5-pin adapter for EF Johnson 5100 series (PRC-127A EFJ)

So yesterday I went ahead and made an adapter for the EF Johnson 5100 series (aka the PRC-127A EFJ) to be able to use 5/6 pin .mil connectors, such as my Liberator 2’s, H-250’s, and an old school single ear headset I can’t find any info on, but it’s all over ebay. Davies Selex summin such or other, so on and so forth. Best I’ve found is this:


It works, the U-94 PTT switch that came with it doesn’t like this thing, but it sorta works for the Lib2 headset. However, this thing works with the PTT that came with the Lib2. And a bigger bonus, It fits under my 3/4 motorcycle helmet with face shield! Now I just have to test it at highway speed to see how it does. Surprisingly, it’s pretty comfortable.

Now, as I was testing, things weren’t always reliable. I figured it was due to the length of wire (near full length of speaker mic’s wire + connector wire + full length head/handsets). That explains why the H-250 wouldn’t reliably key up(now it does). So, without further ado, here is the writeup!



I started with the speaker mic(I had 2, now just 1), and took the back off. It used a T-10 torx for the 4 corner screws, and a T-6 torx for the 2 inner screws holding down the circuit board.


Now, the circuit board isn’t too hard to read. But, I’ve done most of the hard part already. I traced it out, checked the continuity to what I needed, and wrote what color goes to what. Seems to work good.



Now, this cable goes to a PLGR/DAGR to fill comsec. What is nice about it is that it has the end we need on it to go to the various handsets/headsets I want to use. What’s even nicer is that even though I couldn’t check pinout with the plug on(kinda like a serial plug), after taking a risk and snipping the end, it had all the wires I needed, and then some. Plus some insane braided shielding. I would assume to help prevent leakage of comsec being transferred. Yes, you can pick up data running through lines. Voltage detection without having contact with wires. I would guess that is why data can jump from different Cat5 lines to other ones(from what I’ve been told).


Here is the jig I made to test various connections in my attempts to get it working right. Just some nails in a 2×4 and pieces of individual wires from Cat5 I had laying around to connect them. Alligator clips would work better.



Forgot to snap some pictures, but the gist is there. And yeah, my soldering skills are horrible. But some twist and solder action and some heat shrink over each set, and then bigger heat shrink and there we go.

So finding the pinout for the U-238 connector(.mil 5 pin) or 6 pin connector is pretty simple. A couple good sites out there. Both Cryptomuseum and prc68 have good pages:



NOTE: I am not affiliated to either. Just putting up what I used for reference.



Here is the pinout, in case there is any issues with the pictures I wrote it out below. The way I numbered them is probably different than how EFJ or anyone else does. Top down, odds on right, evens on left:

1 – Black – Ground

2 – Yellow/Red – Speaker

3 – White – Mic

5 – Brown – PTT

6 – Yellow/Red – Speaker

8 – Black – Ground

The pinout for the crypto cable is pretty simple:

A – Black – Ground

B – Dark Beige – Speaker

C – Red – PTT

D – Light Beige – Mic

E – Yellow – NOT USED

F – ??? – NOT USED

For some reason, the F pin isn’t connected to the last remaining wire…which I find odd. However, I don’t need it, so it’s a moot point. The connections go as follows:

Crypto Cable —-> Side Port Cable

Black                      Black

Dark Beige             Yellow (I had better luck with yellow than red)

Red                        Brown

Light Beige             White

And there you have it. Not sure what else this would work with, though I’ve read something about this particular variant of EFJ being compatible with a multitude of Motorola stuff, Jedi series from what it said. I dunno, don’t have a Motorola. Though I do have the wideband VHF antenna (136-174Mhz) on this thing, and it works nice.

I will try to edit a video I took during initial wire testing and upload it somewhere (probably the youtubes) so you can see it actually works before you hack up a speaker mic or other accessory.

Now, you may ask why I would want to do this. Or probably not. Either way, I like modularity and the ability to use the same things on multiple systems. As well as battlefield acquisitions. And standardization of stuff(like ammo). So, why not be able to use my Lib2’s? I like em, they work well, and they enhance normal sounds to make hearing things easier. It’s a win/win for me. My original plan when I got them a few years ago was to make a new PTT for my Yaesu, so I could just plug it in and go. Then I thought recently(probably after seeing someone’s youtube video about rewiring a H-250 for their Yaesu) why not just wire in the 5/6 pin plug on the radio to use any type of headset/handset? And there we have it.

I hope that this writeup helps you guys/gals out. I know this isn’t a really common radio to have around, but with my limited google-fu, I haven’t been able to find much other than aftermarket and OEM stuff to go with it, at a stupid high price. Don’t know about you guys, but I ain’t got that kind of money. Maybe next time I’ll have the adapter done for the 857D.


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