Audio Connectors
Cloning - Fill - Retransmission

©  Brooke Clarke 2000 - 2004

U-229 Audio Accessories - Separate web page
U-229 Pinout by Function - seperate web page
Military Audio Accessories - Small Photos

Audio Connectors

Spade Lugs
Phone Tip Plugs
PL-55 1/4 Inch Phone Plug
MIL-C-55116 Connectors
    U-228 & U229
    U-182 & U-183 (U-229 with 6 Terminals)
Remote Channel Programming
Crypto Fill
Squelch Capture Jamming
Radio Wire Integration & Remote Control 

Audio Connectors

Spade Lugs

The Western Electric #D14842 Handset uses spade lugs.

Phone Tip Plugs

Were used on many early headphones.  Both tips are balanced audio connections, or one may be grounded.

PL-55 1/4 Inch Phone Plug

In W.W.II the audio connectors were the same as common civilian connectors.  For example the 1/4" phone plug was used.  Since the jack makes a 1/4" diameter hole into the radio sand, water and other things that should not be in a radio found a way in.  The HS-32 uses this connector.
The tip is the hot audio connection and the sleeve is ground.  The current U-229 family of connectors are sealed.

When a Stereo 1/4" phone plug is used the common assignment is:

The jack that accepts the PL-55 and other 1/4" phone plugs can be obtained with contacts that are switched by the presence of the sleeve.  This can be used to mute a speaker when a headset is plugged in.  [I modified a B&W TV set by adding a switching 1/4" jack so that when I plugged in the headphones the internal speaker was turned off.  It also needed a resistor network so that the volume would be about the same in the headphones as with the speaker, otherwise it was WAY to LOUD.)  This mechanical switching function is not present in the MIL-C-55116 (U-229 family) connectors or the U-77 (10-pin) connectors but can be added by sensing the resistance between pin-A (gnd) and pin-B (headphone).

The identification of phone wires as Tip and Ring comes from the 1/4 plug used on switchboards.


See: M-29B Carbon Microphone U77

All the U-77 type Microphones are carbon.
By the Korean conflict the audio connector was standardized on a single U-77 10-pin connector as used on the PRC-6, PRC-47 and other radios.  This connector is sealed on the radio thus not allowing sand and other stuff to get into the radio.  10 pins makes it expensive both in terms of the connector proper as well as the associated cable.
The H-33F/PT Handset that uses a 10 terminal U-161/U connector.
H-233 Handset, LS-166 Speaker, H-90 Handset , M-29B uses this connector.

MIL-C-55116 Connectors

All the U-229 (see: U-229 Pin Out, U-229 Audio Accessories), microphones are dynamic.
Not only are these connectors water proof at the radio connector (the radio can be submerged) they also have a water tight contact area between the radio and the accessory.  This is done with an O-ring in the audio accessory connector that mates with a smooth barrel on the radio connector.
The 5 and 6 pin version of this connector are all covered by specification MIL-C-55116.
The U-228 and U-183 are typically panel mounted receptacles on the radio and U-229 (5 pin) and U-183 (6 pin) are plugs mounted on audio accessories.
DSCC - MIL-C-55116 specifications - Qualified Parts List (QPL) for the 55116 Connectors -
Approved Vendors are:
  • General Connector     FSCM 25330     (609) 935-7370 - Newark with on line ordering, pg N60 in Catalog 118
  • Nexus Inc.                 FSCM 28986     (203) 327-7300
  • Power Connector      FSCM 0CS66    (516) 563-7878
  • U-228 (chassis) & U-229 (handset)

    The M-80C microphone,  H-250 Handset has the 5 terminal U-229A/U connector with the following pin definitions:
    Audio Function
    Ground, Negative Battery
    headphone audio
    PTT Key  on hook = short, PTT off hook = open ckt
    Microphone audio
    Vehicle B+ on the PRC-68 & PRC-126, 
    Retransmission (switch to ground when receiver squelch opens) 
    on PRC-25, -77, RT-246

    The PRC-25, PRC-77, VRC-12, PRC-74 radios use the U-228 audio connector.
    TM 11-5965-280-15 (ETM 014514.pdf) for the H-189/GR has some information on the U-229 connector.

