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
to your radio incurred as a result
of your reading this website.
Digital VFO for the Ten-Tec Omni A, B, C, D, Argosy & Similar
Radios - Updated!
- Drake TR7 Conversion Suggestions - Under Construction
Omni Information / Modifications
Manuals and Helpful Information
I saw an article written another ham who
is successfully marketing a conversion kit for
Ten-Tec, Heath and Drake radios that replaces their aging PTO's with an
optical encoder driven, DDS equipped VFO. Since
had already built an AD9850 DDS signal generator using
Heron's (N2NAP's) code, I
thought it might be interesting to develop a quality DDS
VFO using additional information published on the internet.
This website shares my results. If
you decide to undertake this project, please
read through the instructions once or twice before
Note: The Freescale code used in this project is available by clicking here.
2. The 'Contributors'
- Giving Credit Where
Credit Is Due
Some DDS code was originally written by George Heron
- N2APB - on a Freescale processor
to control an AD9850 DDS chip as part of a signal generator project.
Porting the software
an MC908JL16 chip was easy. I
bought a Chinese AD9850 DDS chip boards on eBay for $6 (including
shipping). Since it used a 125 Mhz crystal, I changed
multiplication constants in
George's software, and
it worked right off.
The output of the DDS chip was 'wimpy' and needed
amplification. An internet search
produced a 2 transistor amplifier
designed by K8IQY.
Once breadboarded and operational, I had a nice 4
peak-to-peak output. I sold off my old Heathkit
and began to experience the wonders of DDS.
I then reworked the N2APB
code into a dual 5 - 5.5 Mhz VFO arrangement replete with
operation, RIT and LOCK capabilities. Also included was an
interrupt driven tuning acceleration algorithm that significantly
facilities traversing from one band edge to the other. This
tune much like an analog VFO with tuning steps depending upon
the speed and duration of the optical encoder.
wrote an article on equipping a Ten-Tec Corsair with
a DDS VFO. I breadboarded the filter
following the K8IQY
amplifier, and connected it up to my recently acquired Ten-Tec Omni D
at the rear VFO plug. I
also tried it out (externally) on an Argosy I.
It worked really well.
I used Eagle CAD software to generate PCB
artwork, designing the P/C board so that the Chinese AD9850
plug right into it.
M/P code also supports the AD9851 DDS with a 30 Mhz clock, but this DDS
board will probably have to be mounted differently.
Instead of winding toroids for L1 and
the output filter, I used 1.8 uH inductors from Mouser
Electronics. They're high Q devices that mount just like
resistors. Here pictures of the schematic diagram,
the (current) P/C board artwork and
the first completed assembly.
Note: This VFO could
also be housed in a metal enclosure
serve as a versatile dual VFO for those hams who are reluctant to
modify their Omni and / or don't want to take the time (and expense) to
rebuild the mechanical PTO.
3. Parts - Itemized
You'll need about $60 - maybe less if you have
a well stocked junkbox. The most expensive part is the
encoder. I found a couple of nice ones on
for $20 each. It's a fine piece
revolves effortlessly. 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
the web for suggestions.
Note: If demand materializes, I'll order another batch of bare bones P/C
boards which should cost me about $10 each in quantity. With
the assembly source code posted on this website, one should be able to
their own M/P by using Freescale's Code Warrior.
4. Conversion Suggestions
you plan to undertake this conversion, the first thing you need to
address is mounting your optical encoder. To do this, you'll
disassemble the radio to the point of removing the front panel.
You might want to get a small container for all the knobs,
a) Disassembling the
- Remove the top and bottom
covers. On the bottom
you'll probably note that the speaker connection coax is rather short
and has not been provided with a quick disconnect
plug. To save a lot of frustration and possible
just unsolder the wires at the speakers.
- Remove the 2 screws holding
the notch filter
and set it out of the way. There's no need to cut any wires.
- Remove the support bar behind the digital
frequency display and the large screw behing the display itself.
all the knobs. The 3 VOX controls simply pull off.
bandswitch requires a small slotted screwdriver and the remaining
knobs require a very small (.05)
Note: Be sure to
save the 2 felt pads from the main tuning knob as they will come in
- Unscrew the nuts securing the PHONES
and MIKE jacks and retain the flat and lock washers.
- Remove the 4 phillips screws at the
slide the panel (and the trim ring) forward, removing the nut holding
the SPOT button, unplugging the small connector that provides power
to the OT and the ALC lamps. Once done, set the panel aside.
