Digital Frequency Displays for the Ten-Tec
                                   Century 21 Analog Transceiver
display picture

CAUTION - When connected to the 120 volt AC line, there is always the possibility of receiving a 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, please do not attempt it.   I will not be responsible for any accidents occurring as a result of your reading this web page.

Before doing anything, read over these instructions.

Analog Century 21 - Service Manual
Digital Century 21 - Service Manual

1. Introduction

     Neil Hecht founded the Almost All Digital Equipment (AADE) enterprise, and he unfortunately passed away last year. You are encouraged to view his preserved site on the internet Wayback Machine.

    As I had constructed several of his DFD2 units and found them quite well done, I was disappointed that I could not order other units.  So, I developed both the hardware and software to replicate his custom DFD-2 for the Century 21 transceiver.

   I designed and built this unit for my own needs, but I will make a limited number bare-bones boards and pre-programmed PIC processors available for experienced homebrewers.  When these boards are gone - they're gone, and neither will the code be made available.

2. The Century 21 Radio
     The Century 21 CW only transceiver was first introduced in the late 1970's.  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 (optional crystals) bands in the CW portion.  Aside from an infrequent rebuilding of the PTO, it's a virtually trouble free, fun radio.

   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.  

  Note: According to Ten-Tec, it's possible to end up with a plus or minus 3 Khz (or more) deviation on one band, even if the radio has been precisely adjusted.

      I wanted to make the display autonomous by letting the HFO frequency determine whether to ADD or SUBtract the PTO / VFO frequency. However, this was not possible as the 9.0 Mhz HFO crystal is used for both 80 and 20 meters.  So, some form switching is required.  There are no spare contacts on the Century 21's bandswitch to accomplish this (unlike in the Omni series - and in the Digital Century 21 - where a separate bandswitch waver is provided for the display's UP / DOWN switching). 

  Note: So, a miniature SPST (BAND SELECT) switch is mounted on the remote display itself to accomplish this purpose, and is connected to the SELECT pins on the PCB.

3. Upgrading the Century 21 Transceiver

    I 'tweaked' the PIC code in the Heathkit DFD-2 Clone for the Century 21.  The rest of this website describes how to add either an external, or internal DFD-2 clone to the Century 21.  For either upgrade, you'll need to assemble an interface board, described below:

Note: This is also an opportune time to determine if you will add an internal keyer to your Century 21, as a three conductor jack might be your best bet.  This will also solve the raspy sidetone problem. 

4. Adding the Interface Board -
Interface Board Schematic

The logic in the digital display needs both the heterodyne oscillator and the VFO (PTO) to compute the actual frequency.  To accomplish this, a simple MPF-102 source follower is used for each signal

Before wiring up the interface boards, 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.  Solder a ground lead from the interface board to the ground plane on the mixer board.

    Wire up the boards and mount and connect them carefully, as shown here.

     For the external display, you'll need to remote the Heterodyne Oscillator, PTO/VFO signals and 12 volt power to the display. On the Century's rear panel are three (3) RCA jacks.  Two of these supply 12 VDC, while the third is used for the CW key.  Three (3) jacks are needed to remote the digital display - one to provide 12 volt power, one for the buffered PTO (VFO) output and yet another for the heterodyne (HFO) oscillator output.  By mounting a key jack in the plugged hole in the rear of the chassis, and by severing the power lead to one of the RCA jacks, you'll have a power lead and jacks for both the buffered PTO/VFO and Heterodyne oscillator.

5. Building, Calibrating and Connecting the Counter

   Construct, test and calibrate the DFD-2 clone following these instructions.  The Heterodyne Oscillator and VFO connections are used - the BFO connection is NOT used and no chip needs to be installed in the BFO position.  The schematic can also be found here.

6. Making a Suitable Enclosure - External Display

    One thing you can still find at Radio Shack is a good plastic enclosure.  I used the smallest I could find (5 x 2.5 x 2 inches) that would hold the electronics which is stock number 270-1803 ($5.49).  The plastic in these boxes is easily cut with an Exacto (or similar) knife.  Some suggestions:
7. 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 wrong band is displayed, just flip the band select switch.

8. Counter Calibration

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

   The easiest way is to tune in a station whose frequency is known and then gradually 'tweak' C13 until the display mirrors the known frequency.  When doing this, go slowly as the software computes the HFO every 4 seconds while the VFO frequency is computed and displayed every 1/4 second.

    Alternatively, if you have an accurate frequency counter, measure the HFO frequency and jot it down.  Then connect the board's HFO input cable to the radio's HFO output and disconnect the VFO plug.  

   Place a jumper on the TEST connector and the display will now be converted into a very accurate, single input frequency counter in its own right.  Adjust C13 until the frequency on the display exactly matches what you had previously recorded during the HFO measurement, power the board down, remove the jumper from the TEST pins, reconnect the VFO lead, power and you're good to go.

9. Using the Digital Display

    Using the digital display is very easy - just set the bandswitch and note the frequency.  Changing the offset control while in the receive mode will make corresponding changes to the digital display - neat-o!

10. 'Birdies' - Remote Digital Display- Most are either obliterated or severly attenuated by band noise....
11. 'Birdies' - Internal Digital Display
12. - Internal Digital Display
Copyright 2016 -  K3JLS