PRC-138 Intercom Interfacing

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Wire Fox

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So I'm finally coming around to my vehicle-radio setup. I have my PRC-138 as my only functional military radio and do want to set it up to function in my HMMWV. As it is, I have the power delivery functional... The PRC-138 is outfitted with a WKW-7 DC power adapter (powers up via +24V and has an internal BB-490 battery that serves as a UPS-the BB-490s are deprecated, but incredibly easy to rebuild with parts available locally off-the-shelf), which is all then mounted into an RMT-2 shock mount.

When I look at the radio interface terminals on the VIC-3 system, I see a specialized cable where there's generally only the right connection for the VIC-12 family of radios. ...what's the actual correct way to connect a VIC-3 to a PRC-138 or similar radio?
 

aleigh

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That's going to be a bit of a trick. I'm going through a similar problem right now. So while the PRC-138 was originally designed as a manpack radio, Harris did make a variety of accessories for it, including outboard amplifiers to facilitate a variety of vehicle, ground, shipborne, and fixed base operation. At the time Harris also produced a VIC-1, under their label RF-294A.

The weird VIC-12 cable you mention (large and round with an overbuilt retention screw) is present on the VIC-1 (female) and use a male/male cable to jumper cable. On the VIC-3, which has the mini amphenol connectors, the cable are mini on one end and the oldschool on the other. Across the various radios this seems to be an anachronism that presumably for system compatibility the vehicle mount trays use the oldschool audio connectors, just like the VIC-1 and the VIC-3 use the same mount holes. You see these connectors on the PRC-152 VAA and the PRC-119 systems as well.

Harris would have had some cable that went from probably the PA connector to the VIC-1 (contemporary for that era). That actually doesn't entirely help you because you need to go into the VIC-3 so you need the mini connector. So, I think that you are probably going to end up making a cable. You can either (I believe, but I have not cross-checked the pinouts) go from the PA cable on the radio, or just make a cable that goes between the U-229 on the radio and the VIC-3. The latter idea would really work for any military radio.

The question then becomes are the levels coming out of the VIC-3 suitable for the mic input on the radio. I've done this same hook-up with a commercial HAM radio and the speaker-level is fine out of the radio into the VIC-3 but the mic out of the VIC-3 into the radio is pretty hot and needed to be just slightly attenuated to get clean audio. I've not tried going into into a military radio this way but I may be making that exact cable shortly because I've realized the VAA I have is (probably) incompatible with the radio I want to use with it, so we're in the same boat.
 

papakb

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I'm currently powering my 138 thru pins F & K on the front panel PA connector. All of those VIC-1 signals are in that connector and it shouldn't be hard to interface it thru there. Powering the 138 thru that connector also allows you to use the shorter mounts which make the 138 sit properly on the HMMWV shelf.

Kurt
KG6KMJ
 

Augi

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I got my hands on a cable that has the 6 pin connector for the radio port on the VIC 3 on one end and a U329 on the other. It's meant so you cable the front panel audio port on the radio right to the VIC 3. I've only ever seen one.

My friend is using it to connect a Tadiran VHF radio that has no intercom port to his VIC 3.

I made a copy and tried it out with my PRC 104 and it worked fine too. You need a radio with squelch built in tho.

I'll dig up the pinout if I can find where I wrote it down.


Augi
 

86humv

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I got my hands on a cable that has the 6 pin connector for the radio port on the VIC 3 on one end and a U329 on the other. It's meant so you cable the front panel audio port on the radio right to the VIC 3. I've only ever seen one.

My friend is using it to connect a Tadiran VHF radio that has no intercom port to his VIC 3.

I made a copy and tried it out with my PRC 104 and it worked fine too. You need a radio with squelch built in tho.

I'll dig up the pinout if I can find where I wrote it down.


Augi
I copied this off a Harris PDF :
11088-0200-A006
VIC3 Interface Cable This cable allows the AN/PRC-150(C) Manpack to interface receive and transmit audio (with a Keyline) to the AN/VIC3 Intercom System
 

Wire Fox

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That's going to be a bit of a trick. I'm going through a similar problem right now. So while the PRC-138 was originally designed as a manpack radio, Harris did make a variety of accessories for it, including outboard amplifiers to facilitate a variety of vehicle, ground, shipborne, and fixed base operation. At the time Harris also produced a VIC-1, under their label RF-294A.
The weird VIC-12 cable you mention (large and round with an overbuilt retention screw) is present on the VIC-1 (female) and use a male/male cable to jumper cable. On the VIC-3, which has the mini amphenol connectors, the cable are mini on one end and the oldschool on the other. Across the various radios this seems to be an anachronism that presumably for system compatibility the vehicle mount trays use the oldschool audio connectors, just like the VIC-1 and the VIC-3 use the same mount holes. You see these connectors on the PRC-152 VAA and the PRC-119 systems as well.
Harris would have had some cable that went from probably the PA connector to the VIC-1 (contemporary for that era). That actually doesn't entirely help you because you need to go into the VIC-3 so you need the mini connector. So, I think that you are probably going to end up making a cable. You can either (I believe, but I have not cross-checked the pinouts) go from the PA cable on the radio, or just make a cable that goes between the U-229 on the radio and the VIC-3. The latter idea would really work for any military radio.
The question then becomes are the levels coming out of the VIC-3 suitable for the mic input on the radio. I've done this same hook-up with a commercial HAM radio and the speaker-level is fine out of the radio into the VIC-3 but the mic out of the VIC-3 into the radio is pretty hot and needed to be just slightly attenuated to get clean audio. I've not tried going into into a military radio this way but I may be making that exact cable shortly because I've realized the VAA I have is (probably) incompatible with the radio I want to use with it, so we're in the same boat.
I got my hands on a cable that has the 6 pin connector for the radio port on the VIC 3 on one end and a U329 on the other. It's meant so you cable the front panel audio port on the radio right to the VIC 3. I've only ever seen one.
My friend is using it to connect a Tadiran VHF radio that has no intercom port to his VIC 3.
I made a copy and tried it out with my PRC 104 and it worked fine too. You need a radio with squelch built in tho.
I'll dig up the pinout if I can find where I wrote it down.
Augi
Gentlemen, this is all very, very useful information. It sounds like I'm going to have to build my own cable. Likely, I'll just source one of the widely available standard radio cables and a separate U-329 connector and replace the VRC-12-era connector with that U-329. I imagine the pinout isn't anything revolutionary. A copy of TM 11-5830-263-20&P reveals the VIC-3 cable pinouts, so I should be able to reference that to the RF-5200 service manual or to just a general U-329 pinout.

