AN/KY-38 Secure Voice System

Brooke Clarke, 2002 - 2015

KY-38 Front
DC Power Sources
Shock Mount


"The AN/KY-38 Secure Voice System is a member of the NESTOR equipment family,  which consisted of the KY-8 Vehicular unit, the KY-28 aircraft unit and the KY-38 "manpack" unit." . . . from the KY-38 (Nestor) - Crypto Machines Web page

The KY-28 was the airborne Nestor box used the KYK-28 control panel.

"In 1964, production of the Nestor series of enciphered voice communications (KY-8 vehicular mounted, KY-28 aircraft, and KY-38 manpack) was begun, but over a year passed before the KY-8/38 reached Vietnam in sufficient numbers, and even then few existed below battalion level.  The Nestor equipment was sensitive to humidity and heat, and aircraft carrying KY-28s required frequent refitting.  However, most of the problems involved the KY-38 because of its weight and bulkiness.  The initial solution was to divide the communications gear, connected by a cable between two soldiers.  During combat, the weakness of such an arrangement was obvious."

  Ref The Most Secret War, govt. printing office, ISBN 0-16-051284-0, page 74.

"Initially reluctant to produce a cryptographic device for the combat infantryman that would undoubtedly be captured, the National Security Agency in January 1966 nevertheless agreed to develop a security device for portable radios.  The National Security Agency and the U.S. Army Electronics Command designed the PRC-77, a modified PRC-25 that could be connected to the speech security gear." pg. 400

"In late 1967, after 15,000 PRC-25s and 7,000 models of the VRC-12 series radios were in the hands of American troops, and 9,000 PRC-25 models were being issued to the South Vietnamese Army, the Electronics Command produced an improved version of the PRC-25 even before the first PRC-25's had been sent to South Vietnam, the laboratories at Fort Monmouth were working to replace the power amplifier tube, the only vacuum tube in the set, with solid-state circuitry to lessen power requirements.  They also added filters to reduce interference that later plagued communicators on crowded retransmission sites such as Nui Ba Den.

In December 1965 a means for connection with a security device, called an X-mode capability, was added to the specifications, and the new radio was designated the PRC-77.  Although the Army had tested and produced the new radios to give the National Security Agency time to correct deficiencies in the companion security devices.  Having had difficulties linking components of major assemblies in South Vietnam even when they were transported together, the Army wanted to issue the radios and security devices, the KY-38's (KY-28's for aircraft use), at the same time.  The most serious problem encountered when the radios were finally fielded was the breaking of the cables connecting the radio to the security device.  Then 500 replacement cables which had been quickly fabricated in a depot in the United States were lost in the crash of a cargo plane.  The National Security Agency resolved another problem by developing a pressure-relief valve to preclude the buildup of hydrogen gas from the KY-38 batteries that had caused several KY-38's to explode after prolonged operations.  By December 1968 modifications were completed, and over 9,000 new radios were in the hands of American combat radiomen." pg 448

Ref. A Test for Technology, John D. Bergen GPO, 008-020-01035-9

Footnote 10 from An Analysis of the Systemic Security Weaknesses of the U.S. Navy Fleet Broadcast System, 1967-1974, as Exploited by CWO John Walker by  Laura J. Heath, MAJ, USA M.S., Georgia Institute of Technology, 2001

An example of a failed project was the KY-28, a voice radio encryptor designed during the Vietnam War for use in fighter aircraft. More than $110 million was spent to develop and field KY-28s, and over 2,200 were manufactured and installed. Combat pilots refused to use them, however, because the lag between transmission and receipt of the message was a dangerous distraction in combat. The result was that US air to ground communications remained unencrypted and vulnerable to enemy exploitation throughout the war. See Bamford, 95.

My use was the RT-524, AN/VRC-46 , annoying VC & NVA with Naval gunfire.  KY-8 tended to overheat so all the RX was the sync pulse and squelch rush, no audio. The field fix was drilling the top of the KY-8 case full of holes and mounting two (2) cum-shawed Rotron fans on top(non-authorized field change). Heat problem solved, CMS violated, but 5" on target during TET '68.  Off Hue, Danang, Qua Viet, Jan-Feb 1968, USS Buchanan DDG-14

We used a block of ice when we could get one................. Usually in an ice chest filled with soda's of course. RICH WA6KNW


The key is loaded with a KYK-28 which contains a lot of pins set for various depths and a couple of larger plungers to compress the zeroize springs.


