[time-nuts] Higher resolution HP 5065A adjustment

[Skip Withrow]

Hello time-nuts,

I have found that the frequency adjustment on the HP 5065A at 2x10E-12 is
actually a little on the coarse side - at least for my well aged unit.

So, I came up with a very simple modification which I think is an
improvement.  This change involves replacing the current adjustment with a
'fine' control (at 2x10E-14) and mounting the old control on the chassis as
a coarse adjustment.

1. Acquire a 10K Knobpot like the 1K unit on the front panel (Bourns 3600S)
and a 16.5K 1% resistor.

2. Unsolder the front panel knobpot and remove the 3/8" control nut and
washers, then the knobpot.

3. Drill a 3/8" hole in the chassis and mount the old 1K knobpot (I used a
location near the front panel and sort of behind the old location).

4. Place the 10K knobpot in the bracket on the front panel.  Wire the 16.5K
resistor to the 10K pot using one of the same numbered terminals that was
wired before on the old pot.

5. Wire one of the previous pot wires and a wire to the old pot (use the
same pins as previously wired) to the other terminal of the knobpot.

6. Wire the other previous pot wire and a wire to the old pot to the other
end of the 16.5K resistor.

Done.  This should place the new 10K pot and 16.5K resistor in series
across the old 1K pot.  The old pot setting will be slightly higher to put
the unit on frequency (it can be calculated if you like, or just measure
and set the new value).  The front panel control will now have a resolution
of 2x10E-14.  My unit seems to age higher in frequency, so you might want
to start with the fine control around 750 (instead of in the middle at 500).

Now you can happily chase the aging of your 5065A.  I don't have a lot of
data (or time) on it yet, but my guess is that it is in the low parts in
10E-13/day.  Once characterized you should be able to dial in a value for
aging (since last set) and be pretty darn close, or set it against your
GPSDO.  Once set, my unit will stay +/- 5ns for 24 hours when compared to
GPS.  Then the aging starts swamping the current setting, but it is fun
having that kind of control.

Regards,
Skip Withrow