    U-229 Connector
            OpenedU-229 Connector opened to show the internal construction.   The white plastic sleeve has a slot so that it can be removed/installed first/last.   There are two asymmetrical  slots in the connector shell and two ridges on the blue plastic terminal holder so that when the terminal holder is installed in the connector shell the relative rotational position will always be the same.

    U-183 (chassis) & U-283 (handset) (U-228 & U-229 with 6 Terminals)

    U-183/U 5 terminals, panel mount NSN 5935-00-823-0667
    This connector has 6 terminals, A through E are exactly the same as on the U-229 and a sixth terminal is added in the center.  A radio with a 6 terminal U-182 audio connector can use all the U-229 5 terminal audio accessories.  Only when the function of the sixth terminal is being used does the radio need a mating 6 terminal connector. AUDIO 6-Contact-Standard (Conforms to MIL-C-55116)
    Audio Function
    Possible Digital Function
    Ground, Negative Battery Ground, Negative Battery, digital ground
    headphone audio audio +6 VDC when audio present 
    (external handset sense, Spkr mute) 
    WIth a Dc resistance <10 K Ohms PRC-126 or <3.3 K Ohms PRC-68 
    the internal SPKR is muted
    PTT Key  on hook = short, PTT off hook = open ckt data from RT-1439
    Microphone audio data to RT-1439
    Vehicle B+ on the PRC-68 & PRC-126
    Retransmission (switch to ground when receiver squelch opens) 
    on PRC-25, PRC-77, RT-246
    This is used as a power input for the radio. 
    The PRC-126 will run from a 10 - 18 Volt (12 volt nominal) car supply.
    Used to put PRC-126 into FILL (digital) mode, 
    digital data output (PRC-126) 

    Pressing SQ DSBL causes digital data (TTL noise?) to appear

    The RT-1439 radio uses the audio connector for digital data including TRANSEC variables, hopsets and lockout sets.  This is described in
    TM 11-5820-890-30-5 (070228.pdf) in Chapter 2 "Receiver-Transmitter, Radio RT-1439/VRC. Maintenance Instructions" and
    Chapter 18 "Fill Device ECCM MX-10579/VRC and MX-18290/VRC Maintenance Instructions".
    See Figure 2-11 "Fill Circuit Diagram" J3 is the 6 pin U-229 AUDio/FILL connector.

    The MX-18290 does NOT cause the PRC-126 to go into the fill mode.
    The 6 pin connector that's in the PRC-126 and other radios is the 3012761-2 80063 25380.


    Single Band Retransmission

    Retransmission is like two repeaters that are cross connected.  In a retransmission setup either radio can act as the receiver, whichever radio receives a signal first becomes the receiving radio and keys the other to become the transmitting radio.  This mode allows the squad radios to operate in simplex mode allowing them to talk directly to each other, yet also talk to other radios through the retransmission system.

    Note that in this system the squad radio is set for simplex operation at either of the repeater frequencies, but can only talk through the repeater to radios that are set for the other frequency.  For example a squad may have all their radios set for 51.0 MHz so they can talk directly to each other and the headquarters station may be set for 54.0 MHz so HQ can talk to anyone in the squad, assuming a repeater with radios connected with the MK-456 and the radios tuned to 51 and 54 MHz.  Also if a squad radio is out of contact with the other squad radios it can try switching to the retransmission frequency and talk to the rest of the squad.

    For squad radios that can operate on split frequencies (PRC-68B, PRC-126, etc.) the squad radio could be set to Tx on 51 and Rx on 54.  Now the squad radios can NOT talk to each other at close range, but only through the retransmission (or repeater) system.

    Cross Band Retransmission

    This is one of the great strengths of the retransmission system.  Radios operating on different frequency bands can be easily setup for retransmission, such as a VRC-74 (PRC-77) and GRC-213 (PRC-104) since both radio systems have the common VIC-1 connector.  This way a hand held VHF squad radio like and of the PRC-68 family, can be used for 2 way HF communications.

    Manpack Mk-456 Retransmission

    The army has the MK-456 retransmission cable kit (TM 11-5995-202-15 (017891.pdf)) that has a 50 foot long cable that interconnects the audio connectors on two radios like the PRC-25 or PRC-77, RT-246 and others with the same audio connector.  The cable contains a lot of RF filtering and the TM 11-5820-398-12 (018816.pdf)  for the PRC-25 contains a spurious chart showing the transmit and receive frequencies that will interfere if used as a repeater pair.  To use this kit the two frequencies must also be at least 3 MHz apart.  Both repeater radios are in receive mode and the one that receives a signal becomes the source for audio to the other (transmitting) one.

