Troubleshooting the Ten Tec Omni VI Plus
The Symptoms
We recently traveled to Omaha to visit a high-school friend and fellow ham. He had purchased a Ten Tec Omni VI Plus on my recommendation and was eager to get it running on FT8 and other digital modes. To that end, he had also purchased the microHAM DXP USB interface and needed a little help getting it configured. I’ll say more about that in an upcoming post.
During that setup, we noticed a few problems with the Omni, including reduced power output, tripping over-current protection when transmitting, and reduced receiver sensitivity. We also noticed some frequency instability that was visible in the “waterfall” display of WSJT-x in FT-8 mode. These problems were a recent development, and I agreed to help him troubleshoot the transceiver.
Radio Overview
You might wonder if a 20+ year old radio is competitive in today’s market, and worth the trouble. The Ten Tec Omni 6 Plus is a triple-conversion Solid-State transceiver that covers all the HF Amateur bands from 160 to 10 Meters with the exception of 60 Meters. It was first released in 1997 and continued in Ten Tec’s line until 2003.
Frequency is controlled is by precision variable and fixed frequency oscillators, phase-locked to a 20-mHz Temperature-Compensated Crystal Oscillator (TCXO). This provides a very stable, low noise Local Oscillator (L.O.) and accurate tuning. Frequency adjustment is via a rotary encoder with a large knob, and displayed on large LED displays. It has buttery-smooth tuning.
Selectivity is varied by 2 switchable crystal filters in the 9 mHz IF and four crystal filters in the 6.3 mHz IF. Bandwidths of 250, 500, 1800 and 2400 Hz are available.
Dual VFOs with split mode, a built-in CW keyer and all-mode operation (USB, LSB, CW, FSK, AFSK, FM) are included.
Digital enhancements include audio DSP, 100 duplex memories and Computer Aided Transceiver (CAT) control. This allows the Omni 6 Plus to work with digital modes.
RF output is from 0 to 100 watts, ALC stabilized. 100% duty cycle for up to 20 minutes. Continuous duty with optional air cooling on the rear heat sink. It has a sensitivity of .16 uV for 10 dB S+N/N @ 2.4 kHz bandwidth. Sherwood Labs have measured a dynamic range of at least 80 dB.
With Omni 6 Plus radios available for as little as $400-600 on auction sites, I would argue they are not only competitive, but a great value. And in spite of their added complexity when compared to earlier Ten Tec products, still possible for the careful amateur to service.
Transceiver Layout
The Omni VI Plus manual includes pictures of the top and bottom of the radio with the covers off, but the boards are not labeled. The following pics provide that info:
Repair Methodology
In each of the following sections, I describe my process for diagnosing and repair of the Omni VI Plus. Sections are headed with the module name and Ten Tec part number.
A subsequent post will cover the alignment of the radio, necessary after all the changes made during the repair.
Power Amplifier (81611)
Once back home, my first thought was damage to the final amplifier like a failed output transistor. The amp is easy to remove, and ohmmeter checks of the drivers and finals didn’t reveal any shorts or opens. There was no obvious damage either. I even unsoldered legs from the drivers and confirmed they were still functional. No problems were found.
Low Level Driver / N.B. Board (81608)
The next step was to confirm power out from the low-level driver board. With the final amp disconnected from the driver, I confirmed there was output from the driver stage. No problems were found here either.
9 MHz Crystal Filter Board (81782)
In addition to the 9 mHz crystal filters, this board contains Power level, SWR, and Current meter adjustments as well as Final Amp Current trip adjustment. Thinking the Current Trip level (R54) might be set too low, I tried adjusting it. Same problem as before. The radio kept going into overload protection. When this happens, it shuts the whole radio down. The radio is powered off and back on to reset it.
DC Power Input Board (81591)
Next stop, the current sense shunt resistor, R5. This feeds the over-current protection circuitry on the 9 mHz Crystal Filter Board. Maybe it had changed value or even opened up? Nope, everything checked fine here.
Low Pass Filter Board (81592)
The Low Pass Filter board was next. It has been my experience that there are often problems with Ten Tec radios around this system. Situated between the final amp and the radio output connector, perhaps the combination of 100 watts of generated power and static discharge from antenna systems make it more vulnerable to damage.
Ten Tec pioneered the use of diode switching in the mid-70’s and have never looked back. There are dozens if not hundreds of diodes doing the grunt work of switching signals and systems in and out of use.
