How To Fix A Great Ten Tec Omni VI+ Transceiver

The Great Ten Tec Omni VI+ Transceiver

In late 2020, I bought a Ten Tec Omn VI+ transceiver (model 564). It was a big step for me since most Ten Tec equipment I had purchased before was older and pretty inexpensive. See my post on the Omni C as an example. The Omni VI+  represented a step up in performance. Released in 1997 with an MRSP of $2850, It was Ten Tec’s flagship at the time. Although there were some problems with build quality and confusion regarding the differences between the VI, various “option” upgrades and the VI+, a working model with all the filters installed is still an impressive transceiver.

Finding a bargain on eBay

Mine arrived carefully packed and had all four 6 MHz IF filters installed. That left two 9 MHz IF filters slots open for future expansion. Although in generally good shape, the smoked plexiglass window that covers the meter and frequency display was cracked. A quick test showed reasonable receive sensitivity and power out on all bands. Most the knobs worked and seemed to function as intended.

A few problems . . .

There were some problems however. The multifunction meter seemed sticky,  and didn’t indicate reverse power (SWR) on the lower bands. Also, the narrow 250 Hz filter was intermittent, but could sometimes be remedied with a tap on the case. Finally, after installing the radio in my shack, I noticed that it would not connect with MacLogger DX via a CAT connection.

A Broken Meter?

I started with the sticky meter and SWR problem because they seemed related. The top and bottom outer cases slide off after removing four black screws from each side. Removing a small screw near the top on each side and behind the front panel allows it to swing down and out of the way.

Now, the meter is easily accessible. I unsoldered the wires and took it out. There wasn’t any visible damage to the meter — just a little interference between the needle and the scale. A small tweak, a gentle use of some canned air and an adjustment to the zero position screw, and it was as good as new.

First, we have to decipher the manual

Having the meter wires loose made it easier to trace them through the wiring loom. This was necessary because unlike earlier Ten Tec models, the owners manual does not include an overall schematic or block diagram of the radio. I could not find the meter in any of the diagrams. Descriptions and schematics for each of the circuit boards are provided, but the KEYER, PROCESSOR, and METER controls are not shown on any schematic either.

Tracing the wires from the meter to the switch, I found that the FWD and SWR positions are connected to the “9 MHz Filter Board” (81782). There, an op amp (U1a, U1B) and a few transistors (Q10 – Q13) form a DC amplifier circuit. Together, they are a big part of the metering and ALC functions in the transceiver. There are a series of adjustments associated for ALC, Ic, Ic Set, FWD, and SWR. The text in the manual describes how to set these trimmers, which also adjust final amplifier current protection.

The dreaded “maybe” diode

I went through the adjustments a couple of times with the radio set to 20 meters, and it seemed to be working. Nevertheless, there was still little or no SWR indication on 40, 80 and 160 meters. My gut said the two diodes that develop the DC voltage for FWD and SWR on the “Low Pass Filter Board” (81592) were probably faulty, but I couldn’t see how a failing diode would work on some bands and not others. No such thing as a “maybe” diode, right?

I need some SMT skills

Lacking a better idea, that pointed to the IC in the circuit — U1. At this point, I didn’t have the replacement IC, and very little experience with Surface Mount Technology (SMT). I decided it was good enough to use, and besides, there might be other problems. I like to use a newly-acquired radio before extensive restoration work.  I cleaned the pins on all the filters (including the 250 Hz one that was intermittent), buttoned up the radio and starting using it in the shack. In the meanwhile, I would order some SMT op amps, gain some SMT skills and enjoy the radio.

Some time passed . . .

I must not been in much of a hurry, because a couple of years passed. I now had the replacement op amp and had worked on my SMT skills. It was finally time to resolve the Omni VI+ problems. After refreshing my memory on what had already been done, I decided to proceed with the op amp replacement on the 9 MHz filter board. It went well enough, and I was relieved when the board seemed to work ok after the SMT soldering. It didn’t fix the problem though. I then remembered my earlier gut feeling about the Power/SWR sense diodes. The replacement part was a pair of Schottky diodes (BAT41) and they were easily found on Amazon.

Disassembling the radio

Getting the Low Pass Filter Board out of the radio is a challenge. It’s mounted inside a shield at the back of the radio, in front of the final amp. Several wires have to be disconnected including some that are soldered. There are also chassis pieces that must be removed. After that, the BAT41 Schottky diodes are easy to find and replace (see the picture above). Then it’s just a matter of reassembly.

Replacing the SWR Bridge diodes

Once the diodes were replaced and the radio reassembled,  the alignment procedure for the ALC, Ic, Ic Set, FWD, and SWR trimmers was pretty simple. As mentioned earlier, the procedure can be found in the Operator’s Manual for the Omni VI+  on the Low Pass Filter board pages. The adjustments require a dummy load, wattmeter and DC ammeter (20-30 amps full-scale) inline with power to the transceiver. These adjustments are easy and take just a few minutes. The verdict? I should have gone with my gut in the first place. Once aligned, the metering worked as expected.

One other tip: it’s helpful to have a 150 Ω, 100-watt non-inductive load for adjusting the SWR meter. It will provide a 3:1 resistive load needed for this adjustment. I built one using three 50 Ω, 100-watt resistors mounted into an aluminum case to dissipate heat. Both parts can be found on Amazon.

The Millennium DX Dream Station

Shortly after purchasing the Omni VI+, I purchased a Ten Tec Hercules II (model 420) amplifier and matching tuner (model 238). I now had the complete Ten Tec DX dream station of the millennium. I was even able to find the Automatic Antenna Coupler (model 253) a later on. I have had a lot of fun with it — one of the best setups I’ve ever used. The streamlined interface is not cluttered with the  nested menus that plagued so many of the rice boxes of that era. It does have “buttery” smooth digital tuning, great “ears,” great sounding transmit audio  and understated, elegant looks.

The reviews on eHam.com tell the story. Compared to the cost of current transceivers with similar specifications, the Omni VI+ is a bargain.  It is often found on auction sites for under $500. It is CAT controllable and supported by software like MacLoggerDX and WSJT-X. When combined with a Ten Tec amp and automated tuner as mentioned earlier, band switching is completely automatic, making it possible to switch between two frequencies on different bands without manually adjusting anything. Not surprisingly, it is an excellent CW rig . Excellent QSK.

So what about the CAT control problem? Suffice it to say it was a little complicated. While not one of the earliest radios to have CAT control, the protocol was not as refined in 1997 as it is today. Documentation exists, but it is scattered between the manual and various posts I found on the ‘net. I’ll write a separate post describing the integration process and building the needed cables to go between the Omni VI+, the model 253 Automatic Antenna Coupler, the Hercules II amp and either a Windows or Mac computer. Stay tuned . . .