[time-nuts] VCXO in a watch timing machine

Scott McGrath scmcgrath at gmail.com
Tue Feb 5 16:55:30 UTC 2013


Have you actually tried to 'beat' a watch.  Most of these devices use a audio pickup and sense the mechanical impulse from the drive system.   They then indicate whether the watch is fast or slow

I have one which uses the sound card to do the dsp and the secret sauce is the pickup and amplifier.   Yes I work on clocks when I have time....

I would not assume the VCXO is out of spec unless you have a known good unit to compare against.

Encouraging pirate manufacturers by buying their product is another topic for another time.   If you had purchased a used unit you would have been able to get support for it.

Sent from my iPhone

On Feb 5, 2013, at 11:38 AM, john <john at ic0n.org.uk> wrote:

> 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
>> 
>> 
>> 
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com
>>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>> and follow the instructions there.
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.



More information about the Time-nuts_lists.febo.com mailing list