TenTec Century 21 and K1EL K42 Interface

Discussion in 'Straight Keys - CW Enthusiasts' started by KD9FRR, Jan 21, 2018.

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  1. KD9FRR

    KD9FRR Ham Member QRZ Page

    First off, I hope I don’t get flamed for posting this question here; however, I felt that I’d get exposure to the best knowledge base in this forum.

    I have a TenTec Century 21 Analog and a K1EL K42 CW Keyer/Reader. I was wondering if anyone has interfaced the K42 CW Keyer / Reader to work with the TenTec Century 21 analog transceiver...... ?

    If so, I would greatly appreciate it if you could share how you interfaced the Century 21 and the K42. How did you construct the cable to go from the radio to the keyer ?

    I’m currently looking for a TenTec 670 keyer but have been unable to find a unit in acceptable condition.

    Thanks for any help that you can provide....

    Fred KD9FRR
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    For sending CW you only need one connection between your keyer and your Century 21. That will be an 1/8" plug to the keyer with two wires connected and an appropriate plug into the key input socket on the rig. Many rigs of that vintage used 1/4" phone plugs but some of the Ten Tecs I have of similar vintage use an RCA plug for key input. Take a look at the rear panel of your Century 21 and you'll see what type of plug you need. You only need a two wire connection, the PTT connection is not needed for basic CW operation.

    For decoding CW through the K1EL reader you'll also need an audio tap from the rig. On the K42 end the audio cable needs an 1/8" plug, on the rig end you'll need to get a bit creative. Probably the easiest way to make the connection is to tack solder the other end of your audio cable across your speaker wires but one way or another you'll need to tap your receiver audio to feed the CW reader function. If you'll use headphones then tap the audio connection onto the headphone jack prior to the jack switch contact. IOW, place the audio tap where it won't be switched on and off when you plug in headphones. Alternatively you could tap audio from the output of the audio amp on the associated circuit board. The Century 21 manual shows you where this PCB is and where the audio line leaves the PCB under the theory of operation section: http://www.tentec.com/wp-content/uploads/2016/05/Model_Century_21_Analog_Owner_Manual954f.pdf

    I'd probably add a tap to the audio board and add a rear panel connector with an RCA or perhaps a 1/8" phone jack and then use an appropriate patch cable to connect your K42 audio line.

    There are other non-invasive approaches to tapping audio like using a 1/4" phone plug splitter cable if you'll always use headphones but one way or another you'll want to tap audio from the rig to drive the CW reader function.

    You'll also need to power your K1EL keyer and if you want to send CW via a keyboard you'll need to find an older keyboard with a PS2 style connector.

    These connections are described in the K1EL user's guide: http://www.hamcrafters2.com/files/k42man_C.pdf
    K8AI and KD4ZFS like this.
  3. AA4OO

    AA4OO XML Subscriber QRZ Page

    I regularly use a TenTec Century/21. I enjoy using that radio. It's clarity of tone and simplicity is a winner. A stock C21 has a phono plug for key input. Also there is no aux-out, so you'll need to split at the headphone jack unless you open it up and make some modifications. If your headphones are low impedance you're going to hear some buzzing as in the 70's apparently headphones were higher impedance than modern cans. I made a little variable resistor in-line for my headphones to tame it.

    Note that the C21 is a Direct Conversion receiver. This is why it's CW output sounds so beautiful, but you will hear stations both above and below your zero beat. As you work a station you may hear another below (or above depending on how you have the offset) with nearly the same audio pitch, even though it's 700kHz away. Also, the CW filtering on this radio isn't exactly "tight", so you're frequently going to hear a number of CW stations at different pitches on a busy band. That may confuse your decoder, and if you try to use it during a QSO I think you'll miss a lot and it could be quite frustrating.

    Lastly, unless you've modified the sidetone frequency (it's fixed on the C21) you'll find that it is pitched lower than the center of 5kHz selectivity filter. This means received signals in the sweet spot of the selectivity filter will not match the pitch of our sidetone, this may confuse your decoder if you zero it based on your sidetone.

    I think the C21 is a great radio but it may not be the best choice to use with an electronic decoder.
    K8AI and KD4ZFS like this.
  4. AA4OO

    AA4OO XML Subscriber QRZ Page

    That was a typo... the stock C21 has a RCA jack for key input... my personal C21 has been modified to use a 1/4" phono plug.
  5. K8JD

    K8JD Ham Member QRZ Page

    USB connector and a program to communicate with the microprocessor ?:rolleyes:
  6. AA4OO

    AA4OO XML Subscriber QRZ Page

    There are some IC chips in there but luckily no microprocessor to worry about.
  7. KC2SIZ

    KC2SIZ Ham Member QRZ Page

    I love the C21. It's the kind of rig that makes you feel good about the hobby. Ditto for the C22.
    K8AI likes this.
  8. W9GB

    W9GB Ham Member QRZ Page

    The TenTec model 670 Century Keyer is a very simple circuit, perfect DIY project.
    This SAME PC board was used as the Internal Keyer (model 679) for Century 22.
    https://www.dl0bn.de/dc7xj/Anleitungen/TenTec 670 Manual.pdf

    1 - Dual Flip-Flop IC: SN7476
    5 - Transistors: (4) MPS6514 and (1) 2N5087
    1 - Zener Diode: 5.6 Volt
    8 - Diodes: (1) 1N4002, (6) 1N4154

    The manual shows schematic, parts values, and picture of board layout (July 1978).
    http://gmcotton.com/ham_radio/misc manuals/Ten Tec/Ten-Tec model 670 Century Keyer.pdf
    Last edited: Oct 10, 2019
  9. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    Good luck using the reader.

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