PRC-25
The
PRC-25 VHF low band radio came out during the Vietnam ear.
The front panel connector labeled POWER allowed the radio's battery to
be disconnected by taking off the jumper cap and connecting an external
source of the two voltages needed to run the radio. In addition
the Mike input, headphone output and Push To Talk lines are brought so
that the
AM-2060 Audio Amplifier-Speaker/Power Supply sitting on the
MT-1029 Mont can be used for vehicle
applications. Pins A, B, C, and D are the same 1:1 as the AUDIO
connector. It's strange that AUDIO pin E (RETRANS) was not
brought out on POWER pin E, but instead POWER pin E is the +12 - 15
Volts to the Function switch. If you know why they
did this
let me know.
The PRC-25B was made by
modifying a PRC-25
so that it would have wider bandwidth to support multiple telephone
channels. To do this some of the unused pins on the POWER
connecter were used for wide band audio input and wideband audio output.
Z1 (GC U-317/U) Jumper Cap
The cap on the PRC-25 has jumpers that connect the battery common
negative terminal to the radio, the +12 volts from the battery to the
radio ,and connect the low voltage battery to a few different parts of
the radio. If this "Z1" jumper cap was missing you could wire the
male connector in the Power Connector Kit to make a substitute.
This same cap is used on the
PRC-77.
But a different cap probably is used for the PRC10xx series radios so
that they can utilize the low voltage pins for other things.
This cap MUST be connected in order for the radio to work from the
internal battery. When it's disconnected the battery is
disconnected unless a connector is installed that has wiring to the
battery. It's not a good idea to charge the battery through the
POWER connector because there's no way to monitor battery temperature.
PRC-77
These radios were made in large numbers. And in addition a lot of
support equipment was made for this radio. When newer equipment
came out if possible it was designed to use the same accessories as the
PRC-25. The PRC-77 is a form, fit and function replacement
for the PRC-25 with more power, lower battery drain and is crypto
capable. It uses the same POWER connector and all the pin
assignments for DC power, audio and PTT are the same as for the
PRC-25. Note the PRC-77 only uses "
12 Volt"
power so does not use
the low voltage pin. For crypto POWER pin L is reassigned as
/Squelch Disable (read not squelch disable) so when a battery is
installed and the Z1 cap is connected about +2.5 volts is on this pin
enabling Squelch. When a crypto box is connected pin L will be
grounded when crypto data is being passed.
The crypographic audio was first done using the
KY-38
manpack unit that would interoperate with the KY-8 vehicle mounted unit
and the airborne KY-28. The same two wide band audio pins as were
used for multi channel telephony were used for the voice encryption.
When the
KY-57 came out it also connected to
the POWER connector using the same pins as the KY-38. It was also
used in vehicle installations. The KY-58 is the aircraft version
that has a remote control panel.
The newer
MSC2001 voice encryption unit from Germany also uses the POWER connector.
PRC-1077, PRC-1088, PRC-1099
Later radios which had commercial
model numbers like PRC-1077, PRC-1088 and PRC-1099 came out. These used
the same physical size, same battery box and have the same POWER
connector on the front panel and like the PRC-77 the audio, PTT, and DC
power connections are the same as on the PRC-25 and use common accessories. These radios may
reassign the low voltage pin for other uses.
Both ends of the CX-13291 use the same male plug as you would use for an external power cable for the PRC-25.
The CX-13291 Cable is described in TM 11-5820-890-20-1 chapter 4.
