[MRCA] M151 DC Distribution and the MX-7777

[WA5CAB at cs.com]

Ray,

MX-7778/GRC consists of MX-7777(*)/GRC, CX-10613/G, a steel mounting 
bracket, four rubber shock mounts, and the necessary hardware to mount it in the 
vehicle (including one flat ground strap that bridges one of the shock 
mounts).  Internally, MX-7777(*)/GRC contains a dual 50 Amp circuit breaker, a 40 
volt zener diode and a thermostatic switch.  The cover is attached by 
screws, and there is a rubber gasket which is probably in your case stuck to the 
case and cover.

I have PDF's of TM 11-5915-223-12, -12 HR, and -24P.  According to the 
References page in the -12, there never was a -24, -25. -34, -35 or -50.

The MX-7778 was intended as an add-on in vehicles already equipped with a 
battery power junction box with one MS 3102E-22-2S 3-pin female connector 
already mounted.  The mating so-called "Bendix Connector" equivalent to MS 
3106E-22-2P but with the huge back shell is what's on one end of the cable that 
comes with the MX-7778.  The other end is UW1220FA17 (or thereabouts) which 
has the same current rating as the -22-2P or S.  Vietnam vintage Bendix 
backshells are smaller than  the Korean War equivalent but still rather large.  
All of the MX-7777(*)'s that I have had or seen have the UW2020MA00 male 
input connector on the right side and the two 3-pin female connectors on the 
left facing aft.  And nothing on the front end.

I strongly recommend installing the MX-7778 in your mutt if you havent 
already.  It will only take one time forgetting to turn all of the radios off 
before starting the vehicle engine to learn why they were supposed to be 
installed in all vehicles carrying anything newer than AN/GRC-19.

 
In a message dated 06/13/2017 08:12:22 AM Central Daylight Time, 
RAFANTINI at salisbury.edu writes: 
> I have a MX-7777 and the pre-built cables that go from the GRC-106 to the 
> MX but everything I have seen for the M151 shows the direct connection 
> method or what I am doing with a stud in the battery compartment. Looking at 
> the MX have to wonder about running all the power thru the signal connector 
> on the back of the unit, like the big three pin plugs on both sides of the 
> MX but not the smaller input.
> 
>  I tried to open the MX but it appears sealed, want to see what’s inside 
> but imagine that will remain a mystery. If anything if all it is just a big 
> zanier diode or MOV maybe will install that across the stud that’s the 
> distribution point for all the radios or perhaps I can throw together a simple 
> L/C high current filter between the battery and electrical system of the 
> mutt for that function?
> 
>  Would wonder if the batteries themselves tend to work as both transient 
> suppressors and brute force regulators and issues occur when you have the 
> charging circuit from the vehicle somewhat isolated from the batteries and 
> the radio equipment falls in somewhere between. In my layout all the 
> electrical system of the mutt terminate at the starter switch and a #0 cable runs 
> from that point back to the battery where it’s connected directly to the 
> battery. Another #0 cable goes from that point to the slave connector and a 
> third #6 to a 60 Amp fuse and the distribution stud where all the radios are 
> feed from. The one foot jumper from the battery terminal to the stud are 
> the weakest link in the system with its line mounted fuse but would like to 
> think if there is trouble anywhere that’s where I want it to be. The jumper 
> was from the battery tray of a commercial UPS system that we junked at 
> work, the old APC UPS systems are a good source of used parts for short high 
> current wire and fuses.
> 
>   
> 
>  Ray F/KA3EKH
> 
>  
> 
> 
> 

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480
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