Omni VII Model 588 Frequency Calibration

From the Omni VII Yahoo! Group June 17, 2008

This from Tony, W1OT:

I got my answer from Ten-Tec. Remove the top cover and orient the Omni VII per the manual instructions for installing a Collins filter. The adjustment is made by a *tiny* pot labelled REF ADJ. It is on the right side of right-hand circuit board, about 2/3 of the way to the rear. I made an adjustment tool with a blade about .045 in. wide to fit either of the crossed slots.

73, Tony

Posted by Puck, W4PM

June 18, 2008

This is how I determine the amount my O7 calibration is off.

  1. I tune in WWV at 10 mHz
  2. I set the rig to LCW and the STEP to 1 Hz
  3. I push the SPOT button and tune for zero beat.
  4. Assuming the CW offset is equal to the side tone frequency this should give you a pretty good indication of how far the rig is off.

When I got the rig a year ago this method indicated the rig was reading high by 10 Hz. Today this has increased to 14 Hz. In my mind this is not enough to warrant adjusting things at this time.

What do you think of this method? I'm not trying for laboratory grade accuracy.

Puck, W4PM

June 18, 2008

Hi Puck,

A better way than using the Spot is to set up like you did and switch between LCW and UCW and tune until the tones are identical or use a simple spectrum display program, like the one that comes with MMTTY, and change parameters for CW. Then when you switch between LCW and UCW tune until the signal doesn't move. Using this non-scientific method I got a difference of about 5 Hz. In other words at 10,000,005 the display appeared the same on LCW and UCW, while 10,000,004 and 10,000,006 show movement.

BTW the parameters for MMTTY for CW for a 700 Hz offset are, Mark 690 Hz and Shift 20 Hz. To add these values you have to edit the mmtty.ini file.

73 Art W2NRA
"Keep to the Code!"

June 18, 2008

That works too. I get the same results either way - 14 Hz off in my case. A program which is even better than MMTTY for this purpose can be found here:

Thanks Art,

Puck, W4PM

June 19, 2008

I adjusted the little 20 MHz xtal oscillator using the spot button and doing a zero beat on either LCW or UCW while listening to 15 MHz WWV. I used a jeweler's screwdriver (one you can get at Academy in those glasses repair kits at the checkout counter) to make the adjustment. The 20 MHz oscillator is clearly marked on the top board near the upper right corner when the top cover is off and you are standing in front of the rig.

Now the error is within about 1 Hz while listening to time signals on frequencies 5, 7.335, 10, 14.670, and 15 MHz. I put these frequencies in the stored memory slots so all I have to do is hit M>V and dial in one of the time signals and then hit SP. After a few seconds the rig returns to the frequency prior to hitting M>V. I often check the calibration because I am fascinated at how stable the frequency is from a cold start to a hot cabinet. I learned by experience that its best to adjust the 20 MHz oscillator while listening to 15 MHz WWV because that's where the most error was showing up.

73 de k5gp, Gene

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