    * works with simplex radios like the PRC-68 PRC-25/77, VRC-12 that do not have split Tx-Rx capability.

    Example: retrans setup 51 MHz input and 54 MHz output.  People who have radios that can talk and listen on 51, when in range of a retransmission system, can talk to others who are on 54 simplex, like the SLO talk in staff.

    If someone has half duplex capability they can set up for Tx on 51 and Rx on 54 and talk to others who have the same set up.  Others may set up for Tx on 54 and Rx on 51 and can talk to each other, but not to the other group.

    A problem with retrans is that if the repeater is busy?

    If the CU-2194/URC Diplexer is used both radios can be connected to a common antenna. And the 50 foot separation is no longer required.

    On some radios there is a connector labeled RETRANSMISSION that is used with other similar radios to make a simplex repeater.

    The (U-183) retransmission cable used with the RT-1439 is cross wired as follows:

    FM 24-12 - Communications in a "Come-as-You-are" War
    c. The planning range can be further extended with retransmission operations.

         (1) Retransmission, or retrans for short, offers the commander a valuable alternative when multichannel equipment is in short supply or absent. As with NRI, retransmission is often not used to maximum advantage because of lack of knowledge or lack of confidence in its effectiveness. A shortage of multichannel equipment requires better planning and use of all other communications assets; retransmission is no exception.

         (2) A primary application of retransmission is the extension of a particular communications link such as an FM command net or a fire direction net. Another application might be a logistical link from brigade trains to the DISCOM area in the absence of multichannel. Traffic on this link would be for urgent requests for resupply of critical items only such as ammunition or POL, or for a contact team for the maintenance of critical items. Routine traffic should be sent by other means.

         (3) Retransmission should also be used to support anticipated operations or planned moves of important elements. For example, if a brigade CP is moving to a certain location at 1600 hours, a retransmission station could provide a communications link back to the DTOC. Retransmission may also allow the brigade CP to locate in a position that provides better physical security while still maintaining its essential FM radio communications.

         (4) Retransmission sites must be carefully chosen to maximize retransmission distance while at the same time minimizing enemy interception. Several alternate sites should be chosen for each retransmission facility to allow for periodic displacement.

    VIC-1 Based Retransmission

    Another way to achieve retransmission is to use equipment that can be interfaced with the Vehicle Inter Communications equipment (VIC).  The C-2299 Retransmission box can be used stand alone or with the AM-1780 Amplifier to interconnect two radios that are on vehicle mounts (although not necessarily in a vehicle, in my case they will both be indoors.) to allow retransmission.

    Radio Wire Integration Based Retransmission

    By using a GSA-7 (old tube type equipment) at both ends, two radios can be configured for retransmission.  The manual mentions this application.
    In a similar manner the C-6709/G can be used for retransmission by connecting them back to back with 2 pair of field wire.  Have not tried this but it should work.


    There are a number of possible problems with retransmission.

    Wrong R.F. frequencies

    The back of most manuals for the PRC-25 and PRC-77 have graphs showing those frequencies where spurious responses will degrade the performance of retransmission systems.  These frequencies are to be avoided.


    When a powerful signal gets into the R.F. front end of a receiver, even though it is outside the receivers I.F. passband, it can cause the AGC to turn the gain way down.  When this happens the sensitivity of the receiver is greatly degraded and it will no longer properly receive signals that it otherwise could receive.  To cure this problem either the two antennas need to be separated by say 50 feet for a pair of PRC-77 radios, or more for more powerful radios or use a diplexer, like the CU-2194 with the two transmit frequencies following the separation specs for the diplexer.

    RFI - EMI

    Another path for the transmitted R.F. is through the retransmission cabling.  Even if the antennas are properly separated, if the cable allows R.F. a good path, then the De-Sense problem may still be present.   For this reason and also as part of good design the retransmission cable contains chokes and bypass capacitors on all lines.

    Frequency Planning

    Just like for a repeater, the frequencies used need to be selected not only based on the avoidance of problem frequencies, but also they need to be compatible with the users radios.


