©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
           Changing Operating Frequency (Channel)
    3.9 Manual Single Channel Reprogramming
           50 Ohm Antenna or Test Set
    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
9.0 Improvements

1.0 Introduction and General Information

1.1 Equipment Description

1.1.1 Purpose of Equipment

The U.S. Marine Corps accepted the "A" version in 1983.

Radio Set AN/PRC-68A (radio set)  is a hand-held receiver-transmitter that provides ground-to-ground voice communications in the 30 megahertz (MHz) to 79.975 MHz band.  The unit is capable of secure speech operation when it is  used with the Secure Voice Module (SVM).  The radio set can be used with a short or long antenna, with a standard military handset such as H-138/U, H-189/U or H-250/U, or with its built-in speaker-microphone.

This was the second in this series of radios.  Instead of the eight modules used in the PRC-68, the "A" version uses two modules, one on the front side and the other on the back side.

The frequency synthesizer has a microcontroller that converts the Frequency switch positions into serial data for the synthesizer IC and generates the 150 Hz tone and battery saver waveforms. The "A" version adds a EEPROM so that the channel frequency assignments no longer need to be in a sequence.

There are now two new switches inside:

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 internally programmed with ten (10) preset frequency channels.  The chanels must all be in one of the four filter/matching circuit bands listed above.  Later model radios moved the ANT matching switch to the outside of the radio allowing any frequency on any channel.
b.  Warning Tones - The PRC-68A  generates a warning tone for low battery.
C.  Antenna Matching Switch - The PRC-68A does not have an external ANT matching switch.  The internal switch needs to be set for the frequency band in use.  No Field Strength Meter (FSM) is needed when reprogramming the PRC-68A.
d. Liquid Crystal Display - The PRC-68A does not have an LCD.
e.  COMSEC Operation - Provides 16 kb VINSON compatible secure voice operation by simple attachment of the KYV-2,-2A 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-2,-2A or maybe just some other reason like the KYV-2.-2A modules were not available in the needed quantity.

When the battery voltage goes down to 10 volts the speaker will sound a low battery warning.  This may be heard by an enemy that is near by.

1.2 Photos

Front - Speaker Microphone and ID label
Front 1A3 Module -  814636-801 RF/IF with: Back - Push To Talk
Back 1A2 Module - 814635-801 Synth/AF with: Top,- there are just 5 terminals on the audio connector, matching the H-250 handset (6 terminals on later models) pin E on the AUDIO connector is for vehicle power.
When using pin E for power the radio operates with the switch set to OFF!  This is the way they decided to save the batteries.
The antenna connector has a 5/16" x 24 threaded hole.

If someone hands you a PRC-68A radio there is no way, of determining what frequencies is has been programmed for.  On all the later radios there is an LCD that displays the Rx and Tx frequency as the band switch is changed.  The internal switches are only active while programming a single channel and can be changed so that even the last channel programmed may not be given away by the switch positions.  All that you can tell is which of the four frequency bands is in use.

On the PRC-68 you can tell all of the channels by looking at the programming switches.  On all the later models you can see the frequency in the LCD, but on this radio you need a spectrum analyzer or scanner to find out the programming.  Maybe this was the major reason for the PRC-68B?

Bottom - battery snap connectors and SVM plug.

1.3 Table of Specifications

General Specifications

Frequency Range 30.000 MHz - 79.975 MHz
Available Channels 2000 in 25 kHz increments
Minimum Channel Spacing 25 kHz
Preset Channels 10 in one filter band
Modulation Frequency Modulation (FM) 
Operating Temperature Range -40 deg F to +159 deg F (-40 C to +65 C)
Weight (battery, antenna included) 50 oz (1.42 kg)
Size 9.35" (237.5 mm) x 3.8" (96.52 mm) x 1.52" (38.61 mm)
Communications range - long antenna & goose neck 1.0 mile (1.6 km)
Communications range - short (rubber duck) antenna 330 Yards (300 meters)
Typical power consumption
  • 25 mA when in receive mode with the radio quiet (0.8 AH for NiCad batt / .025 = 32 hours of Rx only time) 
  • 40 mA when VOL is set at max and radio driving internal speaker (20 hrs of listen only time)
  • 270 mA when transmitting   (3 hrs of Tx only time)
  • Transmitter Specifications

    Power Output 1 Watt
    Frequency Control Built-in Synthesizer
    Frequency Stability 0.005 percent
    Spurious and harmonics radiation 50 dB below RF carrier level 
    Modulation Limiting +/- 15 kHz Dev max
    148 to 152 Hz Squelch Tone 2.5 to 3.5 kHz deviation

    Receiver Specifications

    Adjacent Channel Rejection -60 dB
    Image Rejection -40 dB
    Sensitivity 0.5 microvolts for 10 dB SINAD
    Squelch sensitivity 0.5 microvolts
    Selectivity, 6 dB down bandwidth greater than +/- 15 kHz
                   , 60 dB down bandwidth less than +/- 50 kHz
    Response to Spurious Signals -60 dB
    Audio Output less than 10 % distortion at 20 milliwatts

    SINAD = (Signal + Noise + Distortion) / (Noise + Distortion)
    Squelch although 150 Hz is sent during 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-68 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.

