NZ Net News 169, 11 Oct 2025

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Featured key

Mac-Key 1938 Model B bug with marbled finish after restoration

Photos: IK4DCT

By Davide IK4DCT

This is the story of an old 1938 McElroy Bug, the Model B. It was given to me by Donald WB6BEE who was confident I could manage to fix it up.

I took Donald at his word, but after dismantling it, with great difficulty due to the rust and oxide everywhere, I realised that the job was going to be difficult and painstaking!

The Mac-Key before restoration

I soaked all the pieces in vinegar, then brushed and polished them as much as possible. I also redid some of the nickel plating. I had to redo the screw seats because they were full of dirt, and loosening everything was difficult. An idea came to me: strip everything with the sandblaster! But the bituminous paint wouldn’t come off – even with the sandblaster! The metal is so porous that, as you can see in the first photo below, it wouldn’t come off completely.

It was then that I noticed that the base, under several layers of paint, was marbled! The challenge almost cost me my family peace, because I tried countless things to imitate this design, making a mess in the garage, equipment, and so on. First, I tried using a comb, trying to spread the paint in different ways, then using a toothbrush with more or less stiff bristles, but the result was always small dots and a few streaks on the piece.

Mac-Key 1938 Model B base restoration - various attempts

1: After cleaning the base there was still some paint that could not be removed.
2 and 3: First attempts to recreate the marbled look.
4: The final result after using an air compressor.

Finally, I found a very simple solution: a compressor and a little sand. After painting the background black, I mixed a little sandblaster sand with a little paint, then tried at least 15 times at different distances and different air pressures. I learned that the paint needs to be of a certain consistency, undiluted. After finally getting the pressure just right, I achieved a result that was similar to the original look.

In the end, I fixed everything, redid the headstock, etc., and the result is quite pleasing and functional.

As you can see from the photo, I’m looking for the upper pivot holder because the original one is damaged at the top and won’t allow me to insert the nut into its original location on the top instead of how I’ve placed it now on the bottom. I need it to be 2 centimeters long. If anyone has an extra one, please contact me.

Here I would like to thank Donald WB6BEE because he gave me the opportunity to spend some time in peace. Thanks friend.

Thanks Davide! Great to see the results of all your hard work, patience and ingenuity. I noticed you have it set for max QRQ. Ted McElroy would approve! [Ed.]

Quick notes

The 1W Game runs 0900-1200 UTC on 11 October on 20 metres. It’s CW-only, with a 1-watt power limit. There’s info in several discussion groups, such as this one. The event is organised by ON6KZ and could be a good chance to work some QRP DX after the NZ Net on Saturday.

The Oceania DX Contest (CW) is this weekend too, from 0600 UTC Saturday 11 Oct to 0600 UTC Sunday 12 Oct. Details.

QTC magazine from the Radio Amateur Society of Australia is a free monthly publication. The latest edition includes an article on the history of QRZ.com plus some very practical tips for running coax up a crank-up tower, and lots more.

Answer to last newsletter’s Photo Flashback question: Naval Communication Station Harold E Holt, located near Exmouth in Western Australia, communicates with submarines using a maximum power of 2 MW in the frequency range 14-28 kHz.

Quiz: If you do not have a voltmeter, how can you determine the polarity of a battery charging cable before you connect it to the battery? Some ideas, from 1930, are later in this newsletter. Use them at your own risk!

Photo flashback

Tuning up a Johnson Thunderbolt 1kW amplifier. Hope they found a knob for that big capacitor on top of the amp!

Tubular tones

By Stephen ZL1ANY

Here is my version of the G4LNA acoustic audio peaking CW filter.

I used 50 mm internal diameter PVC pipe. The second photo shows the speaker, which is about 45 mm diameter, glued to a perspex circle which fits into the pipe fitting and is a friction fit.

Although very simple in mechanical design it performs remarkably well.

If you’d like to make one82, you will need to tailor the distance from the speaker to the outside end of the pipe to your optimally sensitive audio frequency. I found mine by generating a chirp signal using sound generation software (in my case Audacity on Linux but there will be no doubt Windows or Mac equivalents).

Start the chirp at 500 Hz and run up to 800 Hz. Make it last 10 seconds or so. Play the wave form and try to identify where in the signal the sound seems loudest. You can then use this to constrain the start and stop frequency of the chirp and iterate the process to nail down the frequency your ear is most sensitive at. Mine was about 630 Hz. No doubt it will come down as I age further and I might have to lengthen my acoustic cavity.

