century21

Digital Frequency Displays for the Ten-Tec

Century 21 Analog Transceiver

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:

connect the board to the LCD (using 30 gauge wire) and then test the complete assembly,
cut a hole for the LCD in the lid and mount the LCD,
mount the board in the rear of the box leaving space for the interface and power cables,
for the Century 21 installation, mount a small SPST switch on the front panel as this will be used for band selection purposes,
wire this switch to the SELECT terminals,
drill small holes for the interface cables and connect them to the PCB. Use just enough RG-174 to connect your display to the radio.
test and calibrate your display and screw the box closed.

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....

80 meters - 3600, 3705, 3750, 3857, 4000.

40 meters - 7030, 7142, 7211, 7291.

20 meters - 14000, 14062, 14143.

15 meters - 21051, 21118, 21142, 21198

11. 'Birdies' - Internal Digital Display

80 meters - 3500, 3600, 3656, 3681.

40 meters - 7030, 7050, 7065, 7082, 7101, 7142.

20 meters - 14000, 14062, 14143,

15 meters - 21051, 21089, 21118, 21198

12. - Internal Digital Display

remove all knobs and nuts from the front panel, temporarily disconnect the wires to the meter and front panel push buttons; remove the front panel.

remove the 4 spacers from the controls and remove the nuts holding the controls, and place the driver and offset pots back out of the way.

remove the screws holding the front panel and don't lose the 4 fiber spacers securing the PTO to the front panel,

remove the scews holding the (2) panel lamps that backlight the display window and sever the red power wire that runs to the lamp above the meter.

remove the sub-panel.

center the LCD that you plan to use and carefully cut out an opening for it so that the complete LCD will fit flush against the FRONT of the sub panel, like so. Center it above the opening for the PTO. A tin snip cuts the metal very nicely. If you like, you can cut straight down from the top without causing any problems. The software has been adjusted to properly position the frequency information within this 'window'.

carefully dril a hole in the front panel to mount the frequency UP / DOWN switch.

make a corresponding hole in the sub-panel so that the switch connections will not short out.

drill two mounting holes for the LCD (one at each bottom corner). You should not need to remove any of the bottom. Measure twice - cut once.

liberally insulate the metal surrounding the LCD on both sides. That is, wrap plastic electrical tape around all the edges.

affix the LCD to the FRONT of the sub-panel with screw spacers that ensure that there are no shorts to ground. Tape up the front of the LCD so that none of the LCD backnight will shine through when powered up. The same is true for the one remaining lamp above the meter.

wire the LCD to the controller and test it to ensure that it works - use one foot of 30 gauge wire (Radio Shack) for each connection.

temporarily power up the controller and verify that the LCD works.

reposition the two conductor terminal strip on the chassis behind the meter to provide a suitable mounting location for the controller board. Temporarily relocate the board underneath and use care when drilling through the chassis. The controller power will be taken from this terminal strip.

drill mounting holes for the controller - two should suffice. Mount it on standoffs made from 4-40 screws and nuts. Vacuum away any metal filings.

run a new (red) power lead from the remaining lamp over to the other side of the SPOT button.

connect the two terminals on the UP / DOWN switch to the controller board at the SELECT terminals (this is not currently on the schematic).

ensure that the settings on the meter and the current trip potentiometers have not been accidentally changed when you worked in this area. You can find set up instructions in the service manual. I took a shortcut here because I know that the Centuy 21 will put out 40 watts when properly adjusted with a 100 watt resistor - as explained in the manual. So, I connected the radio through an accurate wattmeter and set the trip at 30 watts, just to be conservative.

reassemble, calibrate and enjoy. You'll never be off frequency again.

save all the removed parts in the unlikely event that you want to restore the radio to its original condition. Some black paper would need to be taped to the display window on the sub-panel to 'replace the metal that was cut away.