Important Note:  Unless you are absolutely certain that you know what you are doing, do not perform this conversion.  
I cannot be - nor  will  be - responsible for any damages to your radio incurred as a result of your reading this website. 

A Dual Digital VFO and Display for the Ten-Tec Triton 540


1. Introduction

    Although this site currently describes how this DDS VFO can improve the Ten-Tec Triton 540, it may be readily installed in other Ten-Tec radios including the Corsair I and Corsair II, the 'Omni's', the Delta, the Argosy II and the Digital Century 21 as these radios are all equipped with a factory stock digital display.

    However, if the user is willing to install a home brew digital display, other Ten-Tec radios like the analog Century 21, the Century 22 or the early Argonauts could likewise reap the benefits of this conversion.

Note: This VFO could also be housed in a metal enclosure and serve as a versatile dual VFO for those reluctant to modify their Omni, Triton, TR7, etc and / or don't want to take the time (and expense) to rebuild the mechanical PTO.  Here's just one example.

2. Parts - Itemized Component List

    You'll need about $60 - less if you have a well stocked junkbox.  The most expensive part is the optical encoder.  I found 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.

Note: a 128 step optical encoder will enable a frequency excuirstion of about 5 khz for each complete rotation.  A 256 step encoder will double this to 10 khz.  Either is totally satisfactory and there are most often economical choices on eBay.

   The P/C boards have been designed so that the Chinese AD9850 DDS board can be soldered right to it.  Here are pictures of the schematicthe P/C board 2 artwork, the wire connection points and a completed assembly.

3. Conversion Suggestions - Ten-Tec Triton 540 Radio

    The Analog Triton (540) does not have an internal digital frequency display.
So, if you plan to undertake this conversion, you'll need some form of digital frequency display as the PTO, the circular frequency marker and the slide rule dial will all be gone - removed!  While one could possibly find a used external Ten-Tec digital display, I decided to mount an Almost All Digital Electronics DFD1A device within the radio and on the front panel (more later).

    a) Disassembling the Triton
    b) Removing the PTO and Crystal Calibrator
    c) Pretesting and Mounting the Controller

Before installing, ensure that your DDS VFO board works properly.  Following the schematic, connect up the encoder, a frequency counter to the output and then 12 VDC and ground.  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 Triton 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 Triton's PTO as possible as the Triton's circuitry was designed around this value.  Setting the DDS VFO's output higher than this will generate 'birdies' and needlessly increase the receiver's background noise level.  The Triton's PTO's output is approximately 1.5 Volt peak to peak.  A resistor trimmer (R5) has been provided on the Version 2 circuit board for this purpose.  If you have a 'scope, use it to set the DDS VFO's board right on the money.  If not, just turn R5 slowly until the radio begins to receive 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 OFFSET / RIT connections have yet to be made and the processor's A/D converter (used for the RIT function) will be 'hunting' a bit.

      I mounted my DDS VFO in the space previously occupied by the Noise Blanker.  I removed the plug-in terminal strips and wiring, except for the red lead which I used to power the DDS VFO.  I rerouted the coaxial cables to effectively bypassing the Noise Blanker, and then carefully drilled two mounting holes - one for the voltage regulator and another diagonally across for another grounded mounting.  I placed a piece of perf board between the DDS VFO board and the chassis to eliminate the possibility of any short circuits.  I also installed a small terminal strip to permit the installation of a series wound toroid filter in an attempt to reduce any spurious signals from 'riding' the 12 volt line and to keep any transmitter RF out of the DDS VFO. All-in-all, the board is very securely mounted with good ground connections.  

    While you are drilling the holes, make another one at the base of the DDS VFO board and equip it with a grommet.  This hole will be required for the 4 offset leads and the transmit lead going to the R lead on the Offset board.

