Other Field Expedients

A collection of field expedient stuff:  Improvise, Adapt, Overcome

(Also see field expedient antennas in the Antennas sections)
Field Expedient Antennas

Field expedient keyer paddle:

To go along with my quickie HF mobile installation I needed to make a simple, lightweight keyer paddle for CW. The station Vibrokeyer paddle is too big, heavy, fragile and expensive for this application. More junk box parts:

Some tapped brackets, a chunk of plexiglass, stick-on rubber feet and a hacksaw blade starts the design. A few terminal lugs, a stereo cable and plug plus some heat shrink tubing and it’s ready to go. I used anti vibration nylon-insert machine screws for the contacts but you could just use simple jam nuts to keep the screws from backing out.  Also, you don’t need “jeweled bearings”, or anything else fancy.  I keep the contacts just a few thousandths spaced;  blade barely moves. Doesn’t slide around and there is room for a hose clamp to attach it to the bottom of the radio in the mobile.. Cost? Again, Zero.


Field Expedient Fuse Repair:

Yep, sometimes necessary.  Especially if your gear does not have an emergency Battle Short Switch to bypass the circuit protection under battle conditions like some of the big Navy transmitters have.  Many pieces of older military comms gear use the large AGU – type fuses in the low voltage primary circuits. Those include the GRC-9 dynamotor supplies, RT-68 PP-112 systems etc. The AGU fuses measure 1.5 X 0.4 inches diameter.  These are typically rated for 32 volt service and have a lower voltage drop than, say, a fuse rated for 125/250 V service.  But those large AGU fuses are getting harder to find at your local auto parts store – particularly just prior to your next field-op.  What to do?

I needed a 4 Amp fuse for my VRC-7/RT-70 mobile system.  It is the fuse in the front panel of the AM-65 amplifier chassis that powers the system.  I reasoned that a standard automotive AGC 3AG 32 volt fuse (1.25 X 0.25 inches) should fit INSIDE the larger fuse if I could drill out one end of the blown fuse.  The glass tube has an inside diameter slightly larger than 1/4 inch.  Using a size F drill bit (0.2570 inches), I carefully drilled a hole in the end of the dead fuse and then used a Q Tip to remove the vaporized metal from the blown fuse wire.  You could use a standard 1/4 inch drill bit but you may have to enlarge the hole slightly for a clearance fit.  Be careful not to break the glass tube.

I then cut a small segment from a junk-box spring and loaded it into the AGU fuse and then inserted a 4 Amp, AGC fuse inside the larger fuse – perfect fit.  The spring serves to improve/define the contact with one end of the fuse and also pushes the AGC fuse out sufficiently so you can grab it with pliers to replace if necessary.  (You DID clear the fault that caused the fuse to blow in the first place, didn’t you?)  The fuse holder cap captures everything and the holder socket spring properly compresses the whole assembly. A small wad of tin foil would make a field expedient spring in a pinch.  (nested field expedients?)

Field expedient fuse repair N6CC

Repaired fuse assembly N6CC

“But Billy-Bob, I usually just use a .22 Long Rifle cartridge; they fit into my truck’s fuse clips just fine”

Works great. Presto!  Back on the air.
Then there is a field expedient when you need an FT-243 crystal but only have some HC-49 wire-lead jobs:

FT-243 Crystal Holder hack

A little work with a Dremmel tool makes it happen. These “hacks” usually operate a couple of hundred cps lower than the marked crystal freq since the FT-243 circuit generally provides a 32 pf load capacitance and the HC-49 spec requires an 18 pf load. So it oscillates slightly lower than expected. Usually not a problem in old military rigs on CW or especially AM.

They can be tweaked back to the marked frequency with a small capacitor in series to bring the external load down closer to 18 pf. A small variable cap in the radio works for me. Take a look here for how I implemented that in a crystal controlled AN/TRC-77 transceiver:
AN/TRC-77 modified for HC-49 crystals

HC-49 crystal in an FT-243 holder

Field Expedient Refrigerator: For dire emergencies….

Field Expedient Refrigerator