AN/PRC-25

Overview

PRC-25 TRANSCEIVER is a portable radio PRC-25 which supersedes the legendary PRC-10 during 1962.

Transceiver RT-505

frequency range 30 – 75.95 FM (VHF), 2 Bands,

Power Output 1 - 1.5 watts

Max range 8 km.

For vehicular use (when installed in the AM-2060 Amplifier/Power Supply and Mount MT-1029) is called VRC-53.

Early XC-3 Variant

Patricio Diez LW8DBP recently acquired a rare Experimental model of the PRC-25 made by RCA. Below are detailed pictures

Modifications

AT-272A Adapter for the PRC-25 and -77

I acquired a PRC-25 about 20 years ago when a friend of mine was cleaning a mini-warehouse storage unit after a deadbeat tenant did not pay his rent. I bought an H-250 handset and both the short and long whip antennas to go with the radio, all at pretty reasonable prices.

Recently I acquired a couple of PRC-77’s on ebay. Obviously I needed at least one additional antenna in order to equip all three sets with an antenna. But I found that the antennas now seem to be rather scarce and pricy.

I had acquired an AT-272A whip section, made for a PRC-8-9-10 and I ordered the lower flex base to fit it from Fair Radio Sales for $6.50. The AT-272A base and whip are available from Fair Radio in both new and used condition, but trouble is, the AT-272A will not fit the PRC-25 or PRC-77.

The base of the AT-272A is threaded with a ¼-28 stud. The antenna connection for the PRC-25 and 77 is threaded for a 5/16-24 connection. I decided that it should be possible to make an adapter to enable use of the AT-272A, which is otherwise is almost identical to the later AT-892 whip.

The 50 Cent Solution

I found that Aircraft Spruce and Specialty Company sells brass tubing with a 5/16 inch diameter and a 0.65 wall thickness for only $5.60 a foot and ordered some.

Cut off about an inch of the 5/16 tubing. You will need to “mill” abut a half inch length at one end down to about 0.30 inch diameter or a bit less in order to be able to thread it using a 5/16-24 die. I chucked the tubing in my drill press and as it rotated pressed a file against it in order cut down the diameter; this worked surprisingly well. Then I put the tubing in a vice and threaded the cut down end.

Then you need to cut internal ¼-28 threads in the other end of the tubing. First you will have to bore out the inner diameter; the recommended size is 0.213 inches, made by a No.3 drill bit. You can use a slightly larger drill bit, like the more common 7/32 size, and that will work adequately.

Take care not to grind the outer diameter down more than about a half inch and also do not drill the inner diameter more than about a half inch to keep from weakening the adapter.

The $4.00 Solution

A slightly costlier but more solidly built approach is to use 0.75 inch diameter brass rod to make the adapter. I found I could buy 1 inch long pieces of 0.75 in brass rod on ebay, three for $8.30, including shipping.

Take one of the pieces of brass rod and drill a small hole, about 1/8 inch diameter or so, through it lengthwise. Then you need to enlarge the hole in one end to enable you to tap it for ¼-28 thread, such as with a 7/16 drill bit, about a half inch deep.

The other end of the adapter you need to drill out to enable you to tap it with a 5/16-24 thread. The proper diameter of the hole is 0.2703, but a 9/32 drill bit should work Okay. Don’t drill deeper than about a half inch to prevent

Then take a 5/16-24 bolt and cut the head off, or else buy a 5/16-24 stud, and insert it in the threaded hole. You do not want it sticking out of the brass adapter more than a half inch or else it won’t screw all the way into the radio.

An even simpler to fabricate but slightly more costly version is to get the required special tap and drill bit for a ¼-28 “Helicoil” threaded insert. The drill bit size is also suitable for a hole than can be threaded to 5/16-24 and the other end can be threaded for the ¼-28 Helicoil reinforcement to produce a stronger fitting.

So that’s it! It’s not hard to make one of these adapters and while a drill press is a big help you probably can still accomplish it with no more than a handheld electric drill and a vise.