    Standard Half Duplex

    This is similar to Retransmission but works differently.  Repeaters are very common on the amateur radio VHF and UHF frequencies as well as commercial and emergency services.  It takes 1/2 of the equipment to make a repeater than to make a retransmission system.

    A repeater has a radio acting solely as a receiver that feeds another radio that is acting solely as a transmitter.  They are on different frequencies and may connect to a common antenna if a diplexer is used to keep the transmitted signal out of the receiver.

    For the military there are two down sides of repeaters as compared to retransmission. In both of these cases retransmission continues to work without changing channels.
    This is very important because how would someone know to switch channels?


    A simplex repeater consists of a voice recorder that's put into record mode when a signal is received and goes into play mode when the input signal stops or when it has run out of memory.  The Radio Shack 19-345 works this way.  These are very easy to use since only one transceiver and one antenna are used.  They are also great for drive testing since the test can be done by one person.  Typically used for emergency coms and in sparsely populated areas.

    They cut the available bandwidth in half, since the channel is being used to record the message and then to play it back, a very poor way to rag chew.  Not a good thing to use in a populated area where all available channels are being used.

    Full Duplex

    I don't have any experience with these, but understand that you can get some hand held transceivers that work both VHF and UHF.  By using one band for receive and the other for transmit, there is enough R.F. isolation to prevent de-sense.  This would allow a telephone like conversation where  you can interrupt and talk over the other person.

    Remote Channel Programming

    Either a stand alone keypad can be used to program a radios channels or a programming cable and a computer.  This is done using the 6-pin U-229 audio connector.
    Racal PRC-139 MX-11532/U Frequency Fill Device, MA 6941 PC programming cable


    The idea of cloning is to allow a number of radios to all be programmed with the same channel frequency assignments.  This is a tedious process when done by hand.

    There are a couple of ways of cloning the frequency assignments.

    Racal PRC-139 - Cloning Cable -this is really not a cable with 2 connectors, it is a cloning device that has only one connector.

    Magnavox pre sales sheet for Ancillaries showing the cloning and repeater (retransmission) cables with no bumps, just cables.

    Crypto Key Fill

    The key for a TSEC/KYV-2()  Secure Voice Module has its key loaded filled using an audio connector on the SVM, not the radio.

    The MX-10579/VRC and/or MX-18290/VRC SINCGARS Fill Device may have a clue and will be analyzed shortly.

    TM 11-5820-890-30-5 Chapter 18 has some operation information on the Fill Device.
    eBay Photo  - on the rotary switch: The button in the center is to INITIATE a transfer from the fill device.
    The cable used with the Fill Device is a 6 wire cable wired 1 - 1.

    Racal PRC-139 - Crypto Fill Cable - this looks just like the cloning device
    The Racal PRC-139 and PRC-6725 can also be connected to a computer with the MA6941 interface cable.

    Squelch Capture Jamming(Wiki: Capture Effect)

    On those radios that must see 150 Hz to open the squelch (PRC-25, PRC-77, AN/VRC-xx) there is the possibility that  an enemy can transmit on the frequency of the radio but with no 150 tone.  This will lock up the receiver and not allow normal communications to get through.  

    To test for Squelch Capture, press the SQ DSBL button on the radio.  You should hear static.  If there is no static you are being jammed.

    The SINCGARS radios do send the 150 Hz tone when not hopping.  But I doubt if they need to see the 150 Hz tone when hopping as there would be no point.  Note that a hopping radio is a very good countermeasure to squelch capture jamming.  For example a 100 Watt single channel jammer can put a strong signal on a receiver like the PRC-77 that's tuned to one channel.  But a 100 Watt wide band jammer covering 30 to 80 MHz (2,000 channels) only puts 0.05 watts into each channel.

    AM radios can hear two stations on the same channel when the difference in signal levels varies over a large range.  This is why aircraft radios still use AM.  An FM radio can only hear two stations at the same time if their signals are very close to the same power level.  Once one signal is a few dB stronger the radio "captures" the strong signal and eliminates the weak one.  FM quieting is a way of measuring how quickly this happens.

