Equipping the Ten-Tec Digital Century 21 with a Dual Digital - DDS VFO


CAUTION - To undertake this modification, you need to remove the covers 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 'Ever Ready 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 periodic rebuilding of the PTO, it's a virtually trouble free, fun radio.

    This website shows how to upgrade your Digital Century 21 with an extremely stable, backlash and warble / manitenance free, Dual Digital VFO.  Once done, your Century 21 will work in the SPLIT frequency mode without any external VFO.  Best of all, you'll never have to rebuild that vexatious PTO.

    The converstion consists of removing the existing PTO and installing a suitable mounting to hold the optical encoder of your choice. (Depending upon the length of your optical encoder's shaft, you may have to either use a 1/4 inch extender or cut the sheet metal of the sub panel). Holes are drilled in the chassis to support a mounting 'platform' for the DDS VFO.  The DDS VFO circuit board is then connected to various points within the radio.  

Note: If desired, LEDs may be installed to show the active VFO, the SPLIT function and whether the tuning is LOCKED or UNLOCKED. 

    If you plan to undertake this conversion, kindly read through the procedures described in this website before starting the actual work.

2. Conversion Steps - Schematic  - - - Itemized Component List

    a) Preparatory
b) Removing the PTO c) Installing the DDS VFO Board     d) Mounting the Optical Encoder

    The length of your optical encoder shaft matters.  The easiest way is to place a small metal plate (drilled out for the outside diameter of your encoder and mounted in the existing PTO mounting holes) on the outside of the radio's sub-panel.  Temporarily install your encoder (finger tight) and then the front panel.  If you can satisfactorily attach the tuning knob of your choice, and if it spins properly - that's great.  Chances are - though - you will not be that fortunate.  Your encoder shaft is probably not long enough, so either a shaft extender or some 'metal work' is required.

    To use the tuning knob of my choice whose set screw was too far back to securely grasp the optical encoder mounted on the sub-panel, I mounted the encoder on the back of the front panel by securing it to a piece of scrap aluminum.  I drilled 3 holes though the front of the panel to attach this plate.  The tuning knob nicely hides the screw heads.

    Since the optical encoder (when mounted to the front panel) didn't fit into the PTO opening, I had to enlarge the sub-panel opening using a Greenlee chassis punch.

Hint: While I modified the sub-panel while it was still on the radio, it would be considerably easier removing it from the chassis.  This way, a small jig saw or a nibbler could be used.  Also, another hole needs to be drilled in the sub-panel to accommodate the OFFSET switch.  If I convert another Ten-Tec radio, I'll be sure to remove the sub panel first.

e) Modifying the Front Panel

    At the very least, one additional hole will have to be drilled in the front panel for the OFFSET ON / OFF switch.  I used a Radio Shack miniature SPST toggle, and mounted as shown in the picture.  As noted above, a corresponding hole needs to be made in the sub-panel.

    If you plan to use status LED's, there are several ways to mount them.  For openers, miniature LED's could be mounted in small holes drilled through the red lense just below the digital display.  If I had it to do over again, this is probably what I would do.  However, I simply drilled 4 small holes in the aluminum front panel into which I inserted grommets.  

Note: The LED's themselves are inserted through the grommets and secured with a drop of crazy glue.

    With the LED's placed and the preliminary wiring work done, the front panel may be secured to the radio so that the conversion may be completed


f) Connecting The Front Panel

    Referencing the connection points found here, make the following connections (fishing the wire through an existing sub-panel hole and under the digital display board):
g) Alignment Suggestions and Other Considerations

      Follow the instructions in the service manual to adjust the 2 variable resistors that set the current trip level.  This requires an adjustable high wattage resistor and will ensure that your finals will last.  (I have such a resistor if anyone would like to borrow it - I want it back).

    After making these two adjustments, I found that the 80 meter output power (before tripping) was close top 38 watts - not too shabby.

    To adjust the timebase on the digital display, turn the OFFSET control to OFF and transmit a low level signal into a dummy load.  Using an accurate frequency counter (or the station transceiver, for that matter), adjust the Ten-Tec's time base until the two frequencies match.

    On the underside of the radio there are 3 ground straps that are soldered to the heterodyne oscillator board.  Make sure that these solder connections are firm as an open on one or more of these can cause erratic operation and noise.  Just one of mine was intermittent.

    On the higher frequencies, the digital frequency counter may 'whine' a bit.  This is apparently a known Ten-Tec quirk of the digital Century and some of the older Omni radios, as well.  While this was not objectionable for me, others may feel differently.  A simple solution is just to switch to the two higher selectivity positions and - presto - no more whine!

