[time-nuts] VCXO in a watch timing machine

[WB6BNQ]

Hi John,

OK !  It is good that the other resistors were there.  It is also good that a
lower EFC voltage lowers the frequency.

I am not sure what standard values are available to you in the UK.  I picked a
standard value in the US that would roughly center the pot's range equally around
2.1 volts.

So, I would suggest paralleling a 5.1K 1% metal film resistor across the lower 3K
resistor.  Attaching from the bottom of the pot to that resistor string's ground
point would probably be easier then trying to attach directly across the SMT 3K
resistor.

The 5.1K added resistor puts the bottom resistance value at 1888.9 Ohms.  The
whole string would nominally be 5888.89 Ohms and ups the current in the string
from 0.743 ma to 0.883 ma.  A small change and should not be a problem as to
power dissipation in the upper 3K.


The voltage at the bottom of the pot should be near 1.67 volts and the top of the
pot should be near 2.55 volts.  That should place the range so that the VCXo goes
below 18 MHz.  If not then select the next lower standard value.  Try to obtain
1% metal film at 1/8 watt or second best carbon film at the same ratings.  Do not
use the OLD carbon composition type resistors.

As Bob points out and I also agree, if your unit looks like the type I saw on the
web site, then the 18 MHz oscillator most likely runs the computer system.  That
does not necessarily mean it is also the clock that is clocking the A/D that is
digitizing the sound.  So, varying the 18 MHz may not change your results.  The
important clock, for accuracy, is the one that clocks the A/D inside your unit.

Time will tell.  After you add that resistor and then readjust the 18 MHz based
upon your counter, you will know if the watch is on time the next day.  If not
then it will require further study of the circuitry.

Good luck,

Bill....WB6BNQ


john wrote:

> Hi
>
> Some very good questions - thanks for the responses. No schematic (or
> documentation of any description - Ebay purchase), but I've done some
> dismantling and had a poke around with a multimeter. This is what I
> find:
>
> The power supply provides +5.2V and +/-6V. Strangely, the silkscreen on
> the board power connector says +5V, +8V, -8V and -24V. The power supply
> has no components or wires for this latter voltage, so that's a bit of a
> mystery. The -24V rail disappears off into some components, so maybe
> it's an 'option' on another model? Anyway, let's not get sidetracked.
>
> The 1k pot is sandwiched between two 3k resistors (surface mounted on
> the back, so not immediately obvious). I get 0V - 2.2V - 3V - 5.2V, so
> only 0.8V adjustment range. The lower the voltage, the lower the
> frequency, and vice versa, so I could just short the resistor that's
> connected between ground and the pot?
>
> Modern mechanical watches are relatively impervious to changes in
> temperature - balance springs and balances are made from materials which
> are much better in that regard than their carbon steel forbears, which
> required split bi-metallic balances to compensate.
>
> I agree that 18MHz does seem an odd number. The counter can work with
> watches that beat at 5, 5.5, 6, 7, 8 and 10Hz so you'd think it would
> relate to those in some integer way.
>
> Regards
> John
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