  Mount a small terminal strip on the inside of the VFO enclosure as shown here.  Using a 4-40 screw, you can 're-tap'  the existing screw hold that secures the coaxial grounds on the other side of the VFO enclosure so that the new 4-40 screw can hold both terminal strips.  Once done, run a short piece of coax from the output of the DDS VFO - through the small hole in the VFO enclosure - to the new terminal strip and terminate the shield to ground.  The 2 white leads will be connected together to the coax center conductor on an adjacent terminal strip pin.  This makes for a very stable connection.

    d) Installing and Connecting the Optical Encoder

    The easiest way is to place a small metal plate (drilled for your encoder and mounted in the existing PTO mounting holes) on the outside of the radio's sub-panel.  Install your encoder (finger tight) and then the front panel.  If you can attach the tuning knob of your choice, and if it spins properly - OK!

    In most cases, however, you may have to mount the optical encoder on the front panel with this
small metal plate and then cut away at the sub-panel so that the optical encoder will fit (tin snips will suffice).  This is what I had to do with my Triton.
e) RIT / OFFSET Functionality

    The wires from the OFFSET switch are currently connected to the Control Board which is on the top side of the chassis.  Our goal is to reuse the wiring from the Control Board to the OFFSET control, re-routing them to the DDS VFO board underneath.
f) Installing the Digital Display Controller - Repurposing the Crystal Calibrator Activation Switch

I used an AADE DFD1A digital display board mounted in the space vacated by the crystal calibrator and the LCD display mounted on the Triton's sub-panel.   First, remove the two circuit boards on the other side of the chassis and carefully drill two holes for the 4-40 screws that will support the DFD1A circuit board.  Install a piece of perf board (Radio Shack) between the two stand offs created by the 4-40 hardware.

    Assemble the DFD1A board using the instructions provided in the kit, mounting the pin header and other wiring connections as well as the LCD contrast and TXCO trimmer resistor on the component side of the AADE provided circuit board.  The board is assembled this way to provide access to these points after the DFD1A board has been mounted on the perf board.  I used simple wire straps to hold it in place.  

    The DFD1A makes for an exceptional digital frequency readout.  However, when connecting it up to the Triton 540, a switching mechanism needs to be provided to 'tell' the counter if it should be 'counting' up or down.  Rather than drilling another hole in the radio's front panel, I opted to use (repurpose) the existing push / pull switch on the RF gain control, the one that was used to activate the crystal calibrator.

    As shown on the schematic, this switch provides 12 VDC to activate the calibrator, but the DFD1A board requires a grounded connection.  The actual switch connection on the rear of the RF Gain control has a 12 VDC tie point (4 wires) on one lug, and a voilet wire on the other.  This is the violet wire that runs to the previously removed cryatal calibrator board.   The 12 VDC tie point needs to be removed and a ground substituted.

    Carefully unsolder all 4 wires from the switch, solder them together and cover them with 2 layers of heat shring tubing

    Then connect a wire from the spared terminal to ground, as shown here.

    Verify with your meter that operating the switch provides a solid ground on the violet wire, and then connect the violet wire to the appropriate point on the DFD1A board.  Connect the power and ground leads from the DFD1A module to the 12 VDC connections on the DDS VFO board.                                                                                                                 

    I used the 'Biggie' display provided by AADE.  It fits nicely in the narrow space between the chassis and the front panel.  After carefully measuring and drilling the holes, I mounted the display with 4-40 hardware.

    Before mounting the display, I wired it with 30 gauge wire.  Since the space between the chassis and front panel is mininal at best, I was not able to use pin headers.  Rather, I soldered the 30 gauge wire directly to the holes in the LCD in which the pin headers are normally inserted.  Once the wiring was done and double checked, I provided several layers of plastic tape to guard against shorts and then mounted the LCD.  The wiring back to the DFD1A board was run through a newly drilled - grommet equipped - hole in the front of the chassis.

    Using a short length of RG-174 coax, connect the outout of the Triton's VFO to the input of the DFD1A counter through a 100 pf ceramic capacitor.

