[TenTec] OMNI-VI problem

[Dr. Gerald N. Johnson]

On Mon, 2009-10-05 at 21:12 -1000, Ken Brown wrote:
> Hi Gerry,
> 
> >
> > <SNIP>
> >
> > Actually the synthesizer is up in the 450 to 500 MHz region for better
> > phase noise and then divided down to 5 to 5.5 MHz where its used alone
> > for 80 and 20 and mixed with a crystal for the other bands.
> >   
> Actually the synthesizer generates 4.97 to 5.53 MHz as I said. The way 
> it does this is to use a PLL operating at 200 - 220 MHz. The VCO in this 
> PLL is actually a VCXO using a 39.940 MHz crystal. The 200 - 220 MHz 
> output (a harmonic from the 39.9 MHz VCXO) is divided by by 40 to get 
> the 5.0 - 5.5 synthesizer output. It is done this way for the reason you 
> stated, to minimize phase noise.

I thought it was synthesized at a higher frequency for more phase noise
reduction by division. But I've not studied an Omni V or VI manual
later. I was interested in using one for VHF transverter IF and was not
happy with it going to LSB for all bands for CW. Makes for mixed mode
USB to CW contacts with great difficulty and those are not rare on VHF
and up.
> 
> Maybe it is the Omni V that does not use a crystal oscillator on 80 and 
> 20 meters. The Omni VI uses a 18.0030 MHz crystal oscillator.

Its my Corsair II that doesn't have the mixed mode problem of the Omni V
and VI.
> > Loss of either oscillator will cause a dead receiver.
> >
> > The 160m crystal is 5.8 MHz. 
> The crystal used for 160 meters in my Omni VI XTAL OSC board schematic 
> is 16.3030 MHz. With the 5.0 - 5.5 MHz synthesizer working 16.303 - 
> 5.503 = 10.800 and 10.800 mixed with 1.800 will give a product at the 9 
> MHz IF.
> > Add 1550 that's 7350, quite a ways from the
> > 9 MHz IF, but twice 5.8 is 11.6 subtract 1550, that's 10.050, still
> > outside the filter. 5.8 + twice 1550 is 8.900 MHz. Still a supersonic
> > beat note. Twice a 1600 MHz BC station would mix with the 5.8 MHz
> > crystal to pass through the IF at 9 MHz.
> >   
> I guess that would be the case in an Omni that uses a 5.8 MHz crystal on 
> 160 meters.
> 
> In an Omni VI like mine that use a 16.303 MHz crystal, and the 5.0 - 5.5 
> MHz synthesizer not working, the most straightforward mixes to 9 MHz 
> would be from 25.303 MHz and 7.303 MHz. Since Joe identified the MWBC 
> station that he hears at 1550 kHz, I would say there is some other 
> component involved in some IMD product he is hearing.

Yes, mixing 5 with 5.8 to get 10.8 leaves lots of room for twice either
to also be in the mixer output.
> 
> It is also possible that there is another station with the same program 
> on 7.303/5 = 1.460 and he is hearing the 5th harmonic of that. That is 
> not too hard to believe if you tune around the MWBC dial and hear how 
> much duplication of programming there is on multiple stations. I expect 
> Joe would have heard that one too if it were th

Plus at least the Family Radio religious network has stations on AM, FM,
and most all short wave bands with the same programs, but local hourly
station ID.
> 
> > I'd concentrate on the synthesizer.
> >   
> Yep. First determine whether the synthesizer has any output at all. If 
> it does, then try to determine whether it is making it to the XTAL OSC 
> LO MIXER BOARD. The simplest explanation is a failure of the cable and 
> connectors between boards. I hope it is that simple.

There certainly has been many a TenTec radio fixed by wiggling the cable
connectors. A quarter drop of DeoxIT on each pin helps that process.
Then tightening PC board mounting screws that are often supplying
grounds fixes many a radio.
> 
> DE N6KB

73, Jerry, K0CQ