This is a digital voice encryption  device (not a scrambler) used with wide band military radios, either VHF low band (30-88) or UHF (225-400).

Initially made in conjunction with the VRC-12 Series (RT-246, RT-524, R-442)of vehicular radios.  Later the PRC-25 was redesigned so it too could be used.  The upgraded PRC-25 was called the PRC-77.   Probably  18.75 k bits per second digital data either di-phase or base band.  This is different from the 16 k bps used with the VINSON KY-57.

The NESTOR equipment interfaces directly with the 10 pin X-mode connector on the VRC-12 series radios (called the POWER connector on the PRC-77).  VINSON equipment needs the junction box installed in the MT-1029 when used with vehicle mounted radios.  So the assumption is that there's a cable that connects between the 26 pin connector on the KY-38 and either the 10 pin X-mode connector or the 14 pin POWER connector and no other boxes are needed.

The KY-38 is used with the PRC-41A (225 - 400 Mhz) (not PRC-41) by means of a short CX-10831/PRC-41A cable.  How does the "A" version of the PRC-41 differ from the no change version?  Maybe crypto related?


The transit case is labeled:
National Security Agency
98230 ON015057
Unclassified when all classified
materials are removed

There does not appear to be a nomenclature plate on the KY-38 itself.  There is a stick on label:
Crypto Repair Facility
Navelex SD
Date  2-11-86  Init
Navelexsd 2652/13/74

The Equipment mod record has 27 numbered squares and only 5, 6, 10, 15, 20, 21, 22, 24, 26 and 27 are NOT marked with an "X".


There are 4 connectors on the front panel:

RADIO is a Brundy 16-26 with 26 male pins, the cap is marked Bendix 7217 MS3181-16N and the connector body Brundy BT070 16 26P
PRC-41 to KY-38
            CableInterconnect Cable KY-38 to RT-695A/PRC-41
CX-10831/PRC-41A for use with AN/PRC-41A
p/n 7671975001 s/n 209 order No. 553040 729 00000
Brundy BT06J F 16-26S - mates to KY-38
ITT Cannon- 6850 KPT06J14-19P - mates to PRC-41

Note that the end plugs have splines that mate with the grooves in the connectors and can NOT rotate!  To remove the plug just hold the connector and disconnect the connector like it was mated to a jack.

KY-38 Func
KY-38 Pin
PRC-41 Pin
PRC-41 Func

D, E, F, G, c
Tx ct out ?
Rx pt out ?
WB or norm audio In ?





D, E, F, G, c
Tx pt audio in
Norm Audio Out

+26.5 VDC out

Rx ct in
WB audio Out

D, E, F, G, c

not app
H-33-K (CW key)

Guard out

D, E, F, G, c

H-33-J (rem On-Off)

D, E, F, G, c

AUDIO is a GC U-79/U prior generation U-77 type audio connector  This is consistent with using an H-33 handset.
AUDIO is a U-229 current audio connector consistent with using an H-138, H-189 or H-250 current style handset.
ZEROIZE    is a BNC(f) jack for electrical zeroizing the code key.


There are 4 front panel controls:
RADIO is a three position switch OFF - PLAIN - CIPHER
DELAY is a two position switch IN - OUT (the knob is missing on my KY-38) might be related to turn on time of the radio PTT circuit or turn around time of a satellite link?
VOLUME is a pot 0.........10 (Tx, Rx or both?)
ZEROIZE is a mechanical switch on the key fill door that releases a pin when turned or when the door is opened.


A5 Mechanical Key Holder & A4 (isolated from bus) & A6 Boards (Buss connected)

The Door on the right is for loading the key by means of mechanical programming pins and two spring compression pins inserted into 64 holes and two larger holes.  There are about 20 different depths for each pin key.  When the door is closed after setting the key with the zeroize knob fully counter clock wise a pin on the back of the zeroize knob presses down another pin sticking up from the key holder.

