How to Modernize a Classic Ten Tec 670 Keyer

Updating an old Ten Tec keyer

Over the years, Ten Tec has released a surprising number of CW (continuous wave or Morse Code) Keyers for Amateur Radio use. There have been two basic designs, single paddle and double or imabic paddle models.  Released in about 1975, the Model 670 is a single paddle version, intended for use with the Century 21 CW transceiver. When I saw one on eBay that was just the paddles and case with no internal circuits, I had an idea; I  could use it with the radio I had just refurbishing,  while updating the Keyer for modern use. An auction and a few dollars later, it was on it’s way.

The Ten Tec 670 Keyer is a little dated

The 670 was a simple device by today’s standards. Internally, it boasted one 7476 TTL IC, 5 transistors and a few diodes. It was capable of automatically forming dots and dashes based on the position of the single-lever paddle — left, center or right. It was powered directly from the “Aux Power” on the Century 21 and connected with the “key” input of the transmitter.

By contrast, modern keyers have message memories, various Keyer modes, and adjustable weighting and spacing. One available kit, the Hamgadget Ultra PicoKeyer, does all this and much more. For under $30, this Keyer kit is completely self-contained with its own watch battery power supply — good for at least several months of use. At only about 2 by 3-inches, it seemed the perfect Keyer engine for the empty 670 case.

Designing the Mods for the the Ultra PicoKeyer

I opted for the “no-case” option, and the Ultra PicoKeyer arrived in just a few days. Control buttons, input and output jacks, and speed control are all mounted directly on the circuit board and intended to fit inside the recommended case.

Because the circuit board is much smaller than the Ten Tec 670 case, I was going to have to modify these buttons and connections. I played around with placement for awhile before arriving at a workable arrangement. The board would be mounted against the back chassis panel allowing access to the control buttons. The included 3.5 MM jacks for speed control potentiometer, paddle input, and sound output would not be used, instead making direct connections to the existing 670 speed control, paddles and output cable. The board would also be mounted on plastic standoffs to avoid shorting the circuit board to the aluminum chassis.

Ultra PicoKeyer assembly

The kit itself is a very simple project that beginning kit builders can easily complete. It took about 30 minutes to assemble the battery holder, IC socket, piezo speaker, transistors, control buttons and related parts onto the board. I did not add the 3.5 mm jacks or speed control. One capacitor would be added later to control the output monitor volume.

Ten Tec 670 Chassis Modifications

With the circuit board nearly completed, it was time to modify the chassis. This involved removing the remaining wires and parts from the old Keyer circuitry, drilling four holes on the back panel for the control buttons, and adding a 3.5 mm jack for the monitor output. Two holes were also needed in the chassis bottom for circuit board mounting. Careful measurement and a drill press made short work of these chassis mods.

Final Assembly of the Modified Keyer

After mounting the circuit board in the chassis, and attaching the existing speed control pot, Keyer paddles and output jack, I installed the watch battery. Moving the paddles left and right, nothing happened — disappointing. The piezo speaker should have been merrily beeping away. After a little circuit tracing and thought, I realized that the piezo speaker is connected to the output through one of the output jack switch contacts. Without the jacks in place, no connection is made. This was easily solved with a jumper wire on the circuit board, and everything was now working.

A 50 kilohm speed pot is supplied with the kit, but the 32 kilohm original pot from the 670 worked just fine. I tried the included capacitors to set the output volume, and soldered one in. An ohmmeter set to the lowest range showed that the Keyer output was working — the meter deflected with the same rhythms as the CW tones being heard in the piezo speaker.

With  the chassis cover fastened in place, the 670 front panel looks original. The Keyer control buttons are accessed on the back panel, as is monitor sound output for an external speaker or headphones. There is no external power requirement. It keys the Century 21 transmitter (and several other transceivers I tried) just fine. If the Ultra PicoKeyer manual is to be believed, I may have to replace the watch battery about once a year.

You Can Do This Too!

This was a very simple and satisfying project. The updated Keyer still looks like it’s intended for the Century 21, and an otherwise outdated device has been given a new life. I will have to try the various Keyer modes (iambic, Curtis “A” and “B,” ultamatic, etc.) and the eight message memories will be most welcome. This Keyer even emulates the Vibroplex “bug,” forming the dots automatically while requiring the operator to form each individual dash — helpful in learning how to use a classic bug. This project should be possible for many, because there seems to be an endless supply of Ten Tec keyers on eBay. Buy one and teach an “old dog” new tricks!