Ten-Tec Omni-C Noise Blanker Modification

A major improvement to an excellent radio, just 44 years late!

For the last couple of months I have acquired a new (to me) Omni-C. After going over cleaning of contacts, cleaning - lubricating the PTO and aligning the BFO, have been impressed by its excellent performance, it is very quiet and crystal clear RX. and TX.

It is well built, and of a simple 1 IF analog design, easy to understand and service, qualities almost extinct in present day radios.

All was well and rosy, until the contacts of my local AC grid step down transformer started acting out due to the prolonged dry weather conditions and their suboptimal service.

The result has been severe spark type noise on all bands, 24 – 7.

The NB of the Omni could do a credible job eliminating 90% of the noise in the bands above 30m, but has been completely ineffective below 40m. Its function was well below that of other radios at this QTH, far exceeded by the NB of my FT-901DM.

I had read about this particular issue especially with the later design type of the NB in the Omni models, and based on comments on the net, was looking for the alternative design which has the 3 crystals in place of the 1^st IF coil, which is supposedly better, but as easy to find as hen's teeth these days!

Looking at the NB schematic and operation comments from page 60 of the TT-546 owner's manual.

The 9MHz IF signal. Is split into 2 components, one going through a band pass filter and a Noise gate, whose permeability is controlled via an emitter follower switch transistor (Q2).

The second part of the IF signal is used for the detection and controlled amplification of the noise pulses via IC1 and Q1, which is finally used in controlling the switch transistor (Q2).

I supposed that the lack of effectiveness of the NB was due to insufficient IF level gain in the lower bands compared to above 20m. The Omni has a unique feature of tailoring its sensitivity across the HF spectrum, to avoid amplifying the higher ambient noise of the lower bands.

But this low IF level results in the noise pulses presented at the base of the amplifier Q1 to be insufficient for turning it on, and thus fail to stimulate the emitter follower switch Q2.

After much thought, I decided to increase the sensitivity of Q1 by altering its resting base voltage to a slightly (+) potential. This idea came from the supposition that noise pulses brought from IC1 via C9 could not reach the 0.6V potential to turn Q1 on, due to their dissipation via R5 to ground.

Adding a 2.2M resistor from the 9V potential ( Q 1 Collector ) to its Base, elevates the resting base potential to +0.39V instead of 0V, thus keeping it "primed" for the arrival of the weak noise pulses.

Having a 4M7 trimmer wired appropriately, I tried a 2M resistance, touching it between the appropriate leads of R9 and R5 while receiving...and MIRACULOUSLY the pesky noise was COMPLETELY eliminated!

Additionally, there was absolutely no ill effect in the RX performance for either weak or moderately strong signals. It was like someone had switched the sparks off.

A big AHHA or EUREKA moment intervened.

Played around trying different values for the new resistance, and saw a similar benefit from 1M8 to 2M2. The effect waned for R > 3M.

Finally, I ended up installing 2 M 1/4W resistor (2x1M in series), unable to locate a 2M2 one in my spare parts box. What is amazing about this simple modification besides its effectiveness, is its simplicity. One does not even have to remove the NB board from the radio!

With a 30W soldering iron slender tip, I first tinned the 2 respective contacts of R5 and R9, followed by installing the new resistor and securely soldering its ends after tucking them below the leads of the aforementioned resistors. Fig 2

Time required 5 minutes!

The result must be heard to be believed. The following videos tell the story better that any elaborate text descriptions.

Congratulations your old faithful OMNI has gotten a new life!

Fig1 The unmodified NB board.

Fig 2 The NB board after the modification.

Fig 3 The schematics *page 60 of the TT546 manual

The following YouTube videos demonstrate the radio listening on 40m weak and moderate strength signals after the modification. Note that S5 in 40m band corresponds to S9 in my other radios.

The function of the NB before the mod had been IDENTICAL to the OFF position shown in the videos.

https://www.youtube.com/watch?v=9JsnwBTyY6Qos|

https://www.youtube.com/watch?v=35U-BeTcNXk

Any comments or questions are welcome.

Enjoy !

73,

Marinos, sv9dru / ki4gin