You can first press down the two compression springs then push down each pin and feel the detents.  The actual key loader probably loaded the key in parallel with compressing the springs.  The key pins have just under an inch of movement (0.98") and have 18 positions, no movement to down 17 steps where each step is about 0.058".  After the springs have zeroized the key holder all the key pins are returned to the full up position.  In the zeroize position you can only press the key pins down 3 notches, so maybe a good key would not use these positions.

There is a solenoid as part of the key holder mechanism and maybe if the battery power fails the solenoid lets go and the key is lost or maybe the solenoid is only activated from the front panel BNC zeroize connector?  The ZEROIZE knob on the key load door holds down a pin when the door is closed and latched.  If the knob is turned or the door is opened the key will be lost.

The KY-28 aircraft version has an impact zeroize function in case the plane crashes.  The KY-38 key holding mechanical assembly has a weighted lever that is not actuated by anything.  If this weight is moved up center the springs are released and the key is zeroized.  If the KY-38 was in a vehicle mount then it would take a strong upward blow to zeroize the key holder, like would happen of a jeep ran over a land mine or was parachuted and  hit the ground hard.

The chassis construction is the same type as used on the PRC-25 and PRC-77 but the dimensions of the KY-38 are larger.   There are a number of machined castings probably for either mechanical ruggedness and/or Tempest shielding.  This was an expensive box when new.

A Microswitch at the bottom of the key holder is closed when the springs are compressed (i.e. there is a key loaded).

There are two electrical connections to the key holder.

On the bottom there is a 10 pin plug that mates to a socket wired to the main buss on the bottom of the KY-38 case, has power and ground connections as well as the Microswitch and solenoid firing signals.  The Microswitch is closed when a key is loaded and the springs are compressed.

There is a sub assembly that holds the key pin circuitry that has no electrical connection to the 10 pin connector.  The printed circuit board (marked ONI24038) with the contacts for the key pins has connections on both sides, one side is marked J1 A & B with contact positions numbered 1 to 55 and the other side is marked J2 A & B with positions numbered 1 to 55.  But not all of the positions have traces and not all the traces are connected to anything.  A printed circuit board should be installed on both sides of the A4 mechanical key holder.

There are 4 rows of key pins:

  • Row #1 (close to the MOD 4/9 label) maybe a 16x2 or 16x4 matrix

  • Has a row of 17 wires but only 16 pins
    There are only 4 depth contacts active at down 3, 4, 5 or 6, but depth 3 and 6 are the same and depth 4 and 5 are the same.  All other depths have no connections.
  • Row #2 just like row #1 maybe a 16 x 2 or 16x4 matrix
  • Row #3, 16 pins maybe a 16 x 14 switch

  • Has a row of 17 wires but only 16 pins
    There are 14 depth contacts active at levels 3 through 15
  • Row #4 maybe a 16 x 15 matrix

  • Has a row of 17 wires but only 16 pins
    There are 15 depth contacts active at levels 2 through 16
    Rows 1, 2, 3 and 4 have a common connection through the 17th wire in each of these rows. Why?  

    This appears to be a set of 4 matrix switches that routs the signals.  Note that there are 4 electrically isolated matrix switches.  To avoid cross connects two pins in the same row should not be at the same depth.

    When the zeroize springs are released they lift up two bars that move all the key pins to the top position where there are no contacts (i.e. all open circuits.)  There do not appear to be any active contacts at the top and down 3 levels (positions 0, 1 and 2 are all open) maybe as a safety in case the springs did not push the switches all the way up?

    The A4 printed circuit board is connected on the side of the A5 key holder closest to the end of the KY-38 and is fastened to it with screws.  It does not plug into the buss on the bottom of the KY-38 and probably gets it's positive, negative and ground connection through the PCB on the mechanical pins.

    The A6 PCB plugs into the buss at the bottom of the KY-38 case and is also screwed onto the key holder.



    KY-38 Open

    Loosen the 4 Black captive Phillips head screws under the key load door
    Loosen the 4 captive screws holding the cover onto the front panel and slide the cover off
    Loosen the 4 Black captive Phillips head screws (2 near the cover alignment pin and two down near the battery connector)
    The chassis now will hinge open and the key load mechanics assembly is now free.