    3-3 Frequencies with Reduced Sensitivity

    The following frequencies have receiver sensitivity reduction (the squelch may not quiet).
    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.
    BOLD frequencies are unusable.
    Band 1
    Band 2
    Band 3
    Band 4

    3.4 Description of Radio Set Controls, Indicators and Connectors

    ANT connector 
    Connects antenna (or 50 Ohm cable using 914598-801 adapter.) .
    CHAN selector switch
    Selects one of 10 preset operating channels.
    Turns radio set on (clockwise)  or off (full counterclockwise). 
    The spring loaded SQUELCH DIS position will disable the squelch as long as it is held.
    Controls the audio volume
    AUDIO connector
    Standard U-228/U style 5 pin connector for use with external handset, or SVM key fill divice
    PUSH TO TALK switch
    Enables radio set to transmit when pressed in normal operational mode. 
    Battery Connectors
    Connects battery to radio set.
    SVM connector & Plug
    Provides interconnection between SVM and radio set. 
    Jumper Plug allows radio to operate when SVM is not installed.
    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, PRC-128 but different from the PRC-68B that has a 1/4" X 28 thread.

    3.5.1 Short Antenna

    The Short Big Base Rubber Ducky antenna is used for communications to 300 meters.

    3.5.2 Long Antenna

    Just the tape measure antenna is used for communications out to 1,600 meters.

    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.  The internal Filter and Antenna Matching switches need to be set to the "0" position for 50 Ohm operation.  My 68AA Antenna Adapter has a built in DC return.

    The OE-254 and the RC-292 should work with this radio.

    The OF-185 PS/A needs to be tested with the PRC-68 and PRC-68A.

    The Diplexer, VHF, CU-2194/URC needs to be tested.

    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.

    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.

    3.9 Manual Channel Frequency Programming

    3.10 Manual Split Tx & Rx Frequency Channel Reprogramming

    The PRC-68A does not support split Tx and Rx operation.

    3.11 Reading (up loading) Channel Data

    The PRC-68A does not support data transfer.

    3.12 Filling (down loading) Channel Data

    The PRC-68A does not support data transfer.

    3.13 Repeater Operation

    The PRC-68A does not support repeater or retrnasmission operation in the conventional military sense, but the Radio Shack 19-345 time offset repeater does work.  When using the Radio Shack 19-345 and powering both the radio and repeater unit from a wall wart the current capacity needs to be at least 300 mA.

    3.15 Carry Bag

    Because of it's size the Squad Radio 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.  There are pockets for: radio, antenna, battery and a holder for a handset.

    Front Photo - showing a Squad Radio 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 on the other side a velcro flap to hold the Handset.

    Back  Photo - has two standard military pistol belt clips and will not attach to a standard civilian belt.  The bottom of the main pouch has an insert so that when removed will allow for the extra length of the Secure Voice Module.

    It may be the case that the original Operator's manual would also fit into the main pouch with the radio, but that's just a guess based on the possible manual size.  The reprint manuals are 8.5" x 11" because that's the standard copier paper size.

    3.16 150 Hz Tone Disable

    The PRC-68A does not support disabling the 150 Hz tone during transmitt.  When testing a seperate 150 Hz filter needs to be used.

    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.

    3.18 Frequency Planning

    When deciding on which frequencies to use in the range of a Squad Radio 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. 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 79.95 MHz.  This may be a reason to use or not use the 76 to 88 MHz range.

    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 Squad Radios, but it would be awkward to transmit by using the radio's built in microphone.  The amplified 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.

    3.21 Sub Audible (CTCSS) Tone

    The PRC-68A transmits a 150 Hz tone so that it can talk with older radios like the PRC-25, PRC-77 and AN/VRC-xx radios that need to hear the 150 Hz tone to open their squelch.  This tone is not used by ham radio operators for repeaters, so the PRC-68A can not operate on any repeater that needs a specified sub audible tone.  Also see Squelch Capture.

    There are aftermarket CTCSS tone generators that may be adapted for this use.  More needs to be done.

