[TenTec] OMNI D BFO problem

[Dr. Gerald N. Johnson]

On Sat, 2009-06-06 at 18:29 -0700, James C. Owen, III wrote:
> I would bet that the BFO is off frequency. Check the trimmer for the SB-N 
> crystal. That would be LSB on 80 & 40 meters and USB on 20M and above.
> Actually if is sounds good  on  20m SB-R it should sound bad  on 75 & 40 
> SB-N as the crystal for the BFO will be different. 
> 73 Jim K4CGY

Not so. With the 9 MHz IF and the 5 MHz VFO the BFO remains on the same
side for all N and is on LSB for 80 and 40 and on USB for 20 without
changing the BFO. There won't be an LSB signals on 20, nor any USB
signals on 75 to compare the sound to. But the tuning range and
direction can be checked on CW.

Sounds to me as if the crystal filter has been changed for a narrower
one and only one side of the BFO adjusted. The odd thing is that there's
only one possible band where SB-R matches users and that 18 MHz if the
opposite side injection crystal mod has been installed.

Otherwise the only reason for SB-R to sound good and SB-N to not is if
the user liked CW on SB-R and didn't care about adjusting the BFO for
SB-N.

The classical, none instrumented way of setting the BFO is to tune in a
marker carrier, tune to the LF edge of the pass band until the signal is
20 or 30 dB down the slope which should be right about 300 Hz from the
passband corner. Adjust the BFO for zero beat. Repeat on the opposite
side band. If you start on the correct sideband, it won't take too many
iterations to get both to match. No doubt there is a specified frequency
for the standard filter for each BFO frequency which is probably the
same crystal with two shifters involved. But that requires a frequency
counter or very well calibrated 9 MHz range receiver. Tuning for 20 or
30 dB down or 300 Hz from the corner works on all radios with all
filters.

It would be beneficial to plot the filter response. It might be that the
filter response has changed from an open circuit on the board or in a
crystal or simply that a CW filter has been stuffed in the SSB filter
slot.

73, Jerry, K0CQ
> --- On Sat, 6/6/09, Don Rasmussen <wb8yqj at yahoo.com> wrote:
> 
> > From: Don Rasmussen <wb8yqj at yahoo.com>
> > Subject: [TenTec] OMNI D BFO problem
> > To: tentec at contesting.com
> > Date: Saturday, June 6, 2009, 7:05 PM
> > Hello All,
> > My OMNI D sounds great on SB-R (reversed sideband), but
> > sounds as if the passband is misadjusted for SB-N. That is
> > to say OMNI D does not have IF shift on the front panel but
> > sounds like if it did it would be turned to where the audio
> > is all high's. Anyone have a common cause for this before I
> > go looking inside the set, maybe some BFO issue?
> > TIADon