3. DDS Commands

    When initially powered up, both the A and the B VFO will be set to the lower band edge, that is, 7000, 3500, 1800, 28000 (etc).  VFO A will be enabled.  The user may then tune with VFO A in the normal manner, and VFO A will be used for transmitting.  The receive frequency will vary based upon the OFFSET's control setting (if enabled); the transmit frequency will not change.

    To switch to VFO B, depress (tap) the previous SPOT button briefly, and the system will be using VFO B.  The frequency previously stored in VFO A will not be changed.

        Note: if you have wired up the optional LEDS, the LED for either VFO A or VFO B will be illuminated.

   To enter the SPLIT mode, just tap the function / SPOT button twice (a short followed by a longer tap - like a ' A' in CW) and the radio will enter the SPLIT mode.  The on-line VFO will control reception, while the off-line VFO will control transmitting.

        Note: if you have wired up the optional LEDS, the SPLIT LED will be illuminated.

    To exit the split mode, tap the function / SPOT button twice (another short - long tap sequence) and the radio will revert to the normal mode.  The contents of the on-line VFO will be copied into the off-line VFO.

    To LOCK the system at any point, just hold the function / SPOT button down for 2 seconds and the system will be LOCKED, and cannot be changed until UNLOCKED.  To unlock the system, just tap the function / SPOT button - and that's it!  While LOCKED, the OFFSET control will work.

        Note: if you have wired up the optional LEDS, the LOCK LED will be illuminated.  

    If you are willing to drill another hole in the front panel, an optional button may be added dedicated to the SPLIT function.
 Tap it one time and the SPLIT function is active.  You can then use the main button to switch between the VFO's.  Tapping the SPLIT button again will disable the split function and map the on-line VFO into the standby unit.

    To store the last used frequency before powering down the radio, just operate the LOCK function, release the button and push and hold it again within one second and then release it.  If you have equipped the LED's, they will all flash 3 times to indicate that the frequencies (VFO-A, VFO-B and the split function) have all been stored in flash memory and will be available whenever the radio is next powered up.  

4. Other Concerns / Considerations

    If the user decides to tune up the antenna to make a QSO (say, answering a CQ), and if the antenna SWR is too high - the power supply circuit breaker will trip.  Since the DDS board is powered by the same supply, the desired frequency will be lost when the breaker is reset.  This is one of the drawbacks of using DDS in lieu of the analog PTO when the DDS is powered by the current sensing power supply.  

    Two solutions are possible. 
  1. The DDS board could be powered separately - say by a wallwart supply - and left on all the time.  This way, should the Century's power supply trip out, the desired frequency information will be retained on power up.   Gauche? - yes, but workable.
  2. Alternately and preferably, the operator may gradually increase output power (using the drive control) when tuning up to an antenna.
5. Spurs

    A few DDS spurious signals that can be heard above the background noise with a tuned antenna connected:
6. Digital Display Whine

    There are signals from the digital display at 7291 khz and 28.411 khz. - and possibly others.   Since the extraneous signals from the digital display were few, no attempt was made to ameliorate them as they must exist in the stock, unmodified digital Century 21.

   There as also a subtle (but audible) whine present on all bands, but most pronounced on 40 and 80 meters.  While it could easily be eliminated by switching in the CW audio filters, some folks might find it objectionable when the receiver is running 'wide open'.  Here's what I did to significantly reduce it.

    For openers, the Century 21 digital display board is not shielded (like the ones in the Omni's and other rigs) nor is it - in my opinion properly grounded  The single point of ground is actually the shield of the coaxial cable feeding it.  

Note: I think the 'Tennessee Techs' screwed up on this one.

    I soldered 4 short #22 wires at the corners of the board (underside) and a 5th right at the ground connection of  C12, and then connected these leads to the two front panel ground screws that hold the digital display mounting plate.  I screwed the nuts very tightly.  Almost of the whining noise has evaporated.

   Another way to 'peel this onion without crying' would be to ground all 4 corners of the display board directly to its mounting plate.  This would necessitate drilling out the existing insulated mounting pillars and then replacing them with 4-40 hardware and spacers so that the LED's would mount at the proper level relative to the front panel.  This is probably a better solution, but is also more work - diminishing returns??

  A more elegant solution might be installing an AADE DFD1A to replace the Ten-Tec digital display (after its MK50398 chip fails) and / or use the DFD1A to 'digitize' an analog Century 21 - with the stock PTO or with the DDS VFO as it would work with either one.

  Food for thought (if anyone's hungry).