Many Modifications to the Ten-Tec Omni

By Robert E. Helms

Being an incurable experimenter, I have made numerous modifications to my Ten-Tec Omni-D Series 6. These changes are applicable to moat models of the Omni and some of the Triton 540s and 544s. The improvements are "removable" in that no holes are drilled and irreversible changes are used. The modifications provide squelch, selectable receiver age decay times, 500-Hz crystal filtering for RTTY reception, accurate frequency spotting during cw operation,, full break-in cw with a companion kW aniplifiers automatic frequency control afc) to eliminate VFO drift, power supply over voltage protection and QRP operation with ale control. Each modification is independent and need not be done with another.

Squelch

In all Omnis, except the latest Omni-Cs, the components for the squelch circuit are on the audio board (#8044?) and the i-f/agc board (#80448). The later manuals do not show the schematics to these parts though they are installed. An operative squelch system requires only a control and switch. The zero-beat push switch may be unmounted, insulated and taped inside the radio to allow mounting the new squelch control. A linear 10-kij sub-miniature bushing mounted control with a long shaft and a spst rotary switch is recommended. See Fig. 1 for wiring connections.

Selectable Receiver Age Decay Time

I was disappointed with the age system response for two reasons. When a strong signal appears, the slow attack of the age causes a loud pop, particularly on cw with headsets. Or in a net with strong and weak signals, the slow decay of the age prevents hearing weak signals for several seconds after a strong signal has ceased transmitting. A change of component values and the addition of a switch will cure both problems. I used the Fast/Slow QSK switch for this' purpose since I always used fast QSK. I broke the connection between the switch and the Audio/Sidetone board (#80447) and rewired the QSK switch to the i-f»-agc board (#80448) to give fast and slow age choices. The circuit is changed as shown to de-creasen the attack time thereby eliminating the pops.' The capacitor values shown provide age decay times that: were half (on fast) and double (on slow) the time constant originally provided. Choose the capacitor values according to your taste. The "slow" capacitor is soldered to the board on the positive end and to the switch for a ground. Remove H24, a 680-k.ohm resistor, and use low leakage tantalum capacitors to lessen the load on the age amplifier and detector to reduce the attack time. See Fig. 2 for the circuit.

500-Hz Cw Crystal Filter for RTTX

Luckily, when the 5Q0-Hz crystal is in use on reverse sideband mode (SB-R). the carrier oscillator frequency is such thai the audio output passband will be about 1900- to 2400-Hz. Operation in this manner will provide excellent lsb RTTY operation on the 14- through 28-MHz bands — provided the cw audio filtering centered on 750 Hz is not used. _ Standard 2125-Hz Hark and 2295-Hz Space RTTY high" tones used to operate most common radioteletype modems (terminal units) can be received in this manner. Unfortunately, the Omni Series B front-panel selectivity ' control does not allow the use of the 500-Hz cw crystal filter without also utilizing the internal audio filter. The addition of a switch wired as shown in Fig. 3 will allow the 500-Hz crystal filter to be used in both the 1 ,8-kHz and the 0.5-kHz selectivity control positions. This allows the choice of either bypassing or using any portion of the audio filter in conjunction with the 500Hz crystal filter. The dc control wiring for the crystal filter selection is brought out of the rear panel through the center of the hollow rivets holding the phono jacks to the panel to connect to the switch. A small spst switch on the "L" bracket allows a choice of either the 1 .8-kHz or the 0.5-kHz crystal filter to operate in the 1 .8-kHz position on the front panel control. The^addea switch is shown in the normal or 'unmodified" position in the figure.

Accurate Cw Transmit Spotting

The front-panel zero beat switch is difficult to use except on very strong signals. Even then the true zero beat spotting is seldom achieved since hearing "zero beat" with a product detector equipped receiver is quite a feat! The following alignment procedure change will produce easy, accurate matching of your transmit, frequency to the frequency of the received signal. No parts or hardware change is required. A frequency counter is helpful but a crystal calibrator and an additional receiver will suffice. The following steps must be followed in sequence.

1) Determine the center audio frequency of the sharpest receiver filtering available. Carefully tune in a strong steady carrier 3uch as the spurious response on 28.980 MHz or the external crystal calibrator so the S-meter indication is peaked with all the built in audio and crystal filters in use. Measure the audio frequency across the speaker with a counter.

2) Set the 3idetone oscillator to the frequency determined in Step 1 . Adjust the sidetone pitch (»frequency) to the reading obtained in Step 1 . Lacking a counter, you may"simply match the pitch of the two signals by ear."