- Turn the radio over onto its
the 3 front panel nuts securing the VOX board and the retaining nut in
the rear. Move the VOX board away from its mounting position
you'll need the additional space to rotate the PTO before its removal.
b) Removing the PTO
- Flip the radio over and remove the 4
panel screws holding the digital display and carefully set it back - no wires need be cut.
the 2 front panel screws securing the PTO and twist the PTO
so that you can either clip or unsolder the 3 wires and coax
connections to it. You might want to leave a slight bit of
coded wire on the PTO lugs if you ever plan to reinstall it. Tape the wires and set
the PTO aside.
c) Installing the Optical
next steps will deal with the installation of the Optical Encoder.
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.
You should see a 5.000 Mhz signal (or something very close)
which should change as the encoder is rotated. If you don't
a counter your station receiver will suffice. You can even
connect it directly to your Omni at the rear jack provided for a remote
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 will
generate some 'birdies' here and there and increase the receiver's
background noise level. The PTO's output is approximately .5 Volt
peak to peak. A resistor trimmer (R5) has been provided on the
circuit board for this purpose. If you have an oscillospe, you
may 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
If you monitor the signal on your receiver, you'll probably observe a
rough, warbling note. This is normal as the RIT connections
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
The phase leads can be swapped later (if it tunes
The size (length) of
the 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 chassis. 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.
But if you need a bit more
shaft length, then you can 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
a slight wobble when tuning.
Also, my Ten-Tec knob was not properly balanced as it has definite
heavy spots that caused the tuning to 'creep' unexpectedly - when
connected to a free turning optical encoder (although it works perfectly
with the stiffer resistance presented by the PTO).
This was really a slight design blemish that was corrected in
Argosy series as the engineers then 'balanced' out the innards of the
knob with - believe it
or not - birdshot and glue.
To use the tuning knob of my choice
whose set screw was too far
back to securely grasp the optical encoder mounted on the chassis (as
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
will not fit into the smaller opening for the PTO, I had
the opening using a Greenlee chassis punch.
mounted the encoder with the wiring pins downward and didn't obliterate
the former PTO mounting holes in the very unlikely event
that the radio was to be restored.
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 the whole thing 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
First, remove the 4 screws holding the preselector
and turn the whole assembly backwards. You'll note
example of TEN-TEC's 'frugal' 'point-to-point' wiring
In my case, I was able to flip it back just enough to remove
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
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
to modify the switch assembly. In this way, the Offset
(except while transmitting). You'll have to set
properly when in the SSB mode (so that you're not off frequency), and
adjust it for the proper (750 hz) offset when transmitting.
The controller will apply the offset
specified by the
offset control whenever the tuning knob is turned. Skip ahead
step to reinstall the Offset potentiometer, the preselector,
the digital display and the notch filter.
the 4 nuts holding the switch assembly and push it back.
The mounting screws are inside the radio and they
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
switch. While there,
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 each of the
resistors through the hole and then clipped them both out
with needle nose pliers. They're easy to remove as
watt units. The ideal way to remove these parts would be to
remove the 4 screws holding the board and then bring it up a bit.
However, the stingy wiring practice struck again.
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. Not impossible, but decidedly more work.
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
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.
e) Reinstalling the Front
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 '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
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.
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 itself.
The felt washers will keep the knob from unduly rotating
has been turned and released.
f) Mounting the Controller
decided to mount mine in the right rear corner of the radio, just under
the RCA phono plug array 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.
spot permitted last-minute refinements to
the M/P code without removing the front panel, digital display, etc,
The ideal way to mount the controller would be to drill 4
in both the corners of the P/C board and then into the chassis,
providing a physically secure grounding arrangement.
However, being pressed for time, I mounted a piece of insulating
perf board under my P/C board, and secured this
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
in an existing hole (see picture). Grounding wires at the
corners of the board are connected to convenient locations.
Since the whole board draws close to 140
LM7805 does require a sink of some sorts, and the chassis alone is perfect.
g) Connecting the
the controller is mounted securely, it can be wired up:
- connect the metal tab on the LM7805
voltage regulator to an existing nut that holds a terminal strip (on
the other side of the chassis).
- run short ground wires from each
corner of the board to nearby grounding points. For example,
there are 2 grounding points on the SSB Generator board that can be
- next, connect up the 12VDC supply.
Because I mounted the board underside the chassis close to
the external VFO jacks, I tapped into this 12 VDC source.
When making this connection, I used a T37-2
toroid almost completely wrapped with 24 gauge wire in an
attempt to keep any RF energy from the Omni from interfering with the
DDS. This is
probably not needed, but I had a supply of toroids on hand, so - why
- once these connections are made, turn
the radio on and verify that the red LED on the DDS board lights up.