The speed of sound at normal atmospheric pressure is temperature dependent and can be estimated as:

c = 331 + 0.6*T where c is in m/s and T is in centigrade

At 20C this works out to about 343 m/s.

The length from the speaker to the end of the cavity (including the distance around the right angle bend) should be l/4 where l is the wave length of the sound wave frequency of maximum sensitivity. For me at 630 Hz this is wave length of 343 m/s / 630 Hz giving l of 54.4 cm and a cavity length (1/4 of l) of about 13.4 cm.

I measured the inner and outer internal lengths of the right angle bend section (including the flanges) as 82 and 151 mm respectively, and averaged these numbers of obtain a length of about 11.6 cm.

Subtracting the length of the two flanges (each 3 cm) gave the right angle section an acoustic length of 5.6 cm. So the top pipe section length is 13.4 – 5.6 = 8.6 cm

This worked out remarkably correct, judging by the peaking effect I heard when the thing was put to use. The flange allows a bit of play to tune the length, but my experience is that this was not necessary.

I found the simplicity of this peaking filter elegant. I wonder if the concept has been used in the commercial world in the days of marine telegraphy?

Net numbers

NR34 R ZL1NZ 39/36 AUCKLAND Ø8ØØZ 1OCT25
=
NZ NET
=
SEP QNI VK3DRQ 27 VK4PN 13 ZL1AJY 3 ZL1ANY 29 
ZL1BDS 24 ZL1NZ 3Ø ZL1PX 23 ZL2GD 9 ZL2KE 15 
ZL2LN 16 ZL2TE 6 ZL4BDG 1 ZL4GW 23 ZL4KX 4 
ZL4LDY 9 TOTAL 232 QTC 26
=
ZL1NZ

In the early days…

Our question about battery charging was:

“If a voltmeter is not at hand, how may the polarity of the charging line be determined?”

There are two correct answers, according to Radio Operating Questions and Answers, by Nilsun and Hornung, published in 1930:

“The positive and negative sides of the line may be determined on circuits of 110 volts or less, by dipping the ends of the two wires in a glass of water in which a very small amount of common table salt, potash, or acid electrolyte has been dissolved. Keep the wires about 1 inch apart. When there is current flowing, gas bubbles will form on both wires, but the wire where the greatest amount of bubbles are (sic) being formed will be the negative side of the circuit.

“Another method to determine the polarity of the line is to place the two wires about 1/4 inch apart on a wet piece of blue litmus paper. Where the positive wire touches the wet paper a red mark will appear if current is flowing.”

Net tip: QNV

We continue our look at Q signals with QNV.

It means: “Establish contact with … on this frequency. If successful, move to … and send traffic for … “

It’s a lot, but self-explanatory, I think. The last variable (send traffic for …) is only used if the receiving station is going to perform a relay to a third station later.

Although I don’t think we’ve used it (yet) on NZ Net, a suitable scenario would be:

A station checks in with traffic during the first 5 minutes of the net, i.e. in the period during which we send stations off-frequency to clear their traffic.

The receiving station also checks in.

But conditions are not good, and Net Control cannot be confident the two stations will have a circuit, so Net Control calls the sending station as follows:

(callsign) PSE QNV (receiving station callsign) U5

If the stations fail to make contact or have quite poor signal reports, Net Control might then ask them to wait, and will help them clear traffic later on the net frequency with the aid of a relay station.

» Download the Essential Q Signals

The A-Z of Q Signals

A well-run CW net is a lovely thing to experience. Communication between stations is quick and concise, and the net business is conducted with minimal time wasted.

As CW operators, we have many techniques to communicate quickly and efficiently. And if we know and use these techniques, we can often match or exceed the speed and accuracy of voice communication.

One of our most important tools is the set of Q Signals (or Q Codes), and in this series we look at the “essential” ones, in alphabetical order.

Video

Nothing at all to do with Morse this time, but breaking news from scientists that is just too cool not to share:

The scientists found that the roads on Rapa Nui were designed in such a way as to make moving the stones as easy as possible by assisting the rocking motion.

» Read all about it.

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Suggestions?

If you have suggestions on how to make the NZ Net better, or things you’d like to see covered in these newsletters, please contact ZL1NZ. Articles and photos will be gratefully received!

Thanks for reading, and I hope to hear you soon on the NZ Net!

—

Neil Sanderson ZL1NZ, Net Manager
New Zealand Net (NZ NET)