Omni V PBT & IF/AF Alignment

(the following info was posted to the Omni V Yahoo Group by AA8VE)

My experience with the Omni V has been to keep the rig in it's best possible condition for the best performance. Cleaning all the plug in connections on all the boards will reduce internal noise and improve performance and make sure every running oscillator is in perfect alignment. Get all the BFO oscillators aligned per the manual and getting the PBT oscillator set to 15.3MHz when centered so all the crystal filters are centered. For the best receive and sig to noise ratio the slugs for L18, L19 and L20on the PBT board must be peaked and then fine tune L19 for the best sig to noise ratio. All the slugs and trimmers on the 9MHz filter board should be peaked and centered first. C11 and C13 trim caps can be fine tuned for sig to noise as well but most are just peaked and it's good enough, if the receiver is too hot for weak signal work C11 and C13 can help but only a couple db of drop should be used.

When it comes to really fine tuning for best sig to noise and MDS I find the most benefit from adjusting L19 on the 9Mhz filter board after L18 and L20 have been peaked. For some reason the silk screening lettering on the internal bottom cover is wrong L19 and L20 are reversed on the cover, L20 is on the outer edge of the board but I think they labeled that as L19 on the cover. To adjust MSD (Minimum Discernible Signal) I use the weakest input carrier that I can find like a marker generator crystal oscillator attenuated down so I can just tell it's there above the internal noise floor. Turn the AGC off and set the RF gain back to about 1:00. Then turn the AF gain nearly fully up, with the RF gain fully up you will notice the AF gain hits a point at about 3:00 where the sound won't change any more so you want it set to just below or at that point about 3:00, center the Tone control and set Notch, Fade and BP fully counter clockwise, connect an AC milli-volt meter or a db meter to the external speaker jack which also will disable the internal speaker. Then adjust the strength of the signal generator so you can see a change in the volt meter when it's on and off, just enough signal so it can be seen when the generator is turned on, use a switch in the feed line rather than turning the generator on and off so it will not drift or change pitch. With the generator on adjust the VFO knob for 600Hz tone and PBT so it's at the peak, readjust the signal generator to the lowest signal you can see within reason. I Peak the slugs 18, 19 and 20 with generator on, then adjust L19 only, remember your looking for the highest ratio of signal to noise not the strongest signal so turn the signal generator on and off as the slug is turned one way and then the other till you find the highest ratio or biggest difference when the carrier is on to when it's off, chart the readings as you go. Use which ever filter gives the best ratio reading at the lowest amount of carrier. Basically with L19 you're looking to dampen the internal noise floor while allowing the lowest generator signal possible to pass through. When done recalibrate the S-meter because it will probably not read right, the best S-meter calibrator I've found if the unit offered from Elecraft like XG1 or XG2.

I was told by T*T that L19 offers the best adjustment for signal to noise ratio with all others peaked with everything just peaked for max smoke the Omni V receiver can be too hot and the internal noise can be stronger than the weak signals. Adjusting for best signal to noise ratio is a very subtle thing but every little thing can improve the rig for weak signals, when a receiver is too hot it can be stressing to listen to with hiss and snap sounds. The Omni V has plenty of gain in it's receiver so don't worry about loosing anything if L19 isn't peaked for max noise. Turning the main RF gain back can help reduce noise but then again RF gain will bring down the whole receiver rather then just the internal noise. Also when setting up the signal generator I think it's best to tune the rig so it's getting the tone at the normal CW offset pitch like 600 hz for the Omni V so even the pitch of the carrier on CW will be set to the best sig to noise ratio. Once L19 has been adjusted I then set up the controls as if I were using it normally but leave the signal generator connected and do the same thing adjusting the RF gain pot to find the position that gives the best sig to noise ratio with the RF gain so I know where to set it for weak signal work, my Omni V works best with the RF gain set to about 1:00 so I pretty much leave it there all the time, it gives me a bit of head room for times when I need to do something else to pull a weak signal out, it's different depending on conditions, some times I need more RF gain and other times I need less but it really helps to understand how my rig works to get the most from it.

One of the things that makes the older T*T rigs so good is that they don't have quite as sensitive receiver as the Omni V has, like the Triton's and early Omni's If you ever listened to one of those great rigs you would notice how comfortable they are to listen to, they make the Omni V seem way too hot. This high sensitivity is a good thing of course but it can be a bit much too so adjusting for best sig to noise will get the maximum benefit possible plus it will be more comfortable to listen to when doing weak signal work. This fine tuning will improve the rig and give it an extra little boost in RX performance, it's also something most of these rigs didn't get when new because it can take time to really get them at the top of what they can do. When they were built this extra little bit isn't usually done because of the time involved. By just peaking out everything the rig will still meet it's spec which is good enough for most hams but with a little time and effort they can be even better and there's really no mod work needed.

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