PRC-90

© Brooke Clarke, N6GCE

General Information

This is a survival radio that is carried in the emergency vest of air crew members. It can transmit a beacon (attention getting warble tone) on 243.0 MHz.  Voice on 243.0 or 282.8 MHz and Morse Code in Modulated Continuos Wave (MCW) mode on 243.0.  It will receive voice on 243.0 and 282.8 MHz.  The previous frequencies are for Military emergency communications.  The PRC-90 does not work on the civilian emergency frequency of 121.5 MHz nor does it work on the international distress frequency of 406.025 MHz.

Note: Aircraft radios use AM so that when two transmissions double both transmissions are heard.  This is unlike FM where if one station is stronger than the other (by about 3 dB) the capture effect eliminates the weaker signal and it is not heard.  Since an AM receive will not have an audible output when a pure CW signal is being received, a Modulated CW signal is used and the AM receiver outputs the modulation frequency.

          "The AN/PRC-90 radio set is a dual channel transmitter/receiver capable of transmitting up to 60 nm (line of sight,
depending on receiving aircraft's altitude).  It operates on guard (243.0) or SAR primary operating frequency (282.8) with
a mode for swept tone signal on 243.0 only.  Transmission of beacon or code can be up to 70 nm.  Average battery life is
about 14 hours.
            Radio is equipped with external earphone jacks to assist pilot in hearing radio transmission with helmet on."
from: navygouge.com FAM12
 

 

 
PRC-90-2 with 90BAv3
                    battery adaper
PRC-90-2 with 90BAv3 battery adaper

The Antenna screws onto the radio with the 3/8 x 28 threads as ground and has a socket in the antenna as the hot connection.  The lower 6 1/2" of the antenna is flexible and when the telescoping sections are collapsed the overall length is 12" and extended it's 24" long.  The impedance looking back into the radio is 50 Ohms.  You can use the PRC-90 to BNC(f) adapter in the TS-24() test set to connect the radio to test equipment that's also 50 Ohms.

The earphone is in the black plastic pocket and is for use where enemy troops are nearby.

Maintenance Instructions

The TS-24B Test Set or the AN/PRM-32A (TS-20) can be used to test this radio.

The 9 screws that hold the back on the radio have a drop of olive drab paint/epoxy that covers the screws.

I have 2 of these from eBay both "working", but the 282.8 MHz function has weak audio on one of them.
Note: You can not do a valid test with old BA-1568/U batteries, you need proper voltage in order for the radio to work.

Inside after back cover is removed. - with labels showing modules and PCB removal screw.  When the PCB removal screw is out you can lift the PCB and modules part way out of the housing.  At this point you could trouble shoot and/or remove/replace modules if you had service documents.  If you have service documents let me know.

Battery

It uses a cylindrical battery that has a raised band that will allow the battery to only be inserted with the correct polarity (+ end first into radio).  The original battery was a 14.0 Volt Mercury BA-1568/U that has been replaced by the 10.5 Volt BA-5368/U LiMnO2 battery.
Note the BA-1568/U is rated for 1.0 AH and 14 Hours of PRC-90 operation per TM 55-1680-351-10 for the SRU-21/P Survival Vest.  The battery has a 3 year shelf life in temperate climates.

The pass Fail test for the BA-1568/U on the TS-183 Battery Tester was greater than 11.0 Volts with load 12 (21.0 Ohms).

The official and very expensive replacement is the BA-5368/U LiMnO2.  Mathews, Saft, Ultralife and Bren-Tronics make it. NSN 6135-01-435-7947
 

Label on Mercury Battery:

6135-00-838-0706
14.0 VOLTS
BATTERY, DRY
BA-1568/U
DAAB07-87-D-C036
0988
DURACELL INC.
LEXINGTON, N.C.

DO NOT DISCARD
TURN INTO DRMO

Label on BA1568/U Box:

6135 00 838 0706
BATTERY DRY BA 1568/U
1 EACH
DAAB07 87 D C036
A    9/88 MFD   9/88
TO TEST THIS BATTERY WITH TS-183 ( )/U
USE JACK NO.                      MINIMUM PERMISSIBLE VOLTAGE
        12                                                        11 VOLTS
Unless expressly authorized , this test information shall apply
to Army and Air Force applications only.

 

24 June 2002 - New 90BAv2 Battery Adapter has 1.3 AH capacity and can be used operationally.
There also may be a government official adapter that holds four each 123 photo batteries.

