[time-nuts] troubleshooting a FE-5680A (got two; only one works)

[Peter Bell]

It does seem rather off frequency - the other thing that might be
worth checking is to verify that the PTC thermistor soldered to the
crystal is getting hot, since failure of that would seem likely to
induce frequency errors of about that magnitude.

Regards,

Pete

On Thu, Dec 1, 2011 at 2:01 AM, beale <beale at bealecorner.com> wrote:
>> [Peter Bell] ...check to make sure that the VXCO is actually sweeping through 10MHz - if not, you can try adjusting C217 to centralise the sweep range.
>
> Thanks Pete, you are exactly right. As mentioned, the VCXO was sweeping from 9.999799 to 9.999994 MHz with no lock. I found that the C217 trimmer was nearly at minimum C already, but a tiny tweak to move it to absolute minimum C shifted up the center frequency 30 Hz, which (just) enabled a lock at 10.000000 MHz.  Given the pre-existing C217 setting, I wonder if this unit was marginal even from the factory. I don't know what minimum value C217 has, but I could remove C217 completely which might center it more. Or, maybe there is a way to increase the VCO tuning range, no doubt at a cost in phase noise.
>
> Asking for a replacement unit is another option, but I'm just too impatient for that, so I'm taking it as an opportunity to learn something.  (in fact I did send him a note, but I'm not expecting anything.)
>
> Thanks to Pete and the time-nuts list for the solution!
>
> -john
>
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