Doohickeys and Thingamajigs

Throughout history, every workbench has them; Those indefinable little tools and accessories that every builder and repairman fashions from pieces and parts to get the job done. Here are some of mine.


When you're working down close to the surface inside a baking pan “play pen” looking through a microscope while working with SMD parts you need a stable third hand to hold a board or sub-assembly. Often there is no room for the usual vice or assembly fixture. When you are soldering a cable to a connector you need a stable clamp or two. I find that the common wooden clothspin works quite well. I carve out a couple of holes on one side that are just large enough to insert and glue two of those little neodymium button magnets just below flush to the surface. They clamp onto the pan or steel bench top easily but do not scoot. They also clamp to many steel panels and supports on equipment cabinets for holding cables and connectors in place while soldering. The wood surface makes a good grip and does not sink heat from the piece but takes abuse in stride. If you damage one, they are nearly free to replace.

Handscrew clamp

Tom, K9AC, has been innovating bench vises for years and recently sent his latest idea: Using an off-the-shelf handscrew clamp to hold those PC boards during assembly. Much easier than the watchmaker’s vice that I have been using. Woodworkers are familiar with the age-old handscrew; They are used to hold a couple of pieces together while gluing. I had several 5” clamps in the woodshop and grabbed one for the bench. It adjusted easily for my small boards, was low enough to permit usage under my microscopes, but was thick enough to permit flipping it over with leaded components installed while I soldered the underside of the board. Cheap and easy!

To keep it from skidding I grabbed 8 of those powerful button craft magnets and just dropped them into the pivot holes where they snapped solidly to the ends of the internal swivels. They are not easily dislodged but grab the steel bench (or pan) surface quite well. No glue needed!

Current probe

Just a sliver of scrap un-etched double-sided printed circuit board and a couple of wire clippings from the last time you used a leaded component. Bend the leads into an “omega” shape and solder one on each side of the board.

Insert it into a battery holder either between batteries or between the ends of a cell and the holder contact. Clip an ammeter test lead to each loop and measure the current flow. I make the loops large enough to let me slip a test probe through if I want to just hang the leads there without attaching clips. Filing the end of the strip to a wedge shape makes insertion easier.

This is an excellent way to check the charging rate of the battery pack or monitor the current drain of a battery powered device.

RF probe

Back in the days of vacuum tube equipment a little NE-2 neon bulb stuck in the end of a clear soda straw served as a quick check for RF voltages on antennas and in the drivers and finals of our transmitters. This was often the first steps to diagnose a problem in a hurry.

Later, I was introduced to the scary idea of using an incandescent bulb for doing much the same thing while repairing VHF and UHF mobile transceivers in a commercial environment. These were indicator lamps with big glass bulbs and you used a small lead soldered to the tip of the screw base as a probe while pinching the glass envelope to provide a capacitive return. How I avoided RF burns is something that I still marvel. It worked, but I do NOT recommend it!

In the May 1997 issue of RadCom magazine G4TKV introduced the idea of using a Light Emitting Diode in place of the neon or incandescent indicator, thus enabling use at lower RF levels. Others (GM3OXX and G0GQX, in particular) have made gradual improvements on the idea. Presented here is my current version, the design that has been in use for the past 20 years or so.

Two general-purpose switching diodes in series with the leads of a high-output LED and a little 1 nanoFarad bypass capacitor across the LED itself. The cathode lead is brought out to be grasped by the fingers as a return for the RF but the probe end is insulated by a short piece of heat shrink tubing.

Grasp the plastic portion of the LED together with the bare conductor next to it. Hold the probe tip near a rubber ducky antenna to see if it lights when you transmit.

On the bench it works to “walk” down the chain of RF output stages until you see the signal disappear. A quick check to see if signal is getting to the antenna. I’ve also used it to track down stray RF in the shack by touching cabinets, connectors, and such while transmitting. Open grounds and loss of shielding show up quickly. It’s a fast way to tune your antenna counterpoise in the field.

