Jed Margolin  WA2VEW  March 2022 (but I did this several years ago)

My Ten Tec Omni A (545)

A few years ago I used the AD9850 to make my own external VFO for my ancient Ten Tec Omni A (745) ham transceiver. This was produced in the late 1970s and early 1980s. The Omni A uses a Permeability Tuned Oscillator in its VFO. Permeability Tuned means it used a brass rod going in and out of a coil to vary the inductance to determine the frequency. No doubt it cost less and took up less space than a variable capacitor.

The problem is that back then the Ten Tec engineers used a lubricant in the coil that hardened over time. If you caught it in time you could use a suitable solvent to dissolve the lubricant and use a more modern lubricant. Unfortunately, if you did not catch it in time the lubricant became indistinguishable from epoxy. That is what happened to mine: Frozen PTO Syndrome.

Ten Tec used to sell a repair kit but the company has gone through several hands in the past 40+ years and the latest owners no longer sell the repair kit.

Ten Tec also used to sell an external VFO but it used the same PTO oscillator. I expect they have also suffered the same fate as the internal VFO.

So I made my own external VFO with the AD9850. It uses an optical pot to tune it and has several tuning speeds.

As with most analog rigs of the time the VFO operated from 5.0 to 5.5 MHz. That was heterodyned to produce the frequencies needed for a multiband rig. There was a bandswitch that mechanically turned lots of rotary switch sections to select the right crystal and to tune all of the tuned circuits. The bandswitch also moved the display into view for each band.

Ten Tec’s external VFO doesn’t know what band you are on. Its dial only shows 0.0 to 0.5 and 0.5 to 1.0 MHz. Most ham bands start at an even MHz, like 7.0, 14.0, 21.0, 28.0 MHz and are less than 0.5 MHz wide. The 80 meter band starts at 3.5 MHz and ends at 4.0 MHz. The 28 MHz band is 28.0 to 29.7 MHz so the Ten Tec does it in 0.5 MHz segments.

When I did mine I put a small board with an MSP430G2302 in the transceiver that I connected to an unused section of the bandswitch and sends the band information to my external VFO so it displays the full frequency for the band.

Now some pictures.

Notice the two ganged multisection rotary switches which makes up the bandswitch. There are 10 switch sections to tune all of the circuits for the different bands. This arrow points to my board that reads one of the bandswitch sections (it wasn’t being used) and sends the band information to my external VFO.

This is a closeup.

It mounts with a DB9 connector. It took a lot of metal work. I wouldn’t do it that way again.

The way I send the data is by using a DAC. Then I use an ADC input on the MSP430G2553 in the external VFO to read it. I wasn’t sure if that would work reliably so I also have the 4-bit output coded for the band number. Since the analog method works it only needs four wires: Ground, +3.3V, Analog signal, and a Test input to send the band switch positions sequentially for testing. This comes from a switch on the External VFO so I don’t have to open the Omni to run the test.

This is my VFO with the AD9850 DDS module.

This is the optopot for tuning. It would have to be mounted on something for regular use. And have a knob. And mount the switch for changing the tuning speed.

It seems to work but when I did this (a few years ago) the low bands were dead. Something had gone wrong with the Sun and it was not following the sunspot cycle. (At least it hasn’t gone out, exploded, or turned into a Red Giant.) There is also the matter that my SW reception is very poor anyway. I live in the mountains SE of Reno and even up here I am surrounded by ridges. Also, my mountains are between the Rocky Mountains and the Sierra Nevada so this is not the place to live if you are really into ham DXing. For that you should live on a coast, preferably a Southern coast.

I got into ham radio in 1961. That was way before home computers, the Internet, cell phones, or even cable TV. There were only a few communications satellites in orbit and they required huge dishes.

When I was a teenager I put a Heathkit Twoer on my bicycle. The Twoer was in the front basket. In one of the rear baskets I had a 6V motorcycle battery. This was long before the modern (and tiny) HTs. For my bicycle Click Here. For information on the Heathkit Twoer Click Here.

I used to keep a CB (and a portable antenna) in the trunk of my cars in case I got stuck and needed help. Then those newfangled cell phones were invented (and I got one) so I didn’t need the CB anymore.

By now (2022) there probably aren’t many Omni A’s still working, or people who remember them.

My Ten Tec 215 microphone (made by Electro-Voice) is probably worth more than the transceiver.

So is the Ten Tec 280 power supply. It is a linear power supply and works great. Most of the new power supplies for ham rigs are switching supplies. And even if they are properly filtered they seem to get bad reviews for reliability.

I use my 280 to run my Yaesu FT-840 (early 1990s).

73,

Jed

WA2VEW