This board connects directly with the antenna, and diodes are used to switch the antenna between the transmitter output (through the low pass filter) and receiver inputs. The Omni VI also has a secondary receiver antenna input.
Diodes can easily be tested with a digital Volt-Ohm Meter (DVM) that has a “diode” mode. In this mode, good diodes will read between 300 and 800 ohms when forward biased and open when reverse biased.
Transistors can often be checked the same way. After all, they can be thought of as two diodes connected back to back. In this case, NPN transistors will show diode junctions with positive test voltage applied to the base and negative to the emitter and collector. PNP transistors are simply reversed.
A careful check of all the semiconductors on the LPF board showed two failed parts. Q1 (2N6519) and D8 (1N4007) in the secondary receiver input antenna circuit.
It also occurred to me that the Gas Discharge Protection Device (GDP) across the antenna connector could have failed. I removed it from the SO-239 output connector for the time being.
BPF/Front End Board (81593)
Turning to the receive function, I noticed that there was reduced sensitivity on some bands. Time to check the receiver’s front end.
Receiver antenna switches connect next to the “Band Pass Filter/Front End” board. Since damage was found on the LPF board, it seemed wise to check this system.
There are quite a few diodes here as well (D7 through D30), used to switch individual band-pass filters in and out of circuit. Since those filters are in both the transmit and receive paths, other diodes also switch between the transmit and receive modes.
Several diodes were found to be bad. The two diodes on either end of the 17 mHz filter (D21, D22), and those used to switch between transmit and receive (D7, D10).
After replacement, we now had receive on all bands, with normal sensitivity. SSB was garbled though. Sounded like we weren’t hitting the center of the crystal filter passband. Pass Band tuning didn’t seem to have enough range.
Xtal Osc. – L.O. Mixer Board (81595)
Being a triple-conversion receiver, the mixing scheme for the Omni VI Plus is a bit complicated. To convert the desired receive (or transmit) frequency (e.g. 14 mHz), the band specific Crystal Oscillator (18.0030 mHz), plus the Variable Frequency Oscillator (VFO) frequency (5 mHz) produces the first Intermediate Frequency (IF) of 9 mHz.
Here’s the math: (18.003 mHz + 5.000 mHz) -14 mHz = 9.003 mHz
Together, these modules also have circuitry to compare the resulting transceiver frequency with the 10 mHz master clock and adjust the L.O. in a phase locked loop (PLL). The corrected Xtal/L.O. signal is sent to the BPF/Front End board to produce the 9 mHz IF.
Both the Xtal Osc. – L.O. Mixer and VFO are located in a metal enclosure beneath the FM board and noise blanker/Driver board. I didn’t expect problems on these boards and didn’t find any.
Logic Board (81606)
At this point, I was thinking it was time for an initial alignment. Perhaps the garbled audio would be corrected in the process. Wishful thinking as we will see later.
First step: adjust the Master Clock on the Logic board for exactly 10 mHz at the test point. A high-accuracy frequency counter is required for this step, or zero beating the TP output with WWV while listening on a receiver might work. I’ll have another post describing how I modified an old frequency counter to have a GPS-disciplined timebase.
IF/AF Board (81602)
Next, I tweaked L3 and L5 on the IF/AF board for peak signal. I also attempted to adjust the S-Meter calibration.
An Elecraft XG3 RF Signal source and a switchable RF attenuator were used. The XG3 provides an accurate 50 uV signal that corresponds to S9 on the S-meter while the RF attenuator is used to create an S3 (32 uV) signal.
The process is described in the manual, but I couldn’t match the specification. On closer inspection, I discovered R25 and R29, the trimpots used to adjust the S-meter, were reversed.
Changing them around made it possible to get much closer to an accurate S-Meter.
Pass Band Tuning Board (81781)
Thinking the garbled audio might be in the Pass Band Tuning, As described in the Omni VI manual:
To set the tuning of the voltage controlled crystal oscillator, hook a frequency counter to the test point adjacent to the coax jumper cable. With the front panel PBT knob fully clockwise, adjust C24 for 15.3015 mHz. Now with the PBT knob centered, adjust R18 for a reading of 15.300 mHz.
There is no specific test point, so I connected the counter to the center conductor of the coax that connects two points on the board near L17. I was able to meet the specification, but it didn’t correct the garbled audio. No joy.