12 Jan 2009 - New 90BAv3 batteru adapter is same as 90BAv2 except has threads on bottom to hold radio cap.

The BT-70425 is a 12 Volt Lithium Manganese Dioxide Battery (LiMnO2), for use in the AN/PRC90 Radio set. The original battery for the AN/PRC90 Radio Set was a BA-1568/U Mercury battery. Due to various environmental issues, Mercury batteries have been mandated for elimination by the Environmental Protection Agency (EPA) wherever alternative battery systems exist. The BA-5368/U is a LiMnO2 battery that is a "form-fit-function" replacement.

Bren-Tronics’ BT-70425 is a direct replacement for both the BA-1568/U and the BA-5368/U. The BT-70425 incorporates a patented circuit with standard consumer cells to provide a safe and environmentally friendly replacement. Bren-Tronics developed this unique design feature, to allow a 6.0 Volt battery to operate like the 10.5 Volt battery required in the operation of the AN/PRC-90 radio set. The electronic circuit "steps up" the nominal six (6.0) Volts to the appropriate operating voltage, and maintains a consistent voltage similar to the original Mercury battery.

Testing the BT-70425 is also unique. Using a high impedance voltmeter, a minimum open circuit voltage (OCV) of 6.0 Volts DC will be measured.  To verify that the required 10.5 Volts is available, the electronic circuit must be activated. This is done by applying a load between 200 and 1,000 Ohms . After several seconds, the 10.5 Volts can be read on the same equipment as above. Once the operating or test load is removed, the circuit de-activates and the battery returns to a "sleep" mode, assuring the battery will provide its full and rated capacity. The BT-70425 is available for immediate shipment.

Audio Testing

There is no audio signal at the headphone terminals until the earphone connector is within about 1/8" of the radio so that the magnet on the connector will activate the switch in the radio.

Manuals

PIN         TM/EM Number               Publication Title (partial)
 057428  TM 11-5820-800-13&P  RADIO SET, AN/PRC-90 (NSN 5820-00-782-5308)
 064945  TM 11-5820-1049-23P   RADIO SET AN/PRC-90-2 (NSN 5820-01-238-6603)
 067761  TM 11-5820-1049-12     RADIO SET AN/PRC-90-2 (NSN 5820-01-238-66

Test Sets

AN/PRM-32(A) (TS-20) - Two radios with antennas removed to check each other (good for pass, but if a failure then which radio is bad?)  Works with PRC-90 and PRC-106
TS-24B -  - single radio with antenna in shield box
BT-2B Battery Tester - for many survival batteries (TS-2530/UR and TS-2530A/UR were earlier versions)
TS-183 - Battery Tester

Other Radios in the Family

The PRC-106 is a derivative of the PRC-90-2 and provides both beacon and voice capability on the civil and military distress frequencies of 121.5 and 243.0 MHz. The PRC-103 and PRC-195 are other derivatives of the PRC-90.

Replacement

The PRC-112() is the replacement radio.  It too uses a specialized battery instead of commonly available batteries, but there is an official battery adapter being developed by the government.  It's a shame it does not use the BA-5123 (CR123 photo battery).

This is the radio used in the movie Behind Enemy Lines.
The PRC-112 can send the yelp on the standard beacon frequencies of 121.5 & 234 Mhz or voice on those plus 282.8 MHz.  But it can be programmed to work on and frequency in the 225 to 299.975 Mhz range and if a signal comes in on that programmed frequency and the signal contains the 6 digit survivor ID number the PRC-112 will act as a transponder using the Distance Measuring Equipment (DME) protocol.  A SAR aircraft will then know the distance and using DF the bearing to the survivor.  The transponder also works on 243 and 282.8 Mhz (not 121.5 Mhz).

The switch positions are:
Full CCW = 121.5 BCN, 121.5 voice, 282.8 voice, Channel A, Channel B, 243 voice, 243 BCN.

The KY-913 is used to program the radio, set the variable frequency and set the ID code number.  The top of the KY-913 looks like the battery used on the radio and that's how they connect.  No cable in needed.

Oct 2006 - the current PRC-112 is the "G" version.  It includes a GPS receiver.

This is a completely different philosophy from the 406 MHz beacons that only talk to satellites which in turn talk to a central station far from the survivor.  The signals from a PRC-112 can be received by nearby troops but the 406 MHz signal can not.

Links

AN/PRC-90 Legacy by Alan D. Tasker, WA1NYR


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page created 12 May 2001.