A Simplified Dual Digital VFO for the Ten-Tec Omni A, B, C, D, Corsair, Century 21, Argosy, Triton, Delta, Drake TR7 & Similar Digital Display, 5.0 to 5.5 Mhz VFO / PTO Equipped Radios

omni picture
1. Introduction

    Several years back I developed and offered a stable and drift free dual digital DDS driven VFO that tuned from 5 to 5.5 Mhz and which could be easily integrated into the Ten-Tec Omni, Corsair, Triton, Century 21,  (etc) radios and which was sold on eBay.  This design used a 28 pin Freescale processor that required an external crystal oscillator and other supporting components.  

    The initial design has been simplified by coding a new PIC processor which has been described on this web page and which has been mounted in a mini-box requiring only one simple wire change within the Ten-Tec radio.  This is truly a plug-and-play solution for those folks whose PTO needs rebuilding (Ten-Tec no longer sells the rebuilding kits) and / or for those who don't have the time to attempt a rebuild using the existing PTO parts (sometimes this works - - - and regrettably sometimes it doesn't).

    Of course, this DDS VFO could be installed right in the radio itself by following the general  instructions shown below.

   The balance of this webpage shows how this newer design may be implemented in the Omni series specifically, and in other Ten-Tec radios -  in principle.

2. Parts

    You'll need about $60 - much less if you have a well stocked junkbox.  The most expensive part is the optical encoder.  I found a couple of nice ones on eBay for $20 each.  Truly clever folks may be able to construct their own encoder using photo-diodes and a home built optical interrupter with 'bearing type' parts from discarded potentiometers.  Surf the web for suggestions. The board has been designed so that the Chinese AD9850 board (currently available on eBay) will plug right into it so there is no need to solder any surface mounted devices.

3. Conversion Suggestions - Ten-Tec Omni Radio

    If you plan to mount the DDS controller within the radio, the first thing is determining how to mount your optical encoder.   To do this, you'll have to disassemble the radio to the point of removing the front panel.  You might want to get a small container for all the knobs, parts, screws, etc.

    a) Disassembling the Omni (as an example).
    Note: Be sure to save the 2 felt pads from the main tuning knob as they will come in handy later.
    b) Removing the PTO
     c) Installing the Optical Encoder
Note: a 128 step optical encoder will enable a frequency excursion of about 5 khz for each complete rotation.  A 256 step encoder will double this to 10 khz.  In my opinion, either is totally satisfactory and there are most often economical choices on eBay.

    Before installing the encoder, make sure your board is working properly.  Following the schematic, connect up the encoder, a frequency counter to the output and then 12 VDC and ground (see connections).  You should see a 5.000 Mhz signal (or something very close) which should change as the encoder is rotated.  If you don't have a counter, your station receiver will suffice.  You can even connect it directly to your Omni at the rear jack provided for a remote VFO.  
Important Note: The DDS VFO output level should be set as close to the output of the Omni's PTO as possible as the Omni's circuitry was designed around this value.  Setting the DDS VFO's output higher than this may generate some 'birdies' and needlessly increase the receiver's background noise level.  The Ten-Tec's PTO's output is approximately .5 Volt peak to peak.  A resistor trimmer (R18) has been provided for this purpose.  If you have a 'scope, you may use it to set the DDS VFO's board right on the money.  If you don't have a 'scope, just turn R18 slowly until the radio begins to receive AND transmit SSB  properly on all bands, but no higher.  
 
Also Note: If you monitor the signal on your receiver, you'll probably observe a rough, warbling note.  This is normal as the RIT connections have yet to be made and the processor's A/D converter (used for the RIT function) will be 'hunting' a bit.
    Connect 4 (2.5 foot strands) strands of 30 gauge wire to the encoder.  Tag the power and ground connections (I used knots in the wire).  The phase leads can be swapped later (if it tunes backwards).

    In this case, the size (length) of your encoder shaft matters.  By far, 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 and then the front panel.  If you can satisfactorily attach the tuning knob, and if it spins properly - that's great.

   But if you need a bit more shaft length, either check around for another tuning knob whose set screw is closer to the back of the knob and / or attempt to drill out the old Ten-Tec tuning knob using a 1/4 inch bit and a drill press.  I tried to my old knob out with just a hand drill and muffed the job because the knob had a slight wobble when tuning.  

