How to Build the Great Argonaut CW Filter

The Argonaut CW Filter

With the Ten Tec Argonaut 509 restored and working well, it was time to try it out. The Anderson Power Pole made it easy to connect an 8 amp hour gel cell. I hooked it up to my end-fed half-wave antenna and listened around 20 meters. There were plenty of CW signals and I quickly realized a CW filter was going to be needed.

Unlike the later Triton series, there was no provision for an internal CW filter in the 509. Instead Ten Tec sold a CW filter accessory that attached with a 6-pin Amphenol plug. Only problem was, this accessory, known as the Model 208 is hard to find. When one does appear on eBay or other auction site, they’re upwards of $100 plus tax and shipping. That’s a third of the auction price for a typical Argonaut!

So why not make one?

On the other hand, I had already produced a PC board to recreate the Ten Tec Model 245 internal CW filter and had several of them left. Maybe I could enclose one of those to mimic one of the original external filters. If I was really clever about it, I might even be able to mimic the case and operating controls — so much the better.

Finding parts

Of course, there were a few problems to solve. The typical Ten Tec case for such things consisted of two pieces of aluminum sheet, bent to form a wide “U” shape and interlocked with each other. That didn’t seem too hard.

The 7-pin chassis connector

Next, we needed a way to mount and connect the Model 245 CW filter. Ten Tec’s radios of this era used a plastic 7-pin socket that mounted to the chassis. It would connect with a row of small pins mounted at 1/4-inch intervals. Some boards had one of these connectors on each side, but the 245 had one single row. They transferred 12-volt power and audio to the board while carrying the filtered audio back to the Argonaut.

Switching and accessory plugs

The original Argonaut filter also had a 3-position switch, labeled “Off,” “CW 1,” and “CW 2.” The 208 connected with the Argonaut over a multi-conductor cable that plugged into the 6-pin accessory socket on the back of the radio.

It would also be nice to match the cabinet paint and wood grain vinyl contact paper to the original.

Building the cabinet

I decided to start with the cabinet. It was made of 1/16th-inch sheet aluminum — easily found on Amazon. I don’t have a proper metal shop, so some ingenuity was needed to cut and form the aluminum. Cutting wasn’t so hard, as I learned in a YouTube video that a table saw will work for this.

I simply marked the required size, 4 1/8-inches by 7 3/8-inches for the bottom, and cut the aluminum using a “sled.” There are countless designs on YouTube for table saw sleds, and they’re very easy to build. The top aluminum piece measures 4 1/8-inches by 8 1/32-inches.

Bending Aluminum

With the metal cut to size, it was time to bend it up. Since the case pieces were only 4 1/8th-inch wide, I experimented with bending them in my large shop vise. Even just over 4-inches of sheet is surprisingly hard to bend though. I could do it, but the creases weren’t crisp and not too accurate. What I needed was a metal brake.

A vise-operated metal brake

 

My vise is only 4-inches wide, but the tool is magnetic and adheres to the vise jaws. Operation is simple. With the brake mounted into the vise, you line up the sheet metal bend line with the fingers and screw the vise closed. It produces a clean, crisp bend. Then I just did the same for the second bend. The top and bottom cabinet pieces are slightly different in order to nest. The top is  3 3/4-inches wide and the bottom is 4 9/32-inches wide.

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Vinyl coverings and paint

Once bent, the two pieces “nest” with each other to make a box. Vinyl covering for the top is next. There are quite a few “wood grain” patterns available on Amazon. I used what I already had on hand since I’m not trying to fool anyone with an exact reproduction of the Model 208, but this pattern is pretty close. I showed how to attach the covering to a cabinet piece in an earlier post, so take a look at that for tips. 

I thought finding paint would be a problem, but discovered that Rust-Oleum Gloss Antique White is a reasonable match. I used a surface cleaner, Kleanstrip Prep-All, to ensure good adhesion and a smooth finish. After the paint had dried, I realized that a semi-gloss finish might have been a better match, but hey, I’m not trying to replicate the Model 208. 

Rubber feet are also needed and these round self-adhesive ones are a perfect match. 

PC board connections

Now it was time to mount and connect the CW filter board. I’ve been all over the Internet, but short of cannibalizing another radio, have never found the the 7-pin board connectors Ten Tec used for so many of their radios.

The next option was to fabricate one, and I toyed with ideas like 3D printing or silicon molding the plastic part. Not having a 3D printer (yet) and not having a connector I could use to create a mold, I was stumped. Then I realized that the pins I had used on the filter pc board must have a mating socket. A little searching on Mouser’s site, and I had a match. 

Making the 7-pin connector

The rest was pretty easy. Using a scrap of copper-clad circuit board roughly the same size as the original connector, I drilled 7 holes at 1/4-inch intervals to accept the sockets. I then used a Dremel tool to cut channels between each hole to isolate each socket. I then drilled holes to match the filter board mounting holes. Finally, I soldered the sockets and connecting wires onto each strip of copper. Voilà, a Ten Tec 7-pin connector!

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Finding the obsolete Amphenol connector

Two other parts were needed. A 3 to 4 position rotary switch and the Amphenol connector. Although not currently available from Amazon, something like this will work for the switch. The connector, Amphenol part number 86-71-6P,  is a little more difficult. These were common back-in-the-day, and came in various versions.

They were often used in audio systems, but are no longer made. I ended up finding one on eBay. Try searching for “Amphenol Chicago 6-pin” or with the part number. If you can’t find an additional connector, you can always rewire the plug supplied with the Argonaut — you’ll just have to have the filter accessory plugged in to hear receive audio. 

Final assembly

With the parts in hand, drilling a few holes and final assembly was all that was left. The original Model 208 used a 3 position switch, labeled “Off,” “CW1,” and “CW2.” I haven’t been able to find a schematic for this accessory so I can’t be certain, but I think CW 1 and 2 connected to the output of the second and fourth filters on the board. Since I had a three position switch to use, I connected positions 2, 3 and 4.

The complete Argonaut setup

How does it work? Just like the internal filter on the Triton IV. Excellent. The Argonaut receiver is very sensitive, and some kind of filter is needed to work CW on a busy band. With the addition of the Model 405 amplifier, you’ve effectively got a Triton I with 50 watts output. Very useable. I’m really enjoying this step back into yesteryear — working one of the cool rigs from my teenage years.