    4.0 Theory of Operation

    4.1 8 Modules

    The PRC-68A and all the later Squad Radios use two modules.  On the front side there is the 1A3 RF/IF module and on the back the 1A2 Synth/AF module that also contains the microcontroller.

    The way the modules on the PRC-68A work is different from the newer radios.  It uses a 12.0 MHz deviation oscillator out of the Synth module on pin P2-10 for transmit and the later radios use a 21.4 MHz deviation oscillator out of the Synth module on pin P2-2.  This means that the PRC-68A modules can not be used on any of the newer radios.

    TSEC/KYV-2A  Secure Voice Module     SVM

    5.0 Maintenance Instructions

    For repair advice you can contact Jim Karlow, KA8TUR  He might do a repair, but it is the third priority for him after his job and family.


    The TS-3354 was designed for testing the PRC-68 but can be used with other Squad Radios in this family.


    This is a more modern version of the TS-3354 for use with newer 30 to 88 MHz 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.

    PS magazine - ???

    Audio/Power Test Adapter

    This has the following parts: The above circuit diagram is from TO 31R2-4_810-3 for the PRC-128.
    The 914877-801 is an earlier version of the Audio/Power Test Adapter and is a part of the MK ()/PRC-68 Maintenance Kit.

    Tool Kit TK-101/G

    Manual SC 5180-91-CL-R13
    Contains the spanner wrench for the antenna connector.

    6.0 Parts List

    Part No.
    Radio Set PRC-68A 5820-01-180-8943

    37695 705956-804
    Receiver Transmitter
    5820-01-229-6812 Fig 1 4 80058
    Carrying pouch 8465-01-152-1157     37695 349924-1
    Carrying Harness Sling       37695 348814-2
    Lanyard 5985-00-933-2454 Fig 1 2 80063 SM-B-522304
    Short Antenna 5985-01-096-9396 Fig 1 1 37695  914161-803
    Alignment Tool 5820-01-096-9410     37695 808234-1
    36" Long tape meas Antenna AS-3575/PRC
    (no gooseneck) 
    5820-00-889-3803 Fig 1 4 37695   
    Battery Spacer   Fig 1 3 37695 347344-1 
    ID Plate   Fig 1 5   155035 
    1A1 Frame & Panel assy 5820-01-094-6523 Fig 2 1 37695 917327-803
    1A2 -  Synth/AF module    
    37695 814635-801
    1A3 - RF/IF module   Fig 2 3 37695 814636-801
    1A?10 module cover   Fig 2 7 37695 918267-803
    1A?11 Battery Housing
    Fig 2 9 37695 914153-803
    Antenna Connector Adapter with BNC(f)       37695 914598-801
    Handset 5965-00-043-3463     80058 H-250/U 
    BA-5588/U Battery Dry Lithium/SO2
    replaced by BB-388
    Fig 2
    80058 BA-5588/U 
    BA-1588/U Battery, Mercury  6135-01-088-2708        
    BB-588/U Battery, NiCad including housing 6140-01-091-1536 
    Simple battery Charger          
    Single Station Battery Charger       37695 565604-801
    Five Station Battery Charger       37695 706841-801
    Bren-Tronics model BB-388A/U 6140-01-419-8190        
    AP-388/U Battery to Charger adapter (holds 2 batt) 5940-01-427-8601        
    BA-715 for only the PRC-68,-68A & 68B?          
    PP-8444/U, Universal Portable Charger (UPC) costs $562.52 new 
    TM 11-6130-489-13&P (075017.pdf) is Restricted on ETM
    Silicon Compound, 2 oz 6850-00-177-5094        
    Lint Free Cloth, yard  7920-00-924-5700        
    TSEC/KYV-2A  Secure Voice Module     SVM 5810-01-160-4999      
    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       56698-802
    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 5965-00-892-0972        
    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          
    Silicon Grease 6850-00-177-5094        
    68AA Antenna Adapter
    w/built in Dc return
    na na na na na
    68BA Battery Adapter na na na na an

    7.0 Reference Information

    7.1 Manuals & Literature

    TM 11-5820-882-10 - Operator's Manual, PRC-68A (060365.pdf = Restricted)
    TM 11-5820-882-23&P-1 (062081.pdf = Restricted)
    TM  06827B-24/2- Technical Manual, Maintenance Instructions
    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-68.  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-68A 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 100 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 25 Ohms the radio mutes the internal speaker.

    (For the PRC-68 SPKR mute requires less than 3.3 K Ohms? thismay be a function of attery voltage?)

    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)

    To use external power set the OFF-ON-SQUELCH DIS switch in the OFF position.  That turns off the battery and connects to the External power.
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