small element leaders cannot quickly talk to the mortars without first finding a PRC-119. NOTE: Most squads carry the PRC-126, which does not frequency hop, and is not secure (? what about the SVM?, Brooke comment). This also lengthens mortar section response time.
    Physics of Failure - AMXSY-LA UNCLASSIFIED 12 FEB 97 - Re-designs of ICAM, AN/PRC-126 and ARC-210 radios underway
    Tobyhanna - Tactical Radio Division - Communications Security (COMSEC) Depot Opertions - Other INFOSEC Links -
    C3 in the Maneuver Company -
    "Use of non-secure radios (PRC 126/127s) is not allowed until contact with the enemy..." in search engine, but broken link
    AN/PRC-117F Special Operations Forces radio has applications for digital divisions and beyond -
    Fort Bragg has established a central drop-off point for its 18 units that require repair support for their AN/PRC-126 handheld radios. Fort Bragg mails the radio components to Tobyhanna Army Depot, Pennsylvania, where they are repaired and mailed back to Fort Bragg. This system replaces the former practice of sending the radios through the regular supply system and cuts days off the turn-around time. The program began with the repair of circuit cards but has expanded to include other omponents, such as the PRC-126's frames and panels and its frequency synthesizer modules. Previously, these modules were thrown away rather than repaired.
    LRC Y2K Spreadsheet - the PRC-126 is Y2K compliant, it does not have a clock
    Oral History Interview JCIT 081
    1LT JOHNSON: Well, get back to training, but again that's one thing that we didn't want to lose; lessons that we had learned. We took time and really wrote up a pretty substantial AAR which we got here. I have already noticed some ... one change come out of it. It was very minor point to put on the tape, but on the PRC-126 the little display panel is in green plastic. The idea behind that is so that it reduces the amount of light when you hit the display light at night. But in actuality, it makes it very hard to read any time either during the day or night. We have been trying to do this for awhile, but we just included in this after action report and that's what brought about the changes and everything. Now there is just clear plastic.
    Special Operations Forces Posture Statement 2000 - Appendix C Key Programs & Systems - ** Improved weight/size and power consumption by replacing numerous multi-frequency/banded, hand-held radios currently used (i.e., AN/PRC-68, AN/PRC-126, MX-300, MX-300S, and MZ-300R) with one full range/band radio
    Specialty Defense Systems - Ranger Assault Carrying Kit (RACK) -

    7.3 NIIN Text File (NIIN.txt)

    This is a file that can be used for searching the DRMS  (Defense Reutilization and Marketing Service).
    It contains all of the NIIN numbers listed above in the Parts List.

    7.4 Related Radios

    A power increase of 6 dB (1 "S" unit or 4 times) the wattage) is just noticable by the human ear, so a radio would need 4 Watts to just be louder than the PRC-126.  Why would you want ot carry the weight of a PRC-25 or PRC-77 if you could have a PRC-126?  Adding the OF-185/PRC
     amplifier results in a much more powerful vehicle mounted radio set.

    8.0 Experiments

    8.1AUDIO J5 Pins

    I have not been able to find any public information on Cloning or remote programming of the PRC-126 or any other radio in the Squad Radio family.  Some of the following are experiments shed some light on this subject.

    J5-A (Ground)

    J5-B (Earpiece)

    Speaker Muting

    The PRC-126 J5-B (audio out) is near 0 VDC when the radio is quiet and the DC level jumps to about +6 VDC when the squelch opens.  The reason is to sense if an external earpiece is present.  If so the speaker is muted.
    Remember that the 1/4" phone plug does this mechanically, but with the MIL-C-55116 connectors there is no mechanical switching possible.

    An AC coupled audio accessory with a high impedance DC voltage controled comparator could be used to detect when the squelch opens and be used for retransmission PTT control.

    Pull Down Resistor Value for SPKR Mute

    With resistance values greater or equal to 10 K Ohms between J5-B (Audio Out) and J5-A (ground) the radio stays in internal speaker mode.  With resistance values less than or equal to 6.8 K Ohms the radio mutes the internal speaker.

    (For the PRC-68 SPKR mute requires less than 3.3 K Ohms)

    J5-C (Push To Talk)

    This has the same fuctions as the PTT switch on the radio.

    J5-D (Microphone Hot)

    From J5-D to ground is 150 Ohms.  This matches the impedance of the H-250 handset.
    This also is what provides the pull down resistance to cause the FILL mode to activate when an H-350 handset is connected.

    J5-E (DC Power)

    This is a power input connection.  There is a reverse polarity protection diode in series with this pin going to the OFF-VOL control switch. There is also a series diode between the internal battery and the OFF-VOL control switch.  Whichever voltage is higher will power the radio.  There is no provision to charge the main battery when an external supply is used.

    Although about 10 VDC appears on this pin when the radio is turned on, it drops to 1.6 VDC with a 1 k Ohm load so is not suitable to power external equipment.

    J5-F (Fill - Digital Data)


    When an H-350/U handset is installed on the Audio connector (J5) of a PRC-126 the LCD shows FILL.  This because the H-350/U provides a path to ground for pin F.

    After the PUSH TO TALK (like to send data out) switch (either on the PRC-126 or the H-350) is momentarily pressed the PRC-126  LCD counts from 1 to 10 and ends with donE, taking aobut 30 seconds.  Just before each digit appears in the LCD you can hear a short serial data burst from the microphone (not the earphone) of the H-350.  Removing the H-350 connector while donE is in the LCD causes  "A3125" to show briefly.  The radio never turns on the transmitter because is knows it's in the fill mode.

