PRC-77 Back Pack Squad Radio

©Brooke Clarke, N6GCE 2002 - 2024

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;

remove the battery box to be sure there's not a battery that might damage your DMM
disconnect the POWER jumper cap
measure the resistance from pin A (ground) and L:

If it is an open circuit then the radio is a PRC-25 (2.5 V battery connection)
If it is about 26 k Ohm the radio is a PRC-77 (150 Hz mute for crypto)

measure the resistance between pin A (gnd) and M:

If it is about 89 k Ohms the radio is a PRC-25 (A10 bias)
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.

There are vehicle installagtion kits for specific radios and vehiclws,

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.

Pin A is ground

Pin E has the vehicle "12 Volt" power.

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.


Radio Side J1		Power Input side J2

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 (K4CHE Mod)	?	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

Back to Brooke's Products for Sale, Military Information, PRC-68 Family of Squad Radios, U229 Audio Accesories, Audio Connectors, Electronics, Personal Home, PRC68.com page

page created 16 Oct. 2002.

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	"

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