PRC-77 Back Pack Squad Radio

©Brooke Clarke, N6GCE 2002 - 2013
RT-841/PRC-77 Receiver Transmitter
RT-841/PRC-77 Receiver Transmitter



Background
Which Radio PRC-25 or PRC-77 is it?
Variations
Configurations
Manuals 
Power
    Internal Battery
    External DC
    U-383/VR DC Power Adapter
Audio
Parts List
Test Equipment
Crypto Related
Modules
Links

Background

The PRC-25B was a PRC-25 that had a wide band audio modification made to support digital voice encryption and multicall digital telephony.  The PRC-25B modification established the pin connections for the "POWER" (now also an X-mode) connector used on the PRC-77.  The PRC-77 also replaced the final RF amplifier tube of the PRC-25 with a solid state RF amplifier to increase reliability and lower power consumption.

The outer appearance, radio functional operation and accessories are identical to the PRC-25.  To learn about the PRC-77 you need to find out more about he PRC-25.  The PRC-77 is really a PRC-25B with a solid state final amplifier.

The PRC-68, PRC-68A, PRC-68B, PRC-126 and PRC-128 (with the VHF low band module) hand held squad radios have the same functionality as the PRC-77.  When the KYV-13 Secure Voice Module is added to the hand held radios they will interoperate with the PRC-77 (or PRC-25B) when used with the KY-57 Voice Encrytpion Uint.

The PRC-77 when can inter-communicate with the SINCGARS radios, and when a KY-57 is used with the PRC-77 it can also communicate with the SINCGARS radios when they are in the COMSEC mode oralso using a KY-57.

The Receiver-Transmitter is the RT-841/PRC-77.

There is a 25 kHz channel spacing version called the PRC-2841.

The AM-4306/GRC is one of the external RF amplifiers that can be used with the radio.  The AM-4477 is another of the RF amps.  Both of these are packaged in a housing very much like the radio.

Which Radio PRC-25 or PRC-77 is it?

The stick on label that tells you if a radio is the PRC-25 or PRC-77 often is missing from the radio and so the question becomes which one is it.
You can not tell from a visual inspection of the outside as far as I know, If you have information on this please let me know.

One way to tell is to remove the front panel and chassis from the case, look at the bottom to see if there's a tube at the central rear of the fixed chassis.  But this is some hassle.

An easier  way might be to use a DMM;
  1. remove the battery box to be sure there's not a battery that might damage your DMM
  2. disconnect the POWER jumper cap
  3. measure the resistance from pin A (ground) and L:
    1. If it is an open circuit then the radio is a PRC-25 (2.5 V battery connection)
    2. If it is about 26 k Ohm the radio is a PRC-77 (150 Hz mute for crypto)
  4. measure the resistance between pin A (gnd) and M:
    1. If it is about 89 k Ohms the radio is a PRC-25 (A10 bias)
    2. If it is an open the radio is a PRC-77 (no function)

Variations

RT-841

Stock version. Covers 30.00 to 75.95 MHz in 50 kHz steps

RT-3841/PRC-77

from a Peruvian eBay seller. Standard PRC-77 frequency range (30-74.975) except in 25 kHz steps. Tadiran built. 2 out of 3 that I ordered worked from the get go. Handset connectors need a good cleaning, and battery connector needs replacement in some (Steve Haney hopefully has some left to sell). Comes w/o battery box. The modules are mostly the same as a standard PRC-77. Unique modules needed to effect the 25 kHz spacing are labeled A3xx, i.e. A22 became A322. I have a chart in Microsoft Excel that lists all the module numbers and functions for the PRC-25, 77, and two variants. Yell if you want a copy.
RT-505/PRC-25T
from Greek sellers (My guess is that if they advertise as not tested, it doesn't work). This is a standard PRC-77
(i.e. all transistorized) except for a frequency range of 26-70.950 MHz. It covers 10 meter FM, especially 29.6 MHz (although wide band vs amateur narrow band). Unique modules are A1xx.
Also see: PRC-25B

Tadiran PRC-77

Does anyone know what Tadiran PRC-77 look-alike used unique modules A2xx? Was there an A4xx?