    Note that J5-F needs to be grounded while J5-C is also grounded.  You can not ground J5-F, unground J5-F, then very quickly ground J5-C.

    Pull Down Resistor Value for FILL

    With resistance values greater or equal to 150 K Ohms between J5-F (Fill) and J5-A (ground) the radio stays in normal receive mode.
    With resistance values less than or equal to 100 K Ohms the radio goes into FILL mode. (The PRC-68 does not have an F pin.)

    TTL Data while LCD counts 1 to 10

    This data is in the form of 10 packets.  Each packet lasts about 50 milliseconds.  It takes about 30 seconds for all 10 packets to be sent.  The first bit of each packet is a TTL high that lasts for about 10 mS and pin F is at TTL ground after each packet and prior to the next one arriving.

    Rx Signal

    When there is no receive signal pin F of the AUDIO connector is at + 4.9 Volts (TTL high) and the AC voltage is zero.
    When a signal is being received the DC voltage drops to 3.2 V and the AC signal increases to 2.35 V (indicating digital data).

    If the SQ DSBL button is pressed J5-F goes to 2.18 VDC and 2.55 VAC (indicating digital data).
    It is not clear if this is some kind of data or just TTL noise.
    Why is this?

    The AC could be rectified and used to sense when the squelch is open and be used for a retransmission system.

    Tri-State Logic

    Pin F seems to have FETs connected to the +5 Volt and Ground rails and can drive rail to rail when in Output mode.
    When in Input mode there is a 200 K Ohm pull up resistor to allow sensing when a ground is connected.
    Data Output Protocol (13 Nov. 2000)
    By using the Pico Technology ADC-10 I have analyzed the output data.  There may be a slight problem with this low cost way of getting data, if the computer has some task that preempts the ADC-10 it may miss data.  To help prevent this use <Ctrl><Alt><Del> and end all tasks except for Systray and Explorer in WIN98SE.  Still there was one place where data appears to be missing.

    I sampled once every 1 millisecond for up to 40 seconds and then trimmed the front and end of the data file.
    At the beginning of the file all the leading zeros were trimmed, leaving only a single zero.
    At the end of the file all the 2.4 volt readings (where pin F was acting as an input) and all but one of the trailing zeros were trimmed.
    The remaining file size is about 26,000 rows of data.
    Using Excel (open the data file which was saved with an extension of txt) and use delimited with a space to get it correctly.
    Each of the 10 data frames was copied and pasted into a different column (1 through 10) with the first zero to +5 volt transitions aligned.
    By looking at the data you can see that there is a start pulse and data separation pulses occurring about 440 mS apart.
    It looks like each of the 10 frames consist of 16 bits, 1 start pulse about 3 ms wide and 15 data bits each about 3 ms wide.
    The 10 frames correspond to radio channel numbers of 1, 2, 3, 4, 5, 10, 9, 8, 7, 6 in that order, not in the order of the numbers shown in the LCD.
    There are 9 Words in each frame:
    Rx 10 MHz, Rx 1 MHz, Rx 100 kHz, Rx (0, 25, 50 or 100 kHz),
    empty frame,
    Tx 10 MHz, Tx 1 MHz, Tx 100 kHz, Tx (0, 25, 50 or 100 kHz).
    The first three digits are represented by 4 bits (the remaining 15-4=11 bits are not used) in 1, 2, 4, 8 weighting with positive true logic.
    The 25 kHz is handled with 2 bits in 1,2 weighting i.e. 00 = .000 kHz, 10 = 25 kHz, 01 = 50 kHz and 11 = 75 kHz.

    This protocol can support much more than the PRC-126 uses to output the channel assignments.

    Excel file (PRC-126)with raw data in the left 2 columns plus processed data.

    11 April 2002 - The data protocol for the PRC-128 is  the same as the PRC-126 when a 30 - 88 MHz RF module is installed.  When a 130 -174 Mhz RF module is installed the stop bit after all of the channel 5 data has been sent is 6 ms wide instead of the normal 3 ms wide stop bit.

    MX-18290 on PRC-126

    P1 will mate but does not cause the PRC-126 to go into fill mode.  Opening the MX-18290 box shows why, pins F and B of P1 are not connected to anything.
    P1 is the connector that is used to fill the MX-18290 and J1 plus a 1:1 U-229 cable is used to fill the radio.
    Doing this test using J1 and a 1:1 cable may give different results, but I don't have the 1:1 cable.

    8.2Working Cables

    8.2.1 Cloning Cable 11 Dec. 2000

    The cloning cable is wired as shown below: This works on both the PRC-126 and PRC-68B.
    This is a symmetrical cable so that the radio that has PTT pressed becomes the master and the other radio gets reprogrammed.
    27 Jan 2001 - to make the Cloning Cable I used the U-183 connectors from two "RT-1209/PRC-104 to KY-99" cables that are offered on eBay.  The cable clamp is available from Newark as their p/n 96F4915.  Alpha Wire 1176C (DS-760914) is a six conductor cable that Newark also stocks.

    8.2.2 Retransmission (Repeater) Cable 15 Dec. 2000


    Multi-Use Radio Service - MURS Allocated Freq.  151.820, 151.880, 151.940, 154.570, 154.600 MHz

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