TX Audio Board (81597)
The Beat Frequency Oscillator (BFO) is developed on this board. There are five trimmer capacitors for setting the BFO in LSB, USB, CW (transmit), FSK (Mark) and FSK (Space) modes. All interact with each other, and challenging to set. The manual states:
The BFO oscillator/amplifier is formed by transistor Q6-Q8, and its frequency of operation is determined by ctystals Y1-Y2, diode switches D8-D12, and capacitors C10-C13. . . . .The trimmers can be set with a higgh resolution counter at connector 46 by switching modes in the listed below and adjusting the corresponding trimmer for the frequency indicated:
CW transmit or TUNE, adjust C10 for 9.000400 mHz
LSB receive and so on , . .
At this point, I realized that the Omni VI Plus manual must be wrong. Fortunately, the Omni V and Omni VI (models 562 and 563) are very similar. Looking at their manuals, I see the directions don’t match.
I ended up using the directions for the Omni VI (model 563) manual and got pretty close. They are very fussy adjustments though. In general, one sets the BFO frequency for LSB first, then USB, CW (while the transmitter is keyed) and lastly FSK Mark and Space. You just turn the power output down to zero when keying the transmitter. I did end up replacing a diode and one of the trimmers, but that didn’t make it any easier. You just have to be patient with a steady hand.
During the BFO alignment, I realized I couldn’t trust the accuracy of my frequency counter. This led down the rabbit hole of buying a GPS Disciplined Oscillator and modifying my old frequency counter to use it as a time base. It took some extra time, but was well worth it. As described in the coming blog post, this mod makes my old counter accurate down to the cycle — crucial for this type of alignment.
Time for more Study
At this point, I was at a loss for the garbled receive audio. I checked the transmitter and discovered (thankfully) that it was now putting out full power. However, the transmit audio was garbled in about the same way as the receiver. As we shall see, that’s an important clue.
Having looked at manuals for the Omni V (model 562) and Omni VI (model 563), I noticed that the documentation is more generous for both of them. The Omni VI Plus (model 564) manual, has no “Block” or “Wiring” Diagram. I found a Block diagram in the Omni V manual, and the Wiring Diagram in the Omni VI manual.
Using a print of the block diagram, I decided to map transmit, receive and frequency change paths onto the Omni V block diagram using yellow, green and pink highlighters. What became instantly clear is that the transmit and receive paths have only one thing in common: the SSB filter on the 9 mHz Filter board. Channeling my inner Sherlock Holmes:
“whenever you have eliminated everything else, whatever remains, no matter how unlikely, must be the problem”
Looking carefully at the 9 mHz Filter board, I noticed that the two filters were not in the correct positions — they were reversed. How could that have happened? Then I remembered; I had the board out to inspect the surface-mount circuitry on the reverse side. I must have gotten them backwards when I reinstalled the board.
I put them back into the correct positions and Voila – good, clear receive audio. A little embarrassed for having made such a rookie mistake, I took some solace in correctly reasoning where the problem was.
One more problem . . . .
Having done a partial alignment I decided to try the radio out on FT-8. Sadly, the frequency instability seemed about the same.
WSJT-x Waterfall showing frequency instability
Back on the bench, I noticed that the 10-mHz test point (pin 9 of IC XX) on the Logic Board seemed to be varying quite a bit. Bad crystal? Oven? It was then I noticed the cover on the TCXO was loose. A gentle tug and it came off completely. These are supposed to be sealed units. Without a tight seal, the heater element probably can’t keep up.
The TCXO was added to the Omni VI Plus towards the end of the production run around 2002 or 2003. Opinions at the time varied on its effectiveness, but I think it’s essential for consistent performance on FT-8. Ten Tec appears to have used at least two manufacturers for this part, OvenAir and IsoTherm. They’re mechanically similar, and I suspect more or less equivalent in use. No matter, short of finding a scrap logic board with one to salvage, pure unobtainium today.
Closer inspection of the oven cover revealed it had been glued onto the base. Maybe I could re-glue it? A little super glue and the deed was done. The 20 mHz crystal clock still operated, and it still got warm, so maybe I had dodged a bullet.
After running the radio for several hours, the timebase seemed to settle down some and I was finally able to set it to 10 mHz with the trimmer. Like the BFO trimmer capacitors, it’s jicky, but patience and a steady hand will out.
Alignment is next
This concludes the physical repairs of the Omni VI Plus. Having changed several parts and moved several adjustments, a full alignment is now necessary. I’ll tackle that in the next post.
One Reply to “Troubleshooting the Ten Tec Omni VI Plus”