3) Set the carrier oscillator/BFO on the ssb Gen board (#80449) to provide a transmit frequency offset equal to the frequency in Step 1 . Measure the carrier or BFO oscillator frequency on the ssb Gen board across R17 (220 ohm) with a counter while in the SSB-N receive mode. Next, adjust the BFO frequency with trimmer capacitor C5 "on the board while in the LOCK mode. The desired frequency is the sum of the SSB-N BFO frequency and the frequency obtained in Step 1 . If the external receiver is used in lieu of the counter, proceed as follows to accomplish Step 3. Perform Steps 1 and 2 by tuning to the external crystal calibrator in both the Omni and the external receiver. Adjust the trimmer described above to place the transmitter exactly on the frequency of the calibrate signal by listening to the signal in the external receiver. The Omni is placed in the LOCK mode and the drive advanced just enough to obtain a signal. Repeat all steps to insure accuracy.

Ten-Tec Transceiver Modifications

After these adjustments are done properly, you should be on frequency with another Station. Simply tune in a signal on the receiver so that the 'pitch matches your aidetone pitch exactly. This procedure will also match your radio to any external audio filters designed" for a 750-Hz center frequency that you might desire to use.

Retaining QSK with an Amplifier

In order to retain the excellent full break-in cw capability of the Omni while using an amplifier, two things must be achieved. First, the receiver antenna signals must be obtained from the antenna side of tte amplifier. Second, a means of switching or "keying' the bias on the amplifier must be provided to prevent noise generated within the amplifier fro® interfering with reception.

The receive antenna switching will be described first.

The control signal for- the external antenna switching is obtained from the radio by the wiring change shown in Fig. 4. The interface wiring shown in this figure is connected to the radio through the use of the phono jacks on the rear of the unit for these purposes, I use the EXT T/R jack and the PTT jack along with the existing AUX 12-V dc jack for these connections. Once the amplifier is modified, the EXT T/R control is not required. The schematic for the antenna relay switch box and the system hookup diagram is Fig, 6. The receive antenna select switch on the rear panel of the Omni must be in the RECEIVE or separate position, A shorted plug is attached to the original antenna relay control on the amplifier since it stays energized any time the amplifier is on, Study the timing diagram, Fig. 5, before attempting to adjust and use the circuit.

The pre-delay identified in trace E of Fig. 5 allows time for the relay contacts to close before the transmitter is keyed. This is done by the relay contacts wired in series with the key line to the Omni. The post delay (weighting) shown in trace D holds the* transmitter keyed for an additional period of time equal to the length of pre-delay. * This prevents altering the weight of * the ow which will cause very short aits at high keying speeds, The relay delay in trace B allows time for the transmitter rf output to drop to zero before the relay switches the antenna back on the receiver.

In order to properly adjust the timing in the switch box, make all interconnections except the rf input and receive antenna. Temporarily disconnect the diodes across the receive antenna jack inside the box. The radio should key normally at slow speeds with the timing pots set "to mid-range. Gradually adjust the cw keying speed upward while sending continuous dits. Bote that the relay followa the keying at slow speeds but will stay energized at high speeds. Vary the setting of the 50-k relay delay trimmer pot and note that the relay switching speed will vary. Set the relay delay to the maximum delay position so the relay will stay energized while you verify the weighting or post delay is functioning, "Again, use a keyer to send continous high speed dits, this time observing the transmitter rf output pulses with a scope. Varying the setting of the 10-k weighting trimmer pot should cause the duty cycle or length of the dits to vary, To adjust the weight you should strive for "the same duty cycle as you have when the switchbox is bypassed. A bypass cable with phono plugs between the two jacks on the Omni going to the TR point on the control board and the mode switch "will allow operation without the external switchbox. To adjust the relay delay (50-kn trimmer pot) connect the scope across the receive antenna jack on the box and connect the rf input to she antenna coax with a T adapter. Fig. 7 shows typical waveforms obtained by 40-wpm" dits and various settings of the relay delay control.

Always adjust the weighting first and then the relay delay. If you get the controls misad-justed so the rf output From the transmitter is passed through the relay, it will burn the 15-ohm 1/8-W resistor in the* switchbox receive antenna line. The two protection diodes across the antenna output jack will protect the Omni receiver if this should happen.

Several excellent articles have been written on amplifier 'rf bias switching. ' Fig. 8 shows the simplified circuit used in my Henry 2k~3 amplifier. See the references at the end of this article for detailed data.