    Sub assemblies

    There are two main sub assemblies on the hinged box: The cable connecting the hinged box to the front panel has 13 wires.  5 of these go into the MOD16 box through FL2 through FL6 (tempest filtering) and the remaining 8 wires go into the compartment behind the front panel controls and connectors.  5 of these go through large tempest in line filters.
    Assembly #
    13 pin
     Power Supply (PS box)
    13 pin
    Power Supply (PS box)
    13 pin
    Power supply related (PS box)
    Keying PCB
    MOD 4/9 E-BOR/3
    Mechanical key holder
    Keying PCB
    scrambling? PCB
     scrambling? PCB
    29 op amps
    Dual 3 kHz LPF 
    MOD 12 E-BOM/3
    18 op amps
    MOD 11/13 E-BOL
    dual LPF PCB
    #1  cf=18.75 kHz bw= 20 Hz
    #2 ?
    short PCB
    MOD 16
    Power supply on back of front panel in lid
    another PCB in bottom of box

    Back side of front panel, under metal cover that has foam stips for PCBs is a small circuit board.  Both of the AUDIO connectors are Tempest filtered and have only 4 active wires & ground.  The 5 wires going to cable (plus ground) between the front panel and the hinged part has inline tempest filtering.  In addition 5 more wires join the cable from inside the MOD16 box.  There are 5 wires between the front panel and the small PCB in the MOD 16 box.

    The center of the BNC(f) ZEROIZE connector shows up on pin 2 of the A4 connector XA5 and E11 on the main buss PCB.  Pins 5 & 6 are ground.

    DC Power Sources

    The battery box holds two each BA-4386 or equivalent batteries face to face.  They first plug into a center divider that also has the connector to mate with the bottom of the radio and they the center divider with batteries attached slides down into the battery box.  You can see the blue connector that mates to the radio on the left side of the center divider.  Using two of the same battery type as the PRC-25 also points to its use with the PRC-25.

    The bottom of the KY-38 has a 3 pin power connector (J5) A = RB+ and B & C are combined RB- and Chassis ground.  The adjacent label says CAUTION Do not exceed 15 VDC.  The battery box is missing from this set.

    257477BA Battery Adapter

    MT-3823 Shock Mount


    Note that the MT-1029 used with the AM-2060 & PRC-25 or PRC-77 has connectors for the AM-2060 and will not hold the PRC-25 or PRC-77 directly but this mount holds just the radio or a KY-38.

    The KY-38 was attached to the MT-3823 shock mount.  The KY-38 has indents cast into the front panel like are used on the PRC-25 and PRC-77 for attachment to the AM-2060 amplifier.


    This is a retransmission box for repeating encrypted digital data.  Since it just regenerates the received data and does not convert to plain text, no communication security devices need to be present.  If the retransmission system is captured there is no comprise of secure data.  It's specified to work with either 16 kbps or 18.75 kbps signals depending on an internal jumper.  Also for both NESTOR and SAVILLE families of encryption as well are retransmission of plain voice.

    The MX-9331 can also be connected to the KY-38 and will power the KY-38 through pin E of the X-mode connector on the MX-9331.
    The MX-9331 was typically used as part of the VRC-49 vehicular retransmission system using a couple of RT-524 VHF low band radios.
    It has a couple of 26 pin connectors that go to the two radios and a standard 14 pin X-mode connector to connect and power a KY-38.


    FM 24-19 mentions the KY-38
    FM 24-12 mentiones use with RT-524, VRC-47

    Manual references: 
    TM 11-5820-401-10-2, VRC-44, VRC-47, VRC-48 & R-442, R-442ADec. 1977.
    TM 11-5810-290-14&P
    TM 11-5810-300-12 Operator's and Organizational Maintenance Manual, for, NESTOR, Communications Systems, using, TSEC/KY-8 and TSEC/KY-38, with Radio Sets and Associated Equipment.  For Official Use Only (FUFO).

