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Digital Frequency Counter for the Ten-Tec
                                   Century 21 Analog Transceiver
                                     counter
CAUTION - To undertake this modification, you need to remove the bottom cover of your Century 21 transceiver.  When connected to the 120 volt AC line, there is always the possibility of receiving a dangerous and possibly life threatening shock.  If you have never worked on 'live' equipment and / or if you don't think you are able to complete the modification safely and satisfactorily, please do not attempt it.  

I cannot and will not be responsible for any accidents occurring as a result of your reading this web page.
1. Introduction
     The Century 21 CW only transceiver was first introduced in the late 1970's.  Well received by Novices and experienced hams alike, it resembles the 'EverReady Bunny' in that it just goes, and goes and goes.'   The radio is equipped with a rugged internal power supply and has a circuit breaker on the power ON / OFF switch to protect the final amplifier transistors in the case of an SWR mismatch.  It works on the 80, 40, 20, 15 and 10 meter bands in the CW portion.  Aside from an infrequent rebuilding of the PTO, it's a virtually trouble free, fun radio.

    When introduced, the Ten-Tec engineers offered two options.  The first was a plug-in crystal calibrator and the second was an optional keyer.  Anyone who has used the Century 21 will quickly understand why the calibrator was required.  The frequency determining elements of the Century 21 consist of a conventional 5 - 5.5 Mhz VFO (the PTO) beat against heterodyne oscillator crystals.  For example, the Century 21 uses the following heterodyne crystals:
    When the bandswitch is in the 80 meter position, the circuitry within the Century 21 beats the PTO against the 9.0 Mhz crystal to produce a signal in the 3.5 Mhz range.  In other words, the PTO frequency is subtracted from the 9.0 Mhz crystal.  Conversely, in the 20 meter position, the 9.0 Mhz frequency is added to the PTO to produce a 14 Mhz signal.

    For 40 meters, the PTO is subtracted from the 12.5 Mhz crystal, while the PTO is added for the 15 and meter bands. The PTO's circular dial has been calibrated accordingly.  I would assume that the Century 22's circuitry is similar. 
2. Calibration Issues
     For operation well within a frequency band segment, the Century 21's analog frequency display is adequate. However, to work near the band edges (e.g. to catch that elusive DX on 40 meters) or to be exactly on the recognized QRP frequencies, some form of frequency calibration is required. While the dial displays the frequency in 5 Khz increments, and while the PTO design is very well done, the PTO is not linear throughout its range.  

    When coupled with the fact that the heterodyne oscillator crystals are not equipped with trimmer capacitors, it's well possible to end up with a plus or minus 5 Khz (or more) deviation at any one frequency, even if the radio has been precisely adjusted for one band of operation.
3. Adding a Digital Frequency Counter
  A quick search of the web will show the work done by N5ESE in adapting the Don Hendricks kit as a digital display for the Century 21 transceiver.  At first, I had considered implementing this solution, but was reluctant to cut the faceplate of my mint Century 21.  So, I searching the web, I came across a nifty 5 digit counter circuit developed by Wolfgang Buscher DL4YHF.  'Wolf' has taken ample time to describe this very clever project and even includes copies of the source code for anyone who is interested in 'rolling their own' from scratch.  Even better yet, Sunil Lakhani - VU3SUA has developed a 5 digit frequency counter kit based upon DL4YHF's design.

    The remainder of this website describes my experiences for anyone who would like to either replicate or build upon them. Of course, the counter can be mounted within the Century 21 if you are willing to cut the nice front panel.

4. DL4YHF Counter Design

    I found this design preferable for this conversion because the counter's microprocessor can store in its eeprom both the precise frequency of the heterodyne oscillator crystal and whether the PTO frequency should be added to or subtracted.  This way, there's no need for any interpolation tables or any other steps once the counter has been set up for a particular band.   Just a flick of a switch and a few button presses accomplishes the programming.

    Better yet, the counter will power up already programmed if the Century 21 is set to the same band.

5. Modifying the Century 21 Transceiver - Unplug the Radio and Remove the Bottom Cover

    I decided to see what existing jacks could be used.  On the rear panel are three (3) RCA jacks (if you want to call them that).  Two of these supply 12 VDC and ground, while the third is used for the CW key.  Three (3) jacks are needed to remote the digital display.

    If you'll note, there is a hole with a plastic plug in the rear of the cabinet just below the SO-239 RF connector.  I guess we should be thankful that Ten-Tec did not decide to use another RCA type jack for the antenna connection (like Heathkit did on some of their SB / HW series radios).  If this plastic plug is removed (and saved for possible later use), an ordinary 1/4 inch jack will fit nicely in the hole.  Wired back to the keying lead removed from the KEY RCA connector, this now becomes the key jack and frees up one of the 3 RCA jacks.

    Two of the RCA jacks have 12 VDC on them.  By cutting the jumper between two of them, another jack is spared up.  And that's it!  The remaining functional RCA jack will provide the 12VDC to the display while the other two will provide the frequency signal (over coax) and the third will provide a conduit over which the heterodyne oscillator 'sampler' relay will be operated.

6. Adding the Interface Board - Schematic Information

    The logic in the digital display needs to be able to count both the heterodyne oscillator and the VFO (PTO) to compute the actual frequency.  To accomplish this, an interface circuit was built within the Century 21 radio.  The interface circuitry consists of a small, Radio Shack SPDT relay to switch between the heterodyne oscillator and the PTO (VFO) and a high impedance FET 'source follower' that both isolates and buffers the signals.  Without the FET source follower, the coax from the external counter will pick up all sorts of RF signals and really mess up the receiver.