Several years ago our county law enforcement radio system was made unusable by a mystery signal that blocked the local repeater. I was able to use my doppler RDF to trace the problem to the State Highway Patrol office but...which car in the parking area? I used one of these probes to walk down the line of patrol cars, holding it near each antenna. Within minutes it lit, pinpointing the radio that was stuck in transmit mode.

Sewing Pins

The little straight pins, particularly those with the spherical plastic heads, are quite useful on the test bench. I often keep several, of various sizes, on magnets nearby. They make excellent voltage probe extensions for accessing those hard to reach test points midst tightly-packed components. Just slip an alligator clamp to the end of your test probe and use it to hold the pin while you maneuver it into position. You can use thumb pressure on the enlarged pin head to pierce solder mask and other conformal coatings so that you can contact the conductor trace beneath.

They also make it possible to troubleshoot heavy plastic or rubber insulated wires. Just slip it through the insulation to make contact within. You now have a test point. Just keep in mind that the wire is exposed to accident while you do this! If you work on “live” circuits then always remain alert and mindful.

I usually keep one large pin that I have bent the last 1/4” in a 90 degree “L”. This is useful for retrieving a dropped nut or washer in an impossible corner of the chassis or the like. It is also good for prying up one end of a component or wire when gripped by needle-nosed pliers. Little sharp extensions for my fat fingers.

Need it to be magnetic (for hardware or wire clippings)? Just place one of those button magnets somewhere on the side of the pin and the entire pin becomes magnetic enough to retrieve the hardware.

NanoVNA fixtures

The new NanoVNA project has created quite a stir of excitement. Cheap, versatile, and portable , it is proving to be the obvious solution to antenna testing and filter evaluation. This instrument is also a convenient solution to characterize capacitors and inductors.

Inductor core materials and capacitor dielectrics behave differently at various frequencies. The packages have different topologies and geometries that also contribute to their true values depending upon frequency. Just as we would not consider an electrolytic capacitor in an RF filter, we would not want to use a type 2 mix iron powder core in a 28 MHz filter. These are reasons for identifying the behavior of components at operating frequency. This is where a Vector Network Analyzer shines.

The NanoVNA uses little SMA connectors. These have superior impedance matching compared to PL259, N-fittings, and even BNC at the upper frequency ranges that the instrument can use. They are also quite small so they complement the playing card size NanoVNA. They are threaded, and that permits a careful control of a good connection.

However...They are SMALL! The connectors are brass. They will wear out if used extensively. The female SMA sockets on the VNA are soldered to the printed circuit board and under shielding so they are not easily replaced.

The solution is to always use jumper cables and leave them attached to the little device most of the time. These little RF test leads are easy to replace and also allow the VNA to rest on the bench where it can be more easily read and controlled. When I test antennas, networks, and components I use the jumpers with appropriate connectors and adapters. I perform the normal calibration routines through the cables and adapters so that their variances are accounted for automatically.

Since many of the Chinese break-out boards for synthesizers, active attenuators, and such are furnished with PC board mount SMA jacks, extras end up in the parts drawer. I usually have a surplus of them so these are the things that I use for making my test adapters for the NanoVNA.

The most often used is just the miniature “grabber” test clips with 1” (25mm) leads directly soldered to the SMA jack. The wires just long enough to easily maneuver without adding a lot of inductance errors. One clip connects to one of the common pins, the other solders to the center conductor pin.

Another VNA test “head” is just a 5/16” (8mm) square piece of single-sided PCB scribed to separate the common pins from the center conductor, and soldered to the SMA jack. I have a bag of nickle plated brass press-mount solder terminals (labeled “Smith 2022D”) that are hollow. They look like machined pin sockets and solder in 1/16” holes in the corners of the PC board. [Don’t know where I got ‘em, probably part of a grab bag, sorry!] This arrangement lets me plug in small, leaded capacitors and inductors for quick checks. The leads can pass all the way through the pins to where the slight tension at the junction makes a good conductive fit. The small size of this test head results in minimum contributed parasitic inaccuracies.

When some new challenge appears, look around you and see if you can re-purpose something nearby or cobble together something to get the job done. Then consider keeping it ready for the next time. Now, where did I leave that whatchamacallit doodad?

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