French ER.95.B

(TR-PP-13), 26-71.95, Looks like H-33 handsets will fit? Might be more narrow band than US sets.
An Iraqi Veteran Comes to the United States - appearance patterened after the PRC-77 but different design
The French/Italian TP-PP-13/RV-3 (ER-95A) VHF Radio by J. Feyssac & M. McCabe

PRC-425

from a Portuguese seller, looks brand new, but if you look closely, you can see wear on the outside edges of the top handset connector. Not big wear, but enough to tell you the unit was used at least a little. They have a detachable front panel. Insides are hermetic. I can not get mine to draw current, so I need to look for the secret. Also, frequency range is not known, although Janes says it was 41-51 MHz, but front panel switches are 0 to 3 left most switch, and 0 to 9 for next two switches.

List of variations courtesy of Alan D. Tasker, WA1NYR

Configurations

Man Pack only PRC-77

Just a battery powered PRC-77 on an ST-138 radio carrier or LC-2 pack frame and with the CW-503 bag of accessories.

Vehicle VRC-64

When the PRC-77 is installed in the AM-2060 Audio Amplifier/Power Supply and connected to the AS-1729 Antenna System it's called a VRC-64 Radio System.  The AM-2060 is set upon the MT-1029 Vehicle Mount which in turn is bolted to a vehicle or generic mounting plate maybe with some supporting struts or brackets.

In many vehicle installations the Vehicle Inter Communication (VIC-1) system is used to give crew members access to the radio and intercom.  This is most common in tracked vehicles, but is also used in some wheeled vehicles.

    Vehicle & Man Pack GRC-160

When the PRC-77 radio is carried on the back it's called a GRC-125 Radio System.  Either the ST-138 Radio carrier or an LC-2 Pack frame and a cargo shelf can be used to carry the PRC-77.  When the man pack accessories are carried in a VRC-64 type installation so the radio can be used either way it's called a GRC-160.

Manuals

PRC-77 Radio Set manuals

PRC-77

Man pack configuration.

TM 11-5820-667-12 - Opertor's and Organizational Maintenance Manual, Radio Set AN/PRC-77, (NSN 5820-00-930-3724), (Including Receiver-Transmitter, Radio RT-841/PRC-77), NSN 5820-00-930-3725), 1 Jan 1987 Change 1
TM 11-5820-667-20P Parts that are easily replaced like modules, accessories and front panel includes the AS-2109 Antenna.
TM 11-5820-667-34P - Repair Parts & Special Tools - 1978 - 215 pages w/ info on all the modules
TM 11-5820-667-34P - Repair Parts & Special Tools - 1989 -  74 pages no module information
TM 11-5820-667-34LD - Plastic Laminated Diagrams for Direct and General Support Maint for RT-841/PRC-77
TM 11-5820-667-35 includes schematics
The Change 2 manual is the same as the Change1 manual except the C2 has a lot of added info typically about different versions of the modules.  Change 3 that's on LOGSA has had a lot of informatin removed including the overall schematic and is not of much use.
SB 11-660 - Recoverability Code for Modules

VRC-64 & GRC-160 Manuals

A vehicle only installation of the PRC-77 is called a VRC-64. If the parts needed for a man-pack setup are included with a VRC-64 then it's called a GRC-160.  The following manuals have a lot of info on the AM-2060 & VIC-1 plus related items, but not on the radio itself.
TM 11-5820-498-12 - Operators & Org Maint Man
TM 11-5820-498-20P - Org Maint Parts
TM 11-5820-498-34P - Repair Parts & Special Tools
TM 11-5820-498-35 - DS, GS & Depot maint

Power

Internal Battery

The Z1 (GC U-317/U) jumper plug must be connected for the radio to work from the battery.
If your's is missing I could make a functional equivalent based n the same plug used for the VPA-2577 Vehicle Power Adapter cable, but this would just look like the plug with no wires.  If interested let me know.

Military batteries like the BA-5598/U LiSO2 primary, the BB-586 Ni-Cad will work.  I think that it's hard if not impossible to get a working BA-4386/PRC-25 Magnesium battery.

My 257477BA Battery Adapter allows the use of 10 common "D" size batteries.

The BA-5598/U LiSO2 battery is half the size of the BA-4386/U Magnesium battery so two of them can be used in the battery box, one active and one spare.  Note that only the Chemtronics LS 91 or LS 94 can measure the State Of Charge of LiSO2 chemistry batteries, ALL the other battery testers can NOT measure the condition of these batteries.

