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
|