Restoring A Ten Tec Omni Series Transceiver – Part 2

What have we done on the Ten Tec Omni Series C transceiver so far?

In part one of this restoration, we tackled the two most glaring problems: garbled signals on receive and blowing fuses when the radio was switched to the 30-Meter band. I could now receive and transmit signals in almost all the band switch positions, albeit with varying sensitivity and power output. It had been quite a chase so far, and the remaining repairs would prove equally challenging.

A Little Background on Ten Tec Solid State Transmitters . . .

First, a word about “tuning up” a Ten Tec transceiver. In most early 70’s transmitters,  one tuned the grid and dipped the plate of a vacuum tube-based output stage. By contrast, the Ten Tec 100-watt transceivers, starting with the Triton II, were fully solid state. Moreover, they were broadband — meaning the output amp could cover an entire amateur band without adjustment. This is commonplace today, but in 1973, it was revolutionary.

“Tuning” amounted to adjusting the drive to the final amplifier for maximum output without distortion or damage to the final transistors. For protection, an “Automatic Level Control” (ALC) circuit provided negative feedback to reduce the transmitter’s output and provide a degree of signal processing. The ALC also lit a visual indicator on the front panel when the transmitter reached the 100-watt maximum output.

This makes the transmit chain — from driver through finals and ALC circuitry — one big closed loop. Although very reliable, it can be little tricky to troubleshoot. On the other hand, Ten Tec has used this transmitter scheme with only minor modifications for most of their radios — beginning with the Triton series. If you understand one of them, you can work on any of them.

Troubleshooting the SWR Meter and ALC Light

In the initial assessment, I noticed that the Signal Strength/SWR meter didn’t move during transmit. This is normal with a matched load, but I also tried the radio on a random-wire antenna and should have seen some indication of the mismatch.

Also, the ALC indicator didn’t light at all, and the radio would trip the over-current protection on a Ten Tec Model 262G power supply as I advanced the “Drive” control on the front panel. This is an excellent reason to use a Ten Tec power supply for testing a radio of unknown condition. Without the overcurrent protection it provides, it’s easy to destroy the output transistors — difficult to find and expensive to replace.

The Transmit Chain

In order the troubleshoot the ALC problem, we need to understand  the transmit chain. A 9-MHz SSB signal (level set by the front panel “drive” control) feeds the “TX-Mixer” (module number 80713) through a crystal filter. There, it is mixed with the VFO to set the final transmitter output frequency. “Bandpass  Filters” (80459) are then selectively switched into the TX-Mixer output.

Next, the “Low Level Driver” (80743) applies ALC and amplifies the transmit signal enough to drive the “Final Amplifier” (80491). A “Low Pass Filter” (80966) reduces harmonics. Finally, the 100-watt transmit signal is routed through the “SWR-TR” (80450) board where the SWR and ALC signals are developed. The SWR-TR board also provides antenna switching between the receiver and transmitter.

A Working Theory

Since the transmitter now had output with modulation, I reasoned that the problem was after the TX-Mixer board and Bandpass Filters. Likewise, the Low Level Driver and Final Amplifier must be working to some degree. So either the ALC and SWR signals were not being developed on the SWR-TR board, or they were not being interpreted by the Low Level Driver or SWR meter.

Testing the SWR-TR board

The SWR-TR board seemed like the natural place to start. It’s a little tricky to get to, being in a metal enclosure (shown at right). The Low Pass Filter module is also located there, so the box is crowded. It is possible to remove the SWR-TR board without  disturbing the Low Pass Filter though.

Once out, I could see that it is a pretty simple affair. Referring to the schematics found in the Ten Tec Manual for the 546C, transformer “T1” takes a sample of rf as it passes through on the way to the antenna jack. The secondary of T1 is center-tapped, with one side going to the SWR circuit and the other used by the ALC. Both use a 1N914 or 1N4148  diode and filter cap to produce a DC control voltage.

I de-soldered and tested both diodes, and they were both blown. A trip to the junk box, and I had replacements installed in no time. The board went back into the radio, and I did a quick transmit test. The SWR meter now worked, but the ALC was still behaving strangely. Well, one out of two ain’t bad for a quick fix.

