[time-nuts] VCXO in a watch timing machine

john john at ic0n.org.uk
Tue Feb 5 11:38:36 EST 2013


Hi Bill

I've just checked - the oscillator does indeed go to the XTAL pins of 
the Philips microcontroller. The crystal I thought was for the mc turns 
out to be for the LCD display controller.

I'll have a rummage for some resistors.

Thanks
John


On Tue, 05 Feb 2013 05:20:48 -0800, WB6BNQ wrote:
> 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
>
>
>
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