    Hold off on reinstalling the front panel as it needs to be reworked a bit.

    Hint:  It's not necessary to ground pins 10, 9, 8 and 7 on the LCD board.  A ground connection between pin 1 and pin 5 will suffice.

g) Reworking the Front Panel

   First, remove the plexiglass panel and then 3 incandescent bulbs.  At the very least, a function button needs to be installed on the front panel to switch VFO's, activate / deactivate the SPLIT mode, and so forth.  Additionally, optional status LED's may also be installed, but I dfecided not to install them at this time.  This is a very tight front panel arrangement, not at all like the rather spacious Omni series.

    Were even the smallest push button switch installed flush with the front panel, a corresponding hole would need to be made in the chassis to accommodate the wiring.  So, to simplify matters, I just removed the OT LED and drilled out the opening just a bit to accommodate a miniature Radio Shack normally open push putton switch (Crazy glued in place).  Connect one lug on this switch to ground and the other to the trace on the P/C board that goes back to the OT lead.  DO NOT CONNECT IT TO THE 12 VDC TRACE.

    Next, remove the OT TR board and snip the OT-LITE wire right at its connection lug.  Remove both the Control and SSB GEN boards so that this lead can be run (through an existing plastic grommet) to the DDS VFO board on the other side of the chassis where it will be terminated on the button lead,
as shown here.  Once done, reinstall the insulators and the boards themselves.  Don't turn the power on yet.
h) Reinstalling the Front Panel

    Ensure that the leads for the optical encoder are run properly (no kinks or snags). Move the front panel close to the sub-panel and connect the 3 wire plug that operates the ALC and OT lamps and then place the panel in place, securing it with the previously removed bottom screws.  Using your meter, verify that the push button - when pushed - grounds out the button terminal on the DDS VFO.

i) Testing the Controller
4. 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.

    To switch to VFO B, depress (tap) the FUNCTION 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 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 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 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 button - and that's it!  While LOCKED, the RIT control will work.

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

    An optional button has been 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, operate the LOCK function, release the button and push it again within one second.  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.  

Note: The instruction manual for this processor states that the flash memory can be updated just 10,000 times, so you might want to use this function frugally.  If you want to disable it, simply ground the FLASH INH lead (see the schematic).

5. How the Software Works - The 30,000 Foot View - click here

6. On-the-Air Results

a) Receiving

    The P/C board is mounted in the rear of the chassis, with no shielding and with rather long leads for the Encoder, Offset and Push Button controls, and it works well for me.

Note: One way to cut down on 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).

   With an antenna connected and the preselector peaked, there are a few detectable narrow banded (200 to 300 khz spurs).  The louder spurs are asterisked. 
Note: This 10 meter spur (28987 khz is quite loud) is a known Omni issue.  To tune this frequency, set the bandswitch to the 29 Mhz position and tune downward. Check the service manual for more information.
 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 Triton - generally very good.  Both the SPLIT and QSK functions work properly on CW. 

8. 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 Triton'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.
9. 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 4 unmarked LED's across the top (VFO-A, VFO-B, SPLIT and LOCK), a red and a black push button, the tuning knob for the optical encoder itself, and the RIT control with an activation / deactivation switch.

    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, operate split and lock the dial.  The red button is just a one press access to the split function.

    Note: The unit shown here worked very well with both my Ten-Tec Omni and Drake TR7.  Another ham is using it with his Corsair 1.

Over Voltage Protection

    Should the pass transistor fail on your power supply, the output voltage can quickly rise to 25 volts or so, wreaking big time havoc with your radio's solid state devices.  A simple way to guard against such a failure is to place a zener diode in the radio on the other side of the fuse so that of the voltage should rise to 14.8 volts (for example), the zener will conduct and draw enough current to pop the fuse first.  I used a  1N6275AG purchased from Mouser and placed it in the circuit as shown here .