My QSK system using these circuits allows any computer to copy my cw keyboard at speeds greater than 60 wpm. I find it amazing that" an off-the-shelf Radio Shack relay will give QSK at 40 wpm plus.

VFO AFC

An automatic frequency control system may be applied to the Omni VFO to prevent warm-up drift. A sort of "poor man's synthesizer", the "circuit digitally counts the VFO frequency using the digital display clock signal as a reference and "locks" the VfQ output frequency to a 5-Hz wide segment. Refer to Fig. 9 along with the following theory of operation.

Q1 , U1 and U2 form a frequency counter that counts the VFO output signal. The counter timebase is the digital display time base. The afc counter counts to the nearest 5 Hz -- so. the output from (J2A pin 1 is either 5-V dc (high logic) or 0—V dc (low logic) depending on whether the VFO output frequency is an odd or an even multiple of 5 Hz.

The CA3140T, U3 and the 2 uF capacitor act as an integrator or very low pass filter. When the unit is first turned on, you should disable the afc which centers the output of the afc system in its lock range. Once that is done, the disable switch is opened (system enabled; and the counter output will increase or decrease the charge on the integrator capacitor. That increase or decrease will cause the VFO frequency to vary until it reaches the next 5-Hz step. The counter output will change and the change in charge will reverse. In other words — the VFO frequency is slowly moved up and down in one 5-Hz wide band and not allowed to drift outside of it. A 5-Hz change is just audible if you listen to a low pitch beat note from a carrier.

The afc circuit is built in a shielded box mounted to the brace for digital display in the Omni D. All connections are made with RG-174/U miniature coax. Refer to Fig. 9 for connections made to the nins on the VFO assembly. The 5-Hz clock signal tor the afc unit is obtained from the junction of IC1 pin 4 and IC2 pin 12 on the digi-t,al display timeoase circuit board. Obtain 12-V dc power "for the unit from the terminal on the digital display assembly also. The disable switch may be a spst normally open push button switch. The zero-beat switch can be used if you'd like the afc to function most of the time. The 2-uF integrator capacitor must be a low-leakage type such as a tantalum.

The disable circuit is adjusted by closing the switch and adjusting the trimmer pot. This is done until the frequency of the receiver does not change when the connection is made and broken between the afc output and the VFO offset circuit.

Power Supply Over-Voltage Protection

A weak point in the Ten-Tec transmitter duty cycle and rf output is the thermal design of the Model 252M0 power supply. Full duty cycle RTTY transmit above 10 amps current drawn from the supply is risky even with a fan on the regulator transistor heat sink. The results of over-axing the supply is usually a shorted pass transistor and 22- to 24-V dc applied to your transceiver. The units I have repaired with that condition had blown LM-380 audio output ICs and damaged IC1s on the timebase board in the digital display. The over-current protection applied to the SCR in the original design will net" "protect the radio from over-voltage due t.0 a failure of Q3, Q4 or Q1 . The "crowbar circuit shown in Fig. 1 1 will protect the radio by shorting the supply output to ground thus blowing the added 20-amp fast-blow fuse, This will happen regardless of the reason that the regulated output exceeded the threshold of about 15-6-V dc.

Slight additional cooling for the pass transistor can be obtained by applying silicone heat sink grease to the junction of the heat sink and the rear panel of the power supply chassis. Improved replacement pase" transistors for the model 252KC supply are the 2X5685 (E & B pina are reversed physically) and the ECG387. A ECG181 will about equal the performance of the original 2N6258 type.

X also made considerable wiring changes and added a few components to achieve external voltage regulator sensing from the Vcc terminal on the rf power amplifier assembly. The improvement in regulation was marginal hence the circuit is not included here. It is available upon request.

Alc-Controlled QRP

The range of ale control on my unit was 30-to 95-W rms rf output. A desire to operate battery powered cw/ssd in the 10-W dc input class for Field Day contesting led me to develop this circuit. The ale control line ia routed to an external ale detector attached to the antenna line by a coaxial 'T' adapter. Fig. 12 shows the circuit including the connection to the ale line at the low-level driver circuit board (#80444). The external detector rf and ale connections must be removed to restore full power operation. With the circuit connected, the resistor values shown provide an ale control range of about 750-mW to 12-W rms rf output depending on where the front-panel ale control is set. Mount the components in a small box with the necessary rf connectors.