External DC

  The Straight Plug is an Amphenol Aerospace MIL-C-26482 Series 1 Bayonet, Solder connector   PT06A-12-14P(SR)  where the SR suffix adds Strain Relief that's in Amphenol catalog 12-070. 21 Nov. 2003 - This is used on my VPA-2577 and CLP-77 cables..  Many of the military cables use the U-316 plug that has a long strain relief.

VPA-2577 Vehicle Power Adapter Cable

WARNING - It's hard to read which pin is which letter on this connector.  "B" is the pin closest to the wide locating ridge and "A" is next to "B".  The schematic (back a few pages from the end) in the PRC-25 manual TM 5820-398-35 shows J3 with pin "B" at the top and is correct.
Many of the military cable plugs that mate with the PRC-77 POWER receptacle are the U-316/U.  This cable plug is designed for the conduit type cabling used when encrypted voice mode is in use.  The outer sheath is a hollow conduit and individual wires are used inside.  The conduit provides better shielding for TEMPEST and EMP then the braided shield on some cables.

With the Vehicle Power Adapter cable it's easy to measure the current:
Input
Volts
Tx Current
Amps
Rx Current
Amps
15.3
1.29
60 ma
14.2
1.22
"
13.2
1.14
"
12.2
1.05
"
11.0
0.96
"
10.0
0.85
"

U-383/VR DC Power Adapter

Note this is a battery eliminator for the PRC-77 and similar radios that only need the +15 VDC supply,  It will not work on the PRC-25.  It does have an active voltage regulator inside.

U-383/VR DC Battery
                  Eliminator
U-383/VR U-383/VR DC Battery
                  Eliminator
Radio Side J1

Power Input side J2
U-383/VR DC Battery
                  Eliminator







C 7673/PCC-1 is a supply that will accept either 110 VAC or 12 VDC
 PRC-77 U-383/VR Power Supply adapter made by Artisan Electronics Corp.  This looks like a stock CY-2562 battery box but will take in 12 to 30 VDC and supplies the radio 12 to 15 VDC to allow use from a military vehicle 24 Volt system.
Note it does not have the filament supply to support the PRC-25 but it can be added.
MS3114P 14-5P
Function
A
+10 to 30
VDC
B
Ground
C
nc
D
nc
E
nc

The DC power input cable will use the cable will use a MS3116E-14-5S or MS3116F-14-5S or MS3116P-14-5S connector.
Same connector as on the TVS-2 battery eliminator.

Audio

The audio connector is the standard 5 pin U-229 type and has pin "E" wired with the Retransmission signal that goes to ground to signal that the squelch has opened on receive.

Parts List

Basic Radio Issue

RT-841/PRC-77 Receiver-Transmitter
CW-503 Accessory Bag with:
H-250 handset
AB-591 Spring Base & AT-271 Fishing Pole Antenna
P/O AT-892 Gooseneck & AT-892 Tape Antenna
BA-5598 LiSO2 Primary Battery or equivalent

Optional Accessories

See the PRC-25 accessories, sine the radios are almost identical
MT-3823 Shock Mount

AM-4477 Range Booster optionally with CY2577 DC Power Supply for use in AM-2060 with MT-1029.

Test Equipment

Test Adapter for the POWER connector
F91120 Bit Error Rate Test Set

Crypto Related

KY-57

The KY-57 Voice Encryption Unit that interoperates with the KYV-2() Secure Voice Module on the PRC-68 family of radios
The CX-12991 cable connects the KY-57 to PRC-77 interconnect cable NSN 5995-01-058-2513 W15P7T-91-S7101 cage 55928
Mfr/pn: SM-D-598871, apr '92

Prior to the KY-57, the PRC-77 was used with the KY-38 Voice Encryption Device.
 
PRC-77 Description
PRC-77 Power pin
KY-57 Radio pin
KY-57 Description
Ground
A
G
 Ground
Narrow audio out
B
C
Rx audio in (for PT)
PTT to radio
C
S
gnd to key radio (Send)
audio to radio
D
M
Tx plain audio in
+15 V to radio
E  (jmp F)

 
+ 15 V from batt
F(jmp E)
nc
 
nc
H
nc
 
Mike ground
J
L
Tx plain audio gnd
Retrans out
K


gnd for Crypto1
L
E = T
 
na
M
nc
 
+15 during Tx
N


Wide Band audio out
P
A
Rx cipher in
Wide Band audio in 2
R
D
Tx  cipher out
 
nc
B
 
 
nc
F
 
 
nc
H
 
 
nc
J
 
 
nc
K
 
 
nc
N
 
 
nc
P
 
 
nc
R
 
 
nc
U
 
 
nc
V
 
1 - Grounding pin L disables the handset mike audio so only the external wide band input on pin R drives the modulator.
2 - The modulator is AC coupled and passes roughly 220 Hz to 220 kHz, is not DC coupled.

