[TenTec] Mod: Omni VI N.B. killer

[Ira Franklin]

Thanks Steve, I'm going to try it on my 6.   
Ira

Steve N4LQ <n4lq at iglou.com> wrote:
3/10/2005 TenTec Omni VI Noise Blanker Mod:

Most of know what the side effects of a noise blanker are. Unwanted signals in the passband, thumps, squeaks etc. Noise blankers can reduce or eliminate certain unwanted, short duration pulses but when conditions are good, you certainly want the blanker turned OFF. 
On the Omni VI or VI+ you might notice that the N.B. has no off/on switch. The manual says: "Always leave the N.B. control in the maximum counterclockwise position when not in use. This position is off."

Well This is not quiet the whole truth. It is off UNLESS a strong signal appears within the passband of the Omni's first IF, which is around 20khz wide! When a strong CW dit appears within this passband, the noise blanker's agc voltage will jump to a low enough level (less positive) and suddenly activate the two N.B. stages. The N.B. amps, now at nearly full gain, spews out a sudden pulse as if it were responding to noise and guess what happens next:

Have you ever noticed how strong CW signals, >20db over S9, seem to clip? Yes? Well that's because the blanker has been suddenly activated. The gain of it's 2 stage I.F. amp suddenly jumps sky high and it produces a plus, opening the noise gate and clipping the CW. It's the sudden amplitude change that causes this. I have observed this on four Omni VI's. The work around is to kick on the -20db attenuator, turn down the RF gain or turn up the N.B. level to stabilize the blanker's reaction to the agc thus reducing the pulse level. This is fine but maybe you are trying to hear a weak signal and a 30db over 9 guy is 15khz up band banging away. Kicking on the attn. kills the weak guy and turning up the N.B. invites all kinds of unfriendly visitors to your passband. Somehow we must kill the blanker. The cure is easy:

Remove the top lid. Locate the Low Level Driver/N.B. Board. From the front of the rig, it's the left center board. Remove the board. Locate the 2 I.F. transformers. Next to each you will see a small IC. These are U1 and U2. Next to each of these you should see a 33k resistor. These are R30 and R42. R30 connects to pin 5 of U1. R42 connects to pin 5 of U2. Orange, Orange, Orange is the color code. Remove them and replace them with 16K resistors. Reinstall the board. 
After replacing these resistors and turning the N.B. level max counterclockwise, I was able to apply a needle pegging CW signal from another rig with no side effects. The resistor value was chosen by gradually decreasing the value so that the N.B. amplifiers had zero gain, even when presented with a strong signal. I then checked for proper N.B. operation. My friendly power company supplies me with a constant test signal on 10m. Rotating the N.B. control clockwise quickly and totally eliminated the power line noise. The N.B. knob needed to be advanced about 5 degrees more than before to overcome the extra bias I had created. Mission accomplished.
I believe the typical lab test overlooks problems like this. Labs use steady state signals to measure IMD, not pulses and having a blanker run full time is certainly an invitation for Mr. IMD.

If you feel the need to align the N.B. for maximum performance you may do so by peaking L7 and L8. Observe the voltage on pin 5 of either U1 or U2 while applying a carrier to the antenna input. Turn up the N.B. control slightly then peak each transformer for max negative deflection. They peak quiet sharply.

Steve Ellington N4LQ
N4LQ at iglou.com
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