As a curiosity, does that specialty cable you found include a PTT switch on the cable? I've seen lots of SINCGARS VIC-3 cables that have a U-329 on each end and a PTT in-line. (Possibly intended to use a SINCGARS radio as an intercom station instead of as a typical radio tied into the entire system? eg: a remote station)

Code:
MCS Electrical Interfaces
RADIO INTERFACES - SINCGARS-V, AN/GRC-213, AN/VRC-12
INPUT - 150 ohms ±10%, 220 mVrms
OUTPUT - 820 ohms ±10%, 1.4 Vrms
FREQUENCY RESPONSE - 300 Hz to 4.2 kHz
PTT OUTPUT - Open collector, closure to ground. 35 V max. 50 mA max

FFCS Electrical Interfaces
EXTERNAL PTT SWITCH - Accepts 3 PTT lines: 1 on the Audio Connector and 2 Vehicle PTT lines
AUDIO CONNECTOR - Open circuit +5V, Closure to ground to source >2.5 mA, Source Impedance: 820 ohms
REMOTE PTT CONNECTOR RADIO - Open circuit +5V, Closure to ground to source >2.5 mA, Source Impedance:820 ohms
REMOTE PTT CONNECTOR INTERCOM – Open circuit +5V, Closure to ground to source >2.5 mA, SourceImpedance: 820 ohms
HEADSET CONNECTOR - Compatible with CVC, CAPS, ACAPS, CCH Headsets, H250 and H350 Handsets, VISloudspeaker, and LS-454 loudspeaker
MIKE INPUT - 150 ohms ±10%, Unbalanced
HEADPHONE OUTPUT - 1.6 ohms at 1 kHz, 8 Vpk-pkPTT LINE - Open circuit +5V, Closure to ground to source >2.5 mA, Source Impedance: 820 ohms
FREQUENCY RESPONSE -300 Hz to 4.2 kHz
 
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Wire Fox

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As a curiosity, does that specialty cable you found include a PTT switch on the cable? I've seen lots of SINCGARS VIC-3 cables that have a U-329 on each end and a PTT in-line. (Possibly intended to use a SINCGARS radio as an intercom station instead of as a typical radio tied into the entire system? eg: a remote station)
As I heads-up, I dug into that cable I was thinking of and found a reference to it in the 20&P VIC-3 TM.

Code:
e. Part Number. A3210695, NAME AN/VIC-3 Interface Cable Assy. To SINCGARS Radio (jump-radio cable). This Cable is used for Operator training where voice transmissions are recorded.
Looks like it's a simple cable the dumps the full audio output of the intercom system into SINCGARS radio to be recorded/monitored elsewhere. My guess is the toggle switch acts as a PTT toggle. There's a note on the cable itself that states that it's only to be used on the loudspeaker connector of the CD-82 MCS, so it's likely intended to be a one-way cable, not as an alternative two-way radio interfacing method. Will it function as that anyway? No idea.

One thing that does intrigue me is I have a PC-interface that's meant to sit in the same pinout and capacity as a military radio (So using a cable to jump it to a radio as a PC interface results in an improper pinout and terrible audio levels if corrected). It might be neat to grab one of those cables, pop it on my PC interface, and have a way to push intercom chatter into some meaningful one-way application. Of course, if it works two-way on an FFCS, that's even better.
 

papakb

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I have digitized PRC-138 manuals if anyone needs a copy. That should make interfacing easier. Here's the connector info for J1, J2, and J6:

J1_J2 connector signals.jpg

J2_J6 connector signals.jpg
 

formica17

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The cable you need for connecting your PRC 138 or a PRC 150 is part number 11088-0200-A006. It has the 6 pin U329 connector for interfacing directly on the radio and the mini connector for going into either a RIT or the Radio port on the VIC3 MCS. I'll try to upload the nsn and pictures tomorrow.

As for the cable with two U329 connectors and a PTT toggle, that's what is commonly known as a Jumpnet cable. It plugs from the radio into either the MCS loudspeaker port or a FFCS port so that whatever is going through the internal VIC3 comms can be heard externally by essentially hot-mic'ing a net. It's used during gunneries so that the vehicle crew evaluators can hear the crew commands without being in the truck.

Update: nsn 5995-01-500-8368
 
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