[TenTec] Orion Frequency Accuracy

[k6se at juno.com]

When I checked the frequency accuracy of my new Orion, the LCD readout
when 10 MHz WWV was zero beat was about 20 Hz low (after a long warmup). 
On 15 MHz WWV it was 30 Hz low.  That's not too shabby, but when the
receiver displays down to 1 Hz you tend to want to make it more accurate.

Studying the manual, I determined that the TCXO (Temperature Compensated
Crystal Oscillator) was located on the Synthesizer board, located at A10
(page 4-4 of the manual).  Here's the procedure I used:

1) Remove the Orion's bottom cover.  Use the provided spline wrench (the
manual calls it a "T10 Torx Wrench") to remove the 9 small-headed screws
around the back edge of the bottom cover, then use a Phillips screwdriver
to remove the 4 large-headed screws on the sides of the radio (2 0n each
side).  Pull the bottom cover rearward and lift it off of the radio.

2) Find the small module located near the rear edge of the synthesizer
board.  It should have "45.55 MHz" written on it.  The small hole in the
module is where the oscillator adjustment trimmer is.

3) Connect an antenna and power to the radio and sit the radio on its
side where you can view the LCD display and have access to the oscillator
trimmer at the same time.

4) Power up the radio and tune in WWV on the sub receiver (use the
highest-frequency WWV you can receive for greatest accuracy - WWV
transmits on 2.5, 5, 10, 15, 20 and 25 MHz).  Set the MODE to either USB
or LSB.  Let the radio warm up for at least 30 minutes.

5) Tune the sub VFO so that the displayed frequency is exactly WWV's
frequency.  For example, adjust the VFO so that the readout says
"10.000.000" if you are using 10 MHz WWV.

6) During most minutes, WWV modulates the carrier with an audio tone for
45 seconds.  While the audio tone is being transmitted, change the MODE
to the other sideband.  If the pitch of the tone is not the same when
switching between USB and LSB, the oscillator trimmer needs adjustment.

Note: The pitch of the audio tone alternates between 500 and 600 Hz each
minute.  A 440-Hz tone is transmitted during the 2nd minute after each
hour.  Don't let this throw you off during the adjustment of the
oscillator.  The object is to adjust the oscillator so that the tone is
at the same pitch when switching between USB and LSB.  During minutes
when voice announcements or silent periods are scheduled, no audio tone
is transmitted.

7) Set the MODE to the sideband that the pitch of the tone is highest on.
 Using a suitable alignment tool, adjust the trimmer so that the pitch of
the audio tone is slightly lower (the trimmer is not too tight and
adjusts easily).

8) Listen for any change in the pitch of the tone while switching between
USB and LSB.  If necessary, adjust the trimmer again.  When the trimmer
is adjusted so that there is no discernible difference in the pitch of
the audio tone while switching between USB and LSB, the oscillator is
calibrated.  When you get this close, you may also hear a slowly
pulsating "whoosh".  The slower  the "whoosh" pulsates, the more accurate
the calibration is.

9) Ensure that the frequency display is still "10.000.000" (i.e., you
didn't accidentally bump the VFO knob during the procedure).

10) Replace the bottom cover.

The accuracy of the frequency readout on my Orion drifts about 20 Hz at
10 Mhz (2 PPM) during the first 30 minutes of warmup.  The direction of
apparent drift of WWV is downward which would explain why the factory
adjustment of the oscillator was that amount -- perhaps their lab is a
cooler environment than my hot shack or they don't allow enough warmup
time before making the adjustment.

73, de Earl, K6SE