    Manual References: ST 24-18-2 US Army, Armor Communications Electronics Data, Fort Knox, Kentucky, July 1977
    TM 11-5810-224-10 KY-8 & KY-28 Manual
    TM 11-5810-245-10 KY-38 26 Aug 1971
    TM 11-5810-245-12P KY-38 12 Jul 76 (Ca 13 Jan 78)
    TM 11-5810-245-15 KY-38 13 Sep 73
    TM 11-5810-245-34  KY-38 17 Sep 73
    TM 11-5810-245-34P KY-38 1 Mar 82
    TM 11-5810-245-ESC KY-38 18 Dec 73
    TM 11-5810-247-10 HYL-3/TSEC Digital Regenerative Repeater
    TM 11-5810-209-12P TSEC/KW-7 Teletype


    Crypto Machines
    Crypto Patents
    KY-8 Front


    The KY-8 is a 30 pound unit designed for vehicle mounting.



    The KY-28 is in 2 parts, 
    This is the KY-28 proper that has the same key load pin system as the KY-38, but no controls.

    C-8057/ARC C-8057/ARC  This an instrument panel control for the KY-28.  It has switches for Zeroize, Cipher-Plain and OFF-ON-RLY.  This control supports retransmission (Relay) operations using 2 radios. 5.75' wide, 1.5" high, 3" deep.

    Like many aircraft instruments the very front of the panel is a sheet of clear plastic painted black with the legends engraved.  When the panel lights are turned on they light up the plastic interior, but only the engraved legends glow red because of the red filter that surrounds each lamp bulb.  There's a rubber gasket under the head of the lamp holder so that white light does not leak out into the cockpit.  On this panel there is also some red glow around the screw holes which have had the black paint worn off.  The Cipher-Plain switch is a pull to change type so it will not accidentally change position.

    The Zeroize push button is a normally open type.  When it's pressed the current energizes the solenoid that releases the spring which in turn moves all the key sliders back to their starting position.

    The connector on the rear is an MS3112E-12-10P

    This unit was made by CAGE 67116 which was Computronics in 1974.

    KY-28 C-8157/ARC Remote


    The three lens are missing for the POWER ON, and PLAIN or CIPHER lamps.  If you have some let me know.

    I've heard the lamp covers are the common twist to dim type and were orange, green and red.  Not sure which color to which lamp?

    You can see roughly what they look like in the black and white photo below.
    The C-8157 is another instrument panel control for the KY-28.
    This is a larger control box that includes 3 indicator lamps as well as 4 switches.
    POWERON/CIPHONY, PLAIN/CIPHER, RE-X/REG or Normal and Zeroize.
    C-8157 Control for

    Related in Time Frame

    KY-7 is a digital TTY boxThis photo was in the same section of  ST 24-18-2 US Army, Armor Communications Electronics Data, Fort Knox, Kentucky, July 1977 as the other B&W KY-xx series NESTOR photos.  I think it is a KW-7 (Jerry Proc) used with teletype equipment.
    KYV()-2 Secure Voice Module - digital encryption
    Voice Security Patents -
    PSM-13 Battery Test Set -
    USAF - M151A1 - MRC-108 Combat Forward Air Controller's Communications Jeep by Robin Michael -
    ARC-51 can use the KY-38 as well as most tactical radios made after the PRC-25 (the PRC-25B should work with the KY-38).

    Body of Secrets by James Bamford, ISBN 0-385-49907-8 mentions the KY-38 as being used in Vietnam and later we tried to recall about 700 of them which were staged in a warehouse at Tan Son Nhut Air Base ready to be shipped back to the U.S. but got left when we evacuated.  Keying material for about 12 Months was also left in the warehouse.  (pg 352, 353).


    A History of U.S. Communicatios Security (Vol 1 1973 and 2 1981),  the David G. Boak Lectures, NSA - some KY-8, KY-28, KY-38 info, mostly redacted pages
    The KY-8 was intended for aircraft applications and even though it was much much smaller (2/3 cu ft) than previous voice ciphony boxes was still not acceptable to the Air Force.  Eventually 27,000 NESTOR equipments were made.  The KY-28 required a 3/5 second delay between pushing the PTT button on the microphone and speaking.  This was probably enforced in hardware by putting a sidetone on the audio channel and when the tone stopped you could start speaking.  When interviewed pilots said the delay was about 3 seconds.  i.e. in the heat of an aircraft battle 3/5 of a second is an eternity.

    An Analysis of the systemic security weaknesses of the U.S. Navy Fleet Broadcasting System, 1967-1974, as exploited by CWO John Walker -
    Three key factors in the Soviet attack on the KW-7:

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    Page created 26 Nov 2002.