Then, turn the radio off.
- connect a short length of RG-174 coax
from the AMP
OUT pins on the DDS controller to the connection where you
externally tested your controller.
- turn the radio on again and note the
digital display. It should show a frequency close to the low
end of the band. Then turn it off again. The next
step will be wiring up the optical encoder.
- there are four (4) connections for the
optical encoder - +5VDC, ground and the two phase connections.
You need to be
careful as reversing the power and ground connections could ruin
- using your voltmeter, double-check both the +5VDC and
ground connections on the controller board. They are located on JP-4.
The connection at the TOP
and the one at the BOTTOM
is +5VDC, as shown here.
Make both connections.
- Next, connect the remaining 2 phase leads to the remaining
pins, power up the radio and turn the encoder. If the
frequency changes properly (i.e. clockwise is up), then this is done.
If not, just reverse the two phase leads. You've a
50 / 50 chance on being right the first time. Once done, you
should be able to tune the radio.
note of how the tuning acceleration algorithm works. Unlike
the PTO scenario, one can get to either band edge in a hurry. Connecting the RIT
pot is next..........
- Identify the wires running to the previously rewired 22K
Offset pot. Connect the tagged center wire to
the center pin on the RIT connector on the controller board, as
Connect one remaining wire
to the lug at one end, and the other wire to the other spare lug (only
one wire on each lug). Turn the radio on and vary the OFFSET / RIT
control. If the
OFFSET control works in the proper direction, you're done. If it tunes backwards, then just
reverse the end connections.
- If you've modified the Omni's OFFSET switch to enable /
disable the RIT function and light the OT lamp, you'll have to
make another connection. If not, skip to the next step.
Identify the two wires connected to the front panel offset
switch and ground one side somewhere on the controller board. Connect
the other side to pin 3 on JP6, as shown here.
Verify that the OFFSET function is disabled when the OT
switch is Off. Tune in a station and turn on the OT switch
(either up position). Vary the offset tuning and note the
signal change. Turn the switch off and note that the
frequency is changed back to its original. There should be
about 1.2 khz available on either side of 'top dead center'.
The next connection is for the push button control.....
- Identify the pair of
wires connected to the push button switch
(formerly the SPOT control), and ground one side somewhere on the
control board. Connect the other lead to pin 3 on JP3, as
shown here. Verify that the SPOT switch - when briefly tapped
- will switch from one VFO to the other.
last interconnection is for the lead that will cause the M/P to go into
the transmit mode, and care must be exercised in its
placement as it carries 12 VDC. If placed on
the wrong pin, this voltage will harm the M/P chip. This
connection is made on the Control Board, the one that's just
below the optional filter board. It's to be placed on the 'R'
- the third pin from the right (from the side of the radio).
It's on the back of the board and can be reached without
the filter board.
Note: Be sure that
you have the right pin by
measuring its voltage in receive (12 VDC) and transmit (0 VDC).
- Solder a wire to this pin and run it
through one of the chassis holes for connection to pin 1 of JP-3 the
DDS control board. Be sure you've
identified the proper pin. You might want to bend this pin a
away from pin 2 to avoid the possibility of a cross. Verify
that the radio transmits properly.
When initially powered up as received, both the A and the B VFO
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 RIT (OFFSET) switch is activated, the receive frequency will vary
its setting; the transmit frequency will not change. When
OFFSET is turned off, the original frequency will be restored.
To switch to VFO B, depress (tap) the function
(most often wired to replace the Omni SPOT button) briefly, and the
system will be using VFO B. The frequency previously stored
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
will control reception, while the off-line VFO will control
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
VFO will be copied into the off-line VFO.
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 RIT control will work.
Note: if you have wired up
the optional LEDS, the LOCK LED will be illuminated.
Some additional functionality has been added - see below.
6. How the Software Works
30,000 Foot View
The program itself is just about 2,300
long and resides on a Freescale MC908JL16CPE chip.
It can be used as is on smaller (less memory) versions of the
JL16 family and / or easily ported over to other chips in the powerful,
flexible and feature rich Freescale family. I've tried to
ample comments in the assembler listing to show how the program works.
When first powered on, the program initializes all the variables that
will be used in processing. Variables include the input /
ports, memory location names and the like. The program sends
serial data to the DDS chip to load the initial frequency (5,000.00
khz), and then starts running a continuous tight loop which both checks
the status of the encoder and the panel mounted (formerly SPOT) button.