General Notes

1) Replace the bayonet based meter lamps with higher-voltage types such as #1815 14-V (Radio Shack #272-1118). Replace Omni-A, remote VFO iModel 243) and Omni-D VFO dial lamps with. 12-V frain of wheat1 bulbs with wire leads vRadio ack #272-1141). The 252MQ power supply output lamps is this type also.

2) To obtain maximum transmit rf output on all bands, set the ale set trimmer R2 on the low-level driver board (#80444) as per the manual. Adjust it for 18 amps current from the power supply with the ale knob fully clockwise on the band that draws the least current. To prevent tripping the power supply over-current protection on the other frequencies while operating, use the ale to limit the current to 18 amps with full drive in the lock mode. Do not advance the ale beyond the point that is determined to limit the current to "that value.

3) Onmi-A owners with aerial numbers below 5450500 and Omni-D owners with serial numbers below 546-1500 who have not had their units updated to Series B should insure a Ten-Tec service note SN-1-545/546 is incorporated in the radio. This change removes a 0.01 uF bypass capacitor from the ale line and changes that wire to a piece of KG-174/11 coax between the low-level driver board (#80444) and the SWR-TR board (#80450). This improves the ale attack time and reduces susceptibility to rf.

4) The Omni-C noise blanker and 250-Hz cw crystal filter will work if plugged into older units." The blanker is considerably improved.

5) The drive level is very critical to adjust on ssb due to excessive mike audip amplification. Change R18 on the ssb Gen board (#80449i to 22k and add a 47-k resistor in series with C12 on the board to reduce the gain. Vary the value of the series resistor to match your mike.

6) Rf feedback may cause fming and distortion on ssb transmit when using an antenna tuner on the higher bands. Bypass capacitors will eliminate the problem when used with a heavy ground braid between each item of equipment used with the radio. Bypass the mike audio at the mike jack with 470 pF. Bypass the side of the ac power switch next to the 13-V input with at least a 0.033-uF 600-V capacitor to ground. If the remote VFO is used, bypass the T,line (transmit voltage) on the control board (#80504) in the VFO with a 0.02-uF capacitor. Place a 0.005-uF capacitor across Zener diode D7 and bypass the cathode of the diode to ground with a 0.001-uF capacitor in the Model 252M0 power supply. The use of shielded cable for virtually all connections to the radio will improve RFI immunity.

Summary

This article was written to publicize circuits for adding other operating features to Ten-Tec transceivers, It is recommended that the circuits are built and tested one at a time using the owners manual in conjunction with this article. The sources for basic circuits and ideas are original or can be found in the references at the end of the text. It is through the excellent design of those authors that my Omni has been considerably improved.

References

[1] Frev, "How to Modify Linear Amplifiers for Full Break-in," April 1978, Ham Radio Magazine.

[2] Lawson, "Break-in with a Keyed High Speed Vacuum Relay," February 1973, QST.

[3] "The ETC Alpha 77 Linear Power Amplifier," March 1973. QST.

i 4] Hertzberg, "Improved Break-in with the Collins 75S-3B," February 1974-, QST.

[5] Bryant, "Electronic Bias Switching for RF Power Amps," Biyant, May 1974, QST.

[6] Pluesa, "Fast QSK System Using Reed Relays," December 1976, QST.

[7] Hertzberg, "CW & SSB Break-in with a Vacuum Relay," September 1976, CQ.

[8] Joyce, "Accu-Control ~ QSK for Kenwood TS-820 & R-820," February 1981. QST.

[9j Rolek, "RF Bias Switching for the SB-220," July 1971, QST.

[10] "VFO AFC," June 1971, Ham Radio.

[11] J Various ARRIj Publications including "Solid StatenDesign and the "Radio Amateur s Handbook. "

AMSAT OSCAR Phase IIIB Launch

June 16, 1983 is now the scheduled launch date for the Phase IIIB satellite, according to the latest word from AMSAT president, Tom Clark, W3IWI.

The date has slipped a number of times, so listen to W1AW and the various AMSAT nets for the latest news.

Keep your fingers crossed!

Data Communications Column

This issuti should have included Dave Borden's column on Data Communications. In fact, he wrote it on his computer/word processor and transmitted it to a data communications network. Murphy s Law strikes again! When we signed on the network and read the incoming mail, for some unknown reason our computer did not save to disk. When we signed back on and seemed to have solved that problem, the network then ran out of ports and subsequently said that the host was not available.

Ten Tec Omni Vii

Continue reading here: Lowpass Speech Filter Using Surplus Inductors

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