    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 (as shown), I mounted the encoder on the front panel by securing it to the same piece of drilled P/C board material which itself was mounted on the rear of the front panel using the two existing screw holes.  Here's a picture of the panel.  That's a steel washer under the nut.  It fits perfectly, and it tunes like a dream.

    Since the optical encoder didn't fit into the PTO opening, I had to enlarge the sub-panel opening using a Greenlee chassis punch.  I mounted the encoder with the wiring pins downward and didn't obliterate the former PTO mounting holes. Once you are satisfied that the front panel can be properly installed, add about 2.5 feet of wire to the old SPOT switch, mount it on the front panel and temporarily set it aside.

d) RIT Functionality

    The goal here is to give the controller access to the Omni's offset control (RX-OT - 22K for my radio) and (optional) access to the existing offset switch.

    First, remove the 4 screws holding the preselector and turn the whole assembly backwards.  You'll note another example of  TEN-TEC's parsimonious wiring.  In my case, I was able to flip it back just enough to remove the Offset control from which I clipped all 3 wires.  

    Next, solder 3 wires (about 2.5 feet long) to the control, tagging the center (wiper) lead, and set this control aside.  If you want to use the existing Offset control switch and the OT led to indicate that the offset mode (only on receive) has been activated, one more step is required (see below).  If you don't want RIT on/off functionality, then there is no need to modify the switch assembly.  If the RIT Disable lead is left open (high),  the Offset control will be active (except while transmitting). You'll have to set it properly when in the SSB mode (so that you're not off frequency).

    Skip ahead to this step to reinstall the Offset potentiometer, the preselector, the digital display and the notch filter.

    Remove the 4 nuts holding the switch assembly and push it back.  The mounting screws are inside the radio and they are equipped with vinyl spacers. The goal is to access to the portion of the OT (offset) DPDT switch that controls the tuning voltage being set to the PTO while the offset function is active.

    Looking down from the front of the radio, disconnect the two wires connected to the rightmost side of the offset switch.  While there, disconnect the two wires from the SPOT switch.  They can be tagged and securely wrapped.  Alternatively, they can be removed if you don't plan to revert to the PTO.  

    Two resistors (R2 and R4) on the switch board itself that need to be removed if you want to use the OT switch to activate / deactivate the controller's RIT function.  Here's a picture of one.  They have to be removed for the switch to activate / deactivate the RIT function and light / extinguish the OT LED.  

    I wiggled the board until I could see both 1/4 watt resistors through the hole and then clipped them both out with needle nose pliers. The ideal way to remove these parts would be to remove the 4 screws holding the board and then bring it up a bit.  Needless to say, if you clip out these resistors it will be difficult restoring the rig to its PTO condition without a lot of additional work.

    Carefully solder wires (each about 2.5 feet long) to the two upper, rightmost portions of the offset switch.  Once done, use your voltmeter to verify that the switch shorts when the offset is off, and that this short is removed when the offset switch is in either of the 2 upper positions.  Run this wire as you've done with the previous ones.

    Using needle nosed pliers, carefully replace the vinyl spacers that may have fallen off their screws and bolt the switch assembly up.

    Reinstall the OT control on the front panel and run the wires beneath the chassis.  Before tightening nut on the OT control, connect a meter and set the control to its exact electrical center (11K from the wiper either leg).  You want to ensure that this position will be maintained when the OFFSET knob is installed.  It should point directly up - for a zero offset. Replace the preselector assembly followed by the digital display and the notch filter.

    Drill suitable holes in the front panel if you plan to mount the SPLIT and FAST TUNING buttons, and the VFO a, VFO B and SPLIT LEDS (not shown).

e) Reinstalling the Front Panel

    Ensure that the leads for the optical encoder, the SPOT switch, RIT on / off (if used) and the Offset control are all run properly.  Ensure that the three flat 'black donuts' are mounted over the toggle switches beneath the preselector and that no wiring is kinked / snagged.  Move the front panel close to the chassis and connect the 3 wire plug that operates the ALC and OT lamps and then place the panel AND the trim ring in place, securing it with the 4 corner screws.

  Remount the mike and headphones jacks, placing them in the proper holes along with the large lock washers.