   A SPST switch mounted on the display itself  - when operated - will switch the signal from the heterodyne oscillator to the external display.  The push button on the display is then used to step through the menu options to either ADD or SUB tract the PTO signals from the heterodyne oscillator value.  After this selection is made, the switch is released, the interface board relay is released, and the PTO signal is sent to the display where the software logic displays the actual operating frequency.  It works quite well.

    Before wiring up the interface board, cut a small piece of either perf board or P/C board and verify that it will fit on the rear corner of the heterodyne oscillator board, close to the existing coaxial connection.  The rear screw holding the Ten-Tec board to the chassis will be used to mount the interface board.

    Wire up the board and carefully test it out before mounting it.  If using the small Radio Shack relay, you may want to install a 100 ohm resistor (R15) in series with its operate path to limit the current flowing through the relay winding.  I had the first relay I used short out for some reason, even though the measured current was almost exactly what Radio Shack had specified.  

    Also, don't forget to add the transient suppressing diode (D7) around the relay winding as the transients generated when the relay is released might feed back on the 12 volt power lead and damage the electronics within the Century 21.

    On the bottom of the radio, place isolating .01 mf capacitors C8 and C9 right at the outputs of the PTO and heterodyne oscillators.  Mount the interface board and run miniature coax to the respective connection points on the board.  The mounting screw also provides an electrical ground.

    Next, run a third piece of miniature coax to one of the 2 spare RCA jacks on the rear of the board.  This is the lead that will carry the PTO / heterodyne oscillator signal to the display over another length of coaxial cable.  Label it DISPLAY.

    Connect an internal source of 12 VDC power to the interface board and provide a connection to operate the relay over the last spare RCA socket on the rear of the radio.  Label this connection RELAY.  The undisturbed RCA connection can be labeled POWER.

    Power up the radio and verify that it still works properly.  Carefully ground the heterodyne oscillator relay lead and verify that the relay operates and that the radio is still working properly.

    If you have a frequency counter, connect it to the DISPLAY.  The counter should display the PTO (VFO) frequency.  Move the Century 21's tuning knob and verify that the counter changes accordingly.  Ground the RELAY connection and verify that your counter now displays the heterodyne oscillator frequency. Change the bandswitch position and verify that the counter follows.

    When done, remove the AC power (unplug the radio), disconnect your counter, unground the RELAY connection, and replace the bottom cover.  You are now ready to build, connect and calibrate the display.

7. Building the Counter

    The counter comes with good instructions and is easy to build.  As it didn't come with a schematic drawing, I traced it out and provided a schematic.
Here are the changes that I made during the construction:
8. Making a Suitable Enclosure

    One thing you can still find at Radio Shack is a good plastic enclosure.  I used the smallest I could find that would hold the electronics which is stock number 270-1805.  The plastic in these boxes is easily cut with an Exacto (or similar) knife.

    I mounted the electronics on the front panel of the box, as shown in the attached picture.  To make the board fit, I had to cut down one of the mounting 'pillars' and notch the side of the box.  When it's all said and done, it fits rather nicely.

    The switch to control the heterodyne oscillator relay and the push button to manipulate the counter itself are mounted on the top of the box.  The 3 RCA jack connections for the power, signal and relay leads are mounted on the rear.  These cables will connect to the Century 21.

9. Initial Tests

    Connect the display to your Century 21 and turn the power on - both the Century 21 and your display should light up and some frequency should be displayed.  If the heterodyne oscillator switch is operated, you should see the frequency of the oscillator crystal depending upon the position of the bandswitch, as shown previously.  This frequency will NOT change as the VFO knob is turned.

    If the heterodyne oscillator switch is released, then the display will show some frequency which WILL change as the VFO knob is turned.

    If these two steps do not work properly, then recheck the construction of the counter itself, the inferface circuit mounted in the Century 21 and / or the cabling between the two unuts.

10. Counter Calibration

    The counter needs to be calibrated to insure accurate freequency reporting.  There are two ways to calibrate it.  Whichever you use, let both the Century 21 and the digital display warm up for at least 15 minutes:
11. Using the Digital Display

    Using the digital display is very easy:  
    When the radio / counter is powered off, the offset information will be retained in the display processor (PIC)'s eeprom memory.  So, if you are going to operate on the same band the next time, you're all set.  Just turn the Century 21 on and get to it!

    Note - changing the offset control while in the receive mode will make corresponding changes to the digital display - neat-o!

12. Possible 6 Digit Counter Design

    Depending upon demand and subsequent interest, I may make availabe a true 6 digit version of this counter by designing a new P/C board and modifying the DL4YHF code to make band changing easier (no need to step down through the menu for adding or subtracting).  The PIC can easily be reprogrammed.  Please email me if you are interested.

13. Ordering the Counter Kit from Sunil Lakhani - VU3SUA

    You can reach Sunil at the following address:   http://bitx20.com/

    You might also check to se if he has any frequency counter kits available on eBay.  I bought mine for $22, and that included free shipping.  That's a really tough deal to beat.

14. Modifying the Century 22

    At first blush, the Century 22 would appear to have the same frequency conversion scheme as the Century 21.  If so, then there's no reason why this counter could not be used in conjunction with the Century 22.  Perhaps someone out there would be willing to try it out???

15. Conclusion

    Our thanks to Wolfgang - DL4YHF - for his ingenious design and willingness to share it with his fellow amateurs.  His example personifies the spirit of our hobby.  Also thanks to Sunil Lakhani - VU3SUA - who is marketing this counter as an inexpensive kit.

    I'm sure that Wolfgang would be interested in hearing about your conversions!

73's - Joe - K3JLS





DISCLAIMER - - If you follow the steps outlined herein, you do so at your own risk. I cannot, nor will not, be responsible for any possible damage to radio equipment, personal property, to yourself or to others caused by modifications that you may make to the radio as a result of your reading this.