Ken,
I had to make similar measurements on my Omni VI when I had
temperature-dependant XTAL OSC instability problems. I found that the
best place to measure the control voltage is across R25; you can't get a
sensible measurement on connector 84 because that's carrying a mixture
of the oscillator RF output to the Logic Board plus the returned DC
control voltage.
I asked TT Service for a recommended loop control voltage setting, but I
didn't get a reply. In the end I simply monitored the DC voltage across
R25 with a 'scope and adjusted the inductor cores until the loop lost
lock. I noted the upper and lower voltages where it lost lock and then
set it mid way.
Hope that helps.
Steve G3TXQ
Ken Brown wrote:
> Hi all,
>
> Changing C19 on the XTAL OSC LO MIXER BOARD did not make any big change
> in the power output, and I can still hear a periodic "relocking" sound
> when on 30 meters (maybe on other bands too). By scoping the output of
> the LO board, I can see that on bands with low TX power output, the
> output of the LO is also low (relative to other bands, I don't know what
> it should be), so I am suspecting the that the last TX mixer, U1 on the
> BPF/FRONT END BOARD is starved for LO power. I also see on some bands
> there the LO signal appears on the scope as multiple tones. I am doing
> this all in CW mode, so I don't expect to see anything but constant
> amplitude sine waves anywhere in the transmit signal path. I am not too
> concerned if they are not pure sine waves, but if they look like
> multiple tones, or modulated it seems like something is wrong. Maybe the
> carrier balance in the LO MIXER is off, letting some of the 5 MHz VFO
> signal through.
>
> The next things I'll try is to get all of the XTAL OSCillators adjusted
> for "minimum stress", which is to have the PLL correction voltage as
> close as possible to nominal, whatever that is, and I'll check the LO
> mixer carrier balance. Based on the values of R25 and R26 (which are 1k
> and 470k) I think I can connect a 10 Meg scope probe at connector 84 to
> monitor the DC voltage there, without severely disrupting the circuit.
> Does anyone know what the DC voltage at 84 is supposed to be? I guess I
> can determine the full range of DC voltage there, and then set the XTAL
> osc tuning coils for the center of that range. I'd sure rather have a
> specification to aim for.
>
> By the way, if anyone is wondering why I don't just sent the rig to
> Ten-Tec for service, there are a number of reasons. Shipping cost both
> ways from Hawaii would be many times the cost of any parts that need to
> be replaced. If I had Ten-Tec fix it I would not learn much.
>
> DE N6KB
>
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
>
>
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|