Concurrently, periodic interrupts are programmed to see if
RX-OT switch has been operated and - if it has - to measure its
deviation from 'top dead center'. If changes are detected (by
processor's A/D converter function), the DDS circuit is adjusted
accordingly. These interrupts occur 10 times each second, and
can hear the discrete steps if you turn the offset control quickly
while listening to a CW signal. There's
no 'dead band' in this RIT software.
Another interrupt measures the number of encoder 'pegs' within given
periods. When theresholds are reached, the tuning increment
adjusted accordingly. It's very easy getting from one band
the other with a little practice.
radio is 'keyed', it grounds pin 1 on the M/P which is the main
hardware interrupt point. Once done, the M/P suspends the
two interrupts, and - if operating in the split mode - quickly computes the new transmit frequency and sends
it to the DDS. (If the radio is operating in the simplex mode, the DDS is not updated). Any RIT offsets are - of course - suspended.
When the radio goes back into the receive mode, the M/P loads
the former receive frequency which may or may not be offset.
ability to store the last used frequency in the M/P's flash memory has
been added. To do this, just 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 and will be available whenever the radio is next
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).
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.
The software runs on a Freescale MC68HC908JL16
flash programmable chip. It was written on the Freescale
CodeWarrior platform which can also dynamically debug the code and
'burn' the M/P's internal EEPROM. The JL16 series has 16K of
memory space, but only about 2.3K is used for this application.
The code could probably be significantly downsized (by using
subroutines), but with all this memory - why bother??
If you want to use the CodeWarrior IDE (Integrated
Development Environnment) package, here's a link
so that you can download CodeWarrior 5 - an older version that
works well with Windows XP. It's the one that I used to
the code for this and for the other projects shown on my website.
This zipped file is over 282 mb long and will take a long
download, but it's worth it. It's self extracting and
While this version will not work with
Windows 7 or 8, you may either download the latest CodeWarrior version
from Freescale which is Windows 7 / 8 compatible (and then learn hiow to use it - ugh!), or partition your
Windows 7 / 8 computer so that it will accept a concurrent copy of XP
and then load CodeWarrior 5 onto
After you've downloaded and installed Code Warrior 5, you can then
download the DDS program itself. It's contained in a zipped
called DDS.zip. Just place file this in your My Documents and you're good to
go with Code Warrior.
This will afford you the opportunity to see the complete code
along with the comments and program a copy or copies for your own use.
You can also personalize the code for your own purposes - - have fun!
To program the M/P you'll need to build a simple MON08 type
programmer. Instructions are provided in the MC68HC908JL16
If you just want to program the chip yourself without downloading the
CodeWarrior IDE, then here's a copy of the S.19 file
which you can use with your favorite programmer.
Finally, I'll program a copy for you for $15 (M/P included) for as long
as my supply of these processors lasts.
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. I may mount the P/C
board in a small box secured to the bar that
holds the rear of the digital display, but I'm not sure at this point
as the rig works well enough for me.
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).
There are a few detectable narrow banded (200
to 300 khz spurs) with an antenna connected and
the preselector properly peaked. The
louder spurs are asterisked.
Meters - 1950
khz (this signal appears in the unmodified Omni and it can be minimized by adjusting R23 - see service manual).
Meters (B) - 28978*
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.
Mounting the DDS board closer to the
front panel - say in the location vacated by the old PTO - and / or
enclosing it in a small metal box might further reduce these spurs, but
for my money it works perfectly well thew way that it is.
SSB QSO's were made on 40 meters, and the reports were comparable to
what one would expect from a PTO equipped Omni - generally vey good.
Both the SPLIT and QSK functions work properly on CW.
9. Other Concerns /
If the user decides to tune up the antenna to make
(say, answering a CQ), and if the antenna SWR is too high - the power
supply circuit breaker will trip. Since the DDS board is
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.
10. Using an External Controller - Front, Side, Rear Views
- 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.
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.
- Alternately, the operator may gradually increase output
power (using the drive control) when tuning up an antenna.
Those who may be 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 unit has 4 (currently 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 that was just described. All that's
left to be done is the installation of some rubber feet and weighting
within the enclosure itself.
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. The new code will be published
shortly for anyone who might be interested in replicating this
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
Derived AGC Improvements (Verbatim
from the Ten-Tec Reflector May 29, 2000)
- It Worked for Me
- for a Time - Then Failed!
The audio amp section of IC-1
on the IF-AGC board is biased to provide a huge amount of gain. So much
in fact that without sufficient agc voltage, even weak signals drive it
to distortion. When a strong signal suddenly apprears, the first dit or
static pulse drives this amplifer way into distortion producing a
square wave. The square wave hits the AF stage like a ton of bricks and
nearly blows out the speaker. Also, it's rectified by the agc diode and
applied to each IF amplifier making matters even worse. So here's what
Now readjust the S meter pot
R-20. You'll need to turn it clockwise to increase it's sensitivity.