Let’s look at the Low Level Driver Module

The next candidate for the ALC problem was the Low Level Driver module. Reviewing the description in the 546C Manual, I learned that the dc control voltage from the SWR-TR board is buffered by Q8 and then applied to the ALC driver stages of Q6 and Q7.

The output of the ALC driver is then applied to the Low Level RF Amplifier. The ALC indicator is connected in the collector of Q8. R21, the ALC minimum threshold adjustment, and the front panel ALC control setting determine the overall gain of the amplifier. Good candidates for the problem we were seeing.

Ten Tec points the way . . .

One thing that Ten Tec excelled at was writing manuals. In most circuit descriptions, tables were provided for Transmit and Receive voltages for both the module pinouts as well as the semiconductors on the board. By carefully measuring Q6, Q7, and Q8’s collector, base and emitter voltages during transmit and receive, I could see there were differences from the manual’s example readings. The problem seemed to involve Q7 and Q8. I removed and tested both transistors, but they were fine.

And the winner is . . . a Zener diode?

There was also a zener diode in the base of Q7. It tested OK as a diode, but I wondered if it had changed value somehow. So I unsoldered and tested it out of circuit it with a 9-volt battery and dropping resistor. To my surprise, it was off — measuring less than 3 volts rather than the specified 3.9 volts. I a zener assortment purchased for another project a few months before and I just happened to have the correct value. Back in circuit, a quick transmit test showed the ALC behaving more normally. Now that’s an obscure problem.

Time for Alignment

We were down to the last few problems: uneven transmit power on the 17 and 160 Meter bands, SWR readings out of calibration and the “9.000” display on some 10 Meter bands. So far, I had avoided touching too many of the adjustments in the radio because they shouldn’t have changed and well, there is always the concern of making things worse. However, as I mentioned in Part 1, it was clear that an earlier tech had made some misguided adjustments in the past. Time to suck it up and do a more thorough alignment.

No output on 17 Meters

With the radio switched to 17 meters, I followed the signal through the transmit chain mentioned earlier in this post. The signal seemed to stop in the Bandpass Module (80459). The manual mentions that “these filters are fairly broad and require no field alignment.” Nevertheless, remembering the other “no alignment necessary” adjustments I had needed, I decided to try it anyway. The power came right back up to spec. Our hapless tech had indeed touched these adjustments.

Low output on 160 Meters

The Bandpass Filter didn’t seem to help this problem, and after a little looking around with an oscilloscope, it looked like the VFO output was low on the VFO Amplifier module (80454). Since there were no adjustments on this board, I decided that problem must be on the Oscillator-Mixer module (80711) — just before the VFO amp. I ended up doing a complete alignment of this board since the adjustments interact with each other. It didn’t take long, and did fix the 160 meter power output problem.

SWR Meter out of calibration

This one was easy. I simply adjusted the “SWR Adj” (R3) on the SWR-TR board. A 150-ohm load is required for this adjustment to simulate a 3:1 SWR. I also nulled capacitor C1 on 10 Meters as described in the manual.

“9.000” MHz shows on the display for the 28.5 and 29.0 MHz bands

This came down to defective crystals on the Oscillator-Mixer module (80711) found when doing the alignment. They are still dead in the radio since I don’t have replacements yet. They are soldered in, and require a complicated disassembly to replace. We’ll get back to this in the future.

The Omni “C” now (nearly) meets spec

This was the most complex repair I have yet made on a single radio and presented quite a few head-scratchers. It was well worth it though. With the exception of the two failed 10 Meter crystals, it is back to original specification.

The Omni “C”  has to be one of the best radios Ten Tec ever sold, with great sensitivity, decent selectivity, excellent SSB clarity, and several other features that are still better than many modern radios costing much more than it’s typical $300-$500 used price. It also has Ten Tec’s legendary full QSK CW operation on the 100-watt transmitter. Still one of the best CW rigs. If you can get your hands on one and bring it back to the original spec, you won’t be disappointed.