    3091735 Frequency modulation interfering signal selecting system, Richard K Moore, General Electronic Laboratories, 1963-05-28, -  a jamming countermeasure

    Radio Wire Integration & Remote Control


    Also known as Radio Net Interface (RNI).  The Plain Old Telephone System uses a 20 Hz ringing signal that is too low in frequency to be sent over a voice radio channel (300 - 3,000 Hz).  So to interconnect phone equipment with radios a tone generator is needed to convert the ring signal to 1,600 Hz that will pass through the radio channel.  And at the other end that tone needs to be notched out so it does not show up in the speech audio yet it needs to be detected and used to turn on a ring generator.  The GRA-7 is used for this purpose.  It can be used in three system configurations:
    The German Funkanpaßgerät WT-FM-E1 AP01 is one of this class of devices.
    The GRA-39 does NOT do this by itself, it requires an operator.  The GRA-39 does not send the 1,600 Hz tone over the radio channel that's required for radio control of the ring signal.  The old tube GRA-7 does this without any operators. 

    The C-6709/G replaces the GRA-7 but only when connected to 4-wire switchboards, not 2-wire phone lines or switchboards like the SB-22.  But it could be used for a retransmission system by connecting two of them back to back with 2 pair of field wire between each.  Should work but not tested.  But it can not be used for connection to a single phone line, like the GSA-7.

    Remote Radio Control

    For short distances you can just use an extension cord on the handset.  But this requires shielding to keep RF out of the phone circuit and only works with good connections, not for miles of distance using field wire.

    To avoid using the field wire pair to carry DC, a 3,900 Hz tone is sent from the GRA-39 Remote unit to signal PTT closure.  At the Local unit the 3,900 Hz signal is separated from the voice signal and used to key the PTT on the radio.  The voice signal is highly amplified prior to being sent over the wire pair then attenuated prior to being applied to the radio.  This maintains a good signal to noise ratio for the voice signal.  In this system there are no signaling tones transmitted by the radio, only voice.  There's also RFI filtering to keep the transmitted RF out of the phone circuitry.



                      Refers to transmission in only one direction.   Simplex refers to one-way communications where one
                      party is the transmitter and the other is the receiver.  Note the difference between simplex and half-duplex.


                      Refers to two-way communications where only one party can transmit at a time.  An example of half-duplex
                      communications is almost any two way radio, you can receive from stations but can't transmit at the same time.


                      Refers to the transmission of data in two directions simultaneously. For example, a telephone is a
                      full-duplex device because both parties can talk at once.  In contrast, a walkie-talkie is a half-duplex device
                      because only one party can transmit at a time.


    Beltone 12D Audiometer
    Teledyne Avionics TA-3D Acoustic Impedance Meter - allows applying positive or negative pressure and then measuring the impedance of the ear.
    TS-2839/GY German Audio Test Set
    General Radio Audio Testing equipment

    Freed-Eismann Radio Speaker FE-50 - efficient horn type loudspeaker
    FreqStd Frequency Standard, Audio
    HP204 HP 204B Audio Oscillator from HP 3350 Carrier Test Set (AN/USM-181 Telephone Test Set
    HP241A HP 241A Audio Oscillator w/Radio Buttons
    HP33120 HP 33120 Function Generator -both audio and sub audio frequencies and various harmonic contents because of waveform
    MAA Military Audio Accessories
    M1024 Magnacord 1024 Reel to Reel Tape Recorder
    SDAR Signal Design, Inc. 65630 Audio Recorder RD-609/TSQ-164 Communications Recorder
    Surround Sound 7.1: Sub woofer, Setup,

    TS585 TS-585 Audio Level Meter
    U229AA U-229 Audio Accessories
    U229PO U-229 Pin Out by Function
    U229Y "Y" Cable,  U-229/U
    Voice of the Theater speakers - efficient exponential horn + 15" woofer with voice coils in same vertical plane to avoid phasing problems at crossover frequency


    Electro-Voice - their web page is under heavy construction (8 Nov 2000)
    Military Radio Accessories Specification Page - microphones, handsets, and 5 & 6 pin AUDIO connectors -
         Richard Lacroix's Military Communications Home Page
    Sonetronics - maker of H-350/U and other mil components with a web page with specs for about all mil mics, head and hand sets & speakers, etc.
    Planning SINCGARS RETRANS Team operations - SINCGARS
    FM 24-18 Retransmission chapter -
    Tacticom - Audio
    Component Products - Audio -
    Tactical Command Industries, Inc. - Headsets

    Brooke's Home, Military Information, Electronics Page

    [an error occurred while processing this directive] page created October 17, 2000.