Calibrate it if you wish but don't trust it! You will find the meter's
action is a bit different now.
Now I can actually wear
headphones without getting blased and the overall distortion and noise
level is so low it almost sounds like a direct conversion receiver. You
can still expect some minor blasting from local stations but overall
this seems to be an improvement. At least if you're not happy with it,
just remove the resistor.
Give it a try and let me know
what you think.
Steve - N4LQ
applied this mod and it worked OK - for a while.
However, while I was working on the radio and powering it up
receive totally quit - no noise, no nothing.
However, the transmit was OK, as was the sidetone.
tracing the incoming signals with a signal generator and 'scope, the
problem was tracked to IC-1 (an RC 4558P) on the IF board - pin 7 had
no voltage on it! Replacing
the IC (using a socket) solved the problem. So,
since the mod involves bridging pins 7 and 6 with the 5.6k resistor, I
removed it. Mouser
currently sells this IC for 47 cents.
Chassis Grounding Screws
Up the Audio Response (Ten-Tec Reflector) - Works - But Increases Background Noise / Distortion - I Tried and Removed It!
Here is a simple mod for improved audio on the Omni C. I have not
tested it on other Omni series such as A, B, D etc. but most likely it
would have the same effect. It gives a boost to voice frequencies and a
slight peak around 600-700hz for cw. All signals seem to be much easier
to copy with the now brilliant sounding audio. Parts needed………1ea. 1 meg resistor
Remove bottom cover of rig.
Locate the audio-sidetone board. This
is the one with the sidetone pots. There is no need to remove the board.
Solder the resistor (see
) between pin 2 of IC-1 (LM-380) and
AF-Output. Instead of soldering directly to pin 2 of IC-1, you may want
to solder to R8's (10K) lead which connects to pin 2 and the other end
to C11's negative lead.
I suggest sliding some insulation
over the 1 meg resistor's leads to prevent shorting.
Replace the bottom cover and enjoy.
Intermittent connections are very often frustrating and difficult to
pinpoint / resolve. One such example involves the chassis
grounding method used by Ten-Tec on the RX Trimmer and bandpass boards
on the radio's underside. When the screws holding these
straps loosen, they can be very difficult to tighten as the PC boards
themselves limit access, and narrow needle nose pliers are often
required. Even worse, when the chassis threading becomes
stripped, there's no way that the screws
can be secured.
A simple solution is to insert a short screw in the chassis hole
beneath. Then, solder a short bare wire to the topside of the
board and secure it to the new screw, as shown here.
Heterodyne Oscillator Crystals - Check the Chassis Grounding Screw and the Crystal Pins
I purchased my Omni used on eBay. The seller mentioned that
40 meter band was sometimes intermittent, and would usually work after
the radio had warmed up for a bit. He was right. I
couple of hours on this problem noting that the 80 meter band would
occasionally crap out. Two bad
I put a scope on the output of the oscillator / mixer board and
watched. When the 40 meter band was starting to fail, the
of this board would start to gradually diminish to the point where the
received signal would weaken and then totally disappear.
Sometimes the display would remain on frequency, but most
it would just revert to 1.000 Mhz.
that I could restore operation by tapping on the crystal band and / or
just flexing the board. However, one problem was positively
traced to a loose grounding screw which is partially hidden by the
oscillator / mixer board itself. There was enough of the
'showing' so that I was able to tighten it with a small bladed
screwdriver, and this solved my problem at least for a while. While you're at, try
tightening up all similar grounding screws on the bottom of the chassis.
Here's a picture.
Poor quality? - yes, but it should point you in the right
After a week or so the problem reappeared. So, I turned the rig
off, let it cool and then removed the bottom cover. Upon power
up, 40 meters worked for a while and then began to fail. I found
that I could reproduce the problem by slightly tapping on the 11 Mhz
So, I removed the crystal from its
socket, cleaned the pins and then reinserted it. No good.
There was either a poorly soldered connection on the bottom of
the crystal socket, or the crystal was intermittent. Then I
noticed that the crystal worked perfectly well if it was only partly
inserted into its socket.
Next, using a
toothpich I carefully narrowed the openings on the crystal socket.
I also gently spread the pins of each crystal so that a bit more
force was required to insert them. So far- so good. No
matter how I tap on the heterodyne oscillator board or the crystals
themselves, I cannot repeat the failure.
Time will tell.....