The PRC-77 and a KY-57 can be carried by one man using the LC-2 pack frame, a couple of cargo shelves and 4 each load straps.
When the KY-38 was used, it was more than one man could carry and there  were a lot of problems related to the cable connecting  two soldiers each carrying one of the items.

MSC-2001

This Voice Encryption Unit (VEU) is made by Seimens in Germany and was designed to work with the PRC-77.   This may be related to export restrictions on the KY-57.  It's Ts/Rx and REMOTE connectors are identical to the PRC-77 POWER connector and many of the pins have complementary functions, so a number of them can be wired 1 to 1 for an interconnect cable.

Modules

Module # Module Function SMD # Rev "A" ? SMD # PRC-25? PRC-77? PRC-377    25 kHz PRC-25T     26-71
                 
A1 DC/DC Converter 447269     yes not needed    
A2 PA Tank 447620     yes repl/w A32    
A3 1st Rx RF Amp 447555     yes repl/w A33    
A4 2nd Rx RF Amp 447556     yes repl/w A34    
A5 Rx Mixer 447462     yes repl/w A35    
A6 Tx IPA 447557     yes repl/w A37    
A7 Tx 2nd RF Amp 447558     yes no    
A8 Tx 1st RF Amp 447559     yes no    
A9 VFO 447554     yes repl\w A39    
A10 Interval Oscillator 447407     yes repl\w A40    
A11 Discriminator 447446 yes ? yes yes A11A A11
A12 2nd Mixer 447447 yes ? yes yes A12A A12
A13 53 MHz Filter 447448     yes repl/w A43    
A14 1st Mixer 447449     yes repl/w A44    
A15 1 MHz Spectrum Gen 447450     yes repl/w A45    
A16 Voltage Regulator 447451 yes ? yes yes A16A A16
A17 Phase Comparitor 447452 yes ? yes yes A17A A17A
A18 5.6 MHz IF 447453 yes ? yes yes A18A A18
A19 1/4 Wave Network 447454     yes no    
A20 Sidestep Oscillator 447455     yes no    
A21 Rx IF Limiter 447177 yes 875724 yes yes A21A A21
A22 Speech Amp 447314     yes yes A322 A22
A23 Tone Generator 447316     yes yes A23 A23
A24 Tone Squelch 447318     yes repl/w A54    
A25 Rx Audio 447113     yes repl/w A55    
A26                
A27                
A28 Antenna Loading ?     yes yes ?  
A29 Tx PA ?     yes no    
A30 Filter ?     yes no    
A31 Tx Frequency Control ?     no yes A331 A31
A32 Rx Input ?     no yes A32 A132
A33 Rx 1st RF Amp ?     no yes A33 A133
A34 Rx 2nd RF Amp ? yes ? no yes A34  A134
A35 Rx Mixer 448751 yes ? no yes A35A A35
A36 Tx PA ?     no yes A36 A136
A37 Tx IPA 448745     no yes A37 A37
A38 Tx VFO ?     no yes A38 A138
A39 Synthesizer VFO ? yes ? no yes A39A A139
A40 Interval Oscillator ? yes ? no yes A340 A40
A41         no no    
A42         no no    
A43 53 MHz Filter 448739     no yes A43 A43
A44 1st Mixer ? yes 621272 no yes A44A A44
A45 1 MHz Spectrum Gen 448750 yes ? no yes A45A A45
A46         no no    
A47         no no    
A48         no no    
A49         no no    
A50 Tx Mixer 448753     no yes ?  
A51         no no    
A52         no no    
A53         no no    
A54 Tone Squelch ? yes 875637 no yes A54A A54
A55 Rx Audio 448752     no yes A55 A55
A56         no no    
A57         no no    
A58         no no    
A59         no no    
A60 Wide Band Attenuator ?     no yes ? ?
Table courtesy of Alan D. Tasker, WA1NYR

Links

New Radios Improve Afghan Army Communications -PRC-77 used by Afghan National Army

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