    Place a 1/4 flat washer up against the optical encoder shaft followed by the 2 felt washers removed earlier, and then mount the tuning knob.  If your encoder has little to no torque resistance (as one of mine did), the felt washers will keep the knob from unduly rotating after it has been turned and released.

  f) Mounting the Controller

    I mounted my board in the right rear corner of the radio where the crystal calibrator mounts in the analog Omni A. This is a convenient spot as it has both a source of 12 VDC and existing coaxial cable access to the VFO amplifier board to which the existing PTO is connected.  This spot permitted last-minute refinements to the M/P code (during the debugging phase) .

   The ideal way to mount the controller would be to drill 4 holes in both the corners of the P/C board and then into the chassis, providing a physically secure grounding arrangement.  Being pressed for time, I mounted a piece of insulating perf board under my P/C board, and secured this with one of the existing screws used to hold one of the connectors on the other side of the board.  One of the 2 P/C board mounting screws is located in the corner of this board, while the voltage regulator grounding tab is used for the other.  I installed a longer screw in an existing hole (see picture).  Grounding wires at the other corners of the board are connected to convenient locations.  

 The board could also be mounted in the space where the optional Noise Blanker plugs in (I understad that while they work very well, they are extremely rare).

    Note: Since the whole board draws close to 140 ma, the LM7805 needs a heat sink, and the chassis alone is perfect.

g) Connecting the Controller

    After the controller is mounted securely, it can be wired up:
Note: Be sure that you have the right pin by measuring its voltage in receive (12 VDC) and transmit (0 VDC).
5. Commands

    When 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.  If the RIT (OFFSET) switch is activated, the receive frequency will vary based upon its setting; the transmit frequency will not change.  When the OFFSET is turned off, the original frequency will be restored.

    Note: The RIT function will not work when in the SPLIT mode.

    To switch to VFO B, depress (tap) the function button briefly, and the system will switch to 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, tap the SPLIT button.  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 SPLIT button.  The contents of the on-line VFO will be copied into the off-line VFO.

6. On-the-Air Results
a) Receiving

    With the P/C board mounted in an external aluminum mini-box, receiving quality is fine with a couple of 'spurs', as shown below.  One would have to assume that the DDS VFO would perform just as well within the OMNI if it were mounted in a small aluminum box or ome made out of soldered P/C board.

    With an antenna connected and the preselector peaked, there are a few detectable narrow banded (200 to 300 khz spurs).  The louder spurs are italicized. 
Note: This 10 meter spur (28987 khz is quite loud) is a well known Omni design issue.  To tune this frequency, set the bandswitch to the 29 Mhz position and tune downward. Check the service manual for more information.
Note: One way to cut down on any spurious mixing products is to follow the instructions in the service manual to adjust both R23 and R2 on the oscillator / mixer board. Perform the adjustments shown in Step 3 and in Step 7 (Mixer Balance).
 b) Transmitting

    Several SSB QSO's were made on 40 meters, and the reports were comparable to what one would expect from a PTO equipped Omni - generally very good.  Both the SPLIT and QSK functions work properly on CW. 

7. 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.  

    Three 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 Omni's power supply trip out, the desired frequency information will be retained on power up.   Gauche? - yes, but workable.
  2. If the AIRPAX (or equiv) circuit breaker used to safeguard the radio's finals were to be installed in the Omni proper, then the DDS VFO could be powered on the 'line side'.  This way, the circuit breaker's tripping would not cut the power to the DDS module.
  3. Alternately, the operator may gradually increase output power (using the drive control) when tuning up to an antenna.
8. Using an External DDS VFO Controller - Front, Side, Rear Views

    Those reluctant about digging into their radio to mount the DDS VFO P/C board, encoder (etc), may opt to build the whole thing in a separate enclosure as shown in the above pictures.  As you'll note, the prototype unit has 3 unmarked LED's across the top (VFO-A, VFO-B, SPLIT), a red and a black push button, the tuning knob for the optical encoder itself, and the RIT control with an activation / deactivation switch.  It's easily connected to any OMNI (A,B,C,D), Corsair (etc) radio.

    The rear panel shows the power connector, the VFO output and a third phono jack into which the transmit signal from the radio is to be plugged, as was done with the internal modification (R lead) that was just described.  

    The black button is the multi-function unit that lets the